The field of exoplanet atmospheric characterization has recently made considerable advances with the advent of highresolution spectroscopy from large groundbased telescopes and the commissioning of the James Webb Space Telescope JWST. We have entered an era in which atmospheric compositions aerosol properties thermal structures mass loss and threedimensional effects can be reliably constrained. While the challenges of remote sensing techniques imply that individual exoplanet atmospheres will likely never be characterized to the degree of detail that is possible for solar system bodies exoplanets present an exciting opportunity to characterize a diverse array of worlds with properties that are not represented in our solar system. This review article summarizes the current state of exoplanet atmospheric studies for transiting planets. We focus on how observational results inform our understanding of exoplanet properties and ultimately address broad questions about planetary formation evolution and diversity. This review is meant to provide an overview of the exoplanet atmospheres field for planetary and geoscientists without astronomy backgrounds and exoplanet specialists alike. We give special attention to the first year of JWST data and recent results in highresolution spectroscopy that have not been summarized by previous review articles.
['Astrophysics - Earth and Planetary Astrophysics', 'Astrophysics - Solar and Stellar Astrophysics', 'Physics - Geophysics']
Transiting Exoplanet Atmospheres in the Era of JWST
{'Transiting Exoplanet Atmospheres in the Era of JWST': 'Eliza M.-R. Kempton 1 and Heather A. Knutson 2  \n1 Department of Astronomy, University of Maryland, College Park, MD 20742, USA  \n2 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA', 'ABSTRACT': 'The *ield of exoplanet atmospheric characterization has recently made considerable advances  with  the  advent  of  high-resolution  spectroscopy  from  large  ground-based telescopes and the commissioning of the James Webb Space Telescope (JWST). We have entered  an  era  in  which  atmospheric  compositions,  aerosol  properties,  thermal structures, mass loss, and three-dimensional effects can be reliably constrained. While the  challenges  of  remote  sensing  techniques  imply  that  individual  exoplanet atmospheres will likely never be characterized to the degree of detail that is possible for solar  system  bodies,  exoplanets  present  an  exciting  opportunity  to  characterize  a diverse array of worlds with properties that are not represented in our solar system. This review article summarizes the current state of exoplanet atmospheric studies for transiting planets. We focus on how observational results inform our understanding of exoplanet  properties  and  ultimately  address  broad  questions  about  planetary formation, evolution, and diversity. This review is meant to provide an overview of the exoplanet  atmospheres  *ield  for  planetary-  and  geo-scientists  without  astronomy backgrounds, and exoplanet specialists, alike. We give special attention to the *irst year of  JWST  data  and  recent  results  in  high-resolution  spectroscopy  that  have  not  been summarized by previous review articles.', '1.1. A Historical Perspective': "As soon as the *irst exoplanets were discovered in the 1990s, the quest to characterize these unique objects in more detail began in earnest. Images from science *iction movies come to mind when we picture alien planets, but these discoveries provided us with an exciting new opportunity to actually measure the atmospheric properties of extrasolar worlds. The *irst transiting exoplanet was discovered in 2000 (Charbonneau et al. 2000). These are planets with orbits that track directly in front of their host stars as viewed by an Earthbound observer, producing a small dip in the amount of light received, known as a transit. We focus on transiting planets in this review article because they present  special  opportunities  for  measuring  atmospheric  properties.  The  clever  techniques  for transiting  exoplanet  atmospheric  characterization  that  have  been  developed  by  the  astronomical community (described in detail in Section 1.3) are all premised on using the known orbital geometry of the system to extract the planetary signal from the combined light of the planet and host star. These techniques have been applied with great success over the last two decades to measure a host of atmospheric properties.  \nThe *irst detection of an exoplanet atmosphere occurred in 2002 (Charbonneau et al. 2002). The planet, HD 209458b, was the only known transiting planet at the time (although that was not the case for long), and it belongs to a broader class of exoplanets referred to as 'hot Jupiters'. Such planets are aptly named for their large sizes and small orbital separations - HD 209458b orbits its Sun-like host star every 3.5 days at an orbital distance of 0.05 AU, and it has a radius of 1.35 Jupiter radii ( R J ). By measuring  a  small  amount  of  excess  absorption  during  transit  at  the  wavelength  of  the  sodium resonance  doublet  (589.3  nm)  with  the  STIS  instrument  on  the  Hubble  Space  Telescope  (HST), Charbonneau  et  al.  (2002)  inferred  the  presence  of  gaseous  sodium  in  the  planet's  atmosphere. Although subsequent studies of HD 209458b's sodium absorption signal with ground-based high resolution spectrographs have revealed that this measurement may be biased by deformations in the stellar  line  shape  due  to  the  planetary  transit  (Casasayas-Barris  et  al.  2020,  2021),  this  *irst  \natmospheric measurement unquestionably marked the birth of a new *ield of exoplanet atmospheric characterization studies.  \nNot long after, came the *irst measurements of exoplanetary thermal emission via secondary eclipse (Deming et al. 2005; Charbonneau et al. 2005), which occurs when a planet passes behind its host star. Then, in 2007, the *irst phase curve observations of thermal emission versus orbital phase were  obtained  for  the  hot  Jupiter  HD  189733b  (Knutson  et  al.  2007).  The  thermal  emission measurements were all made with NASA's Spitzer Space Telescope, which became a workhorse for infrared (IR) characterization of exoplanet atmospheres before it was decommissioned in late 2020. Other  important  *irsts  include  the  measurement  of  escaping  gas  from  an  exoplanet  atmosphere (VidalMadjar et al. 2003), and the *irst robust detections of molecules and (more tentatively) highaltitude  winds  using  a  novel  cross-correlation  spectroscopy  technique  with  high-resolution spectrographs  on  ground-based  telescopes  (Snellen  et  al.  2010).  More  details  on  all  of  these observational techniques can be found in Section 1.3. All of the aforementioned observations were of hot Jupiter targets. The *irst atmospheric spectrum of an object smaller than Neptune was obtained in 2010 for the planet GJ 1214b (Bean et al. 2010), ultimately indicating the presence of a thick layer of clouds or haze (Kreidberg et al. 2014a). In 2018 the *irst thermal emission measurement was made for a rocky, terrestrial exoplanet, LHS 3844b, disappointingly indicating the lack of any atmosphere at all (Kreidberg et al. 2019).  \nToday,  exoplanet  atmospheric  characterization  has  become  its  own bona  8ide sub-*ield  of astronomy. Detections of several dozen atomic and molecular species along with clouds and hazes have  been  claimed  in  the  literature  for  over  100  individual  exoplanets 1 (see  e.g.  Burrows  2014; Madhusudhan et al. 2016; Deming & Seager 2017; Madhusudhan 2019). We note that some of these detections  have  been  made  at  high  statistical  signi*icance,  whereas  others  are  more  tentative  or ambiguous, so we encourage the casual reader of the exoplanet atmospheres literature to do so with  \na  critical  eye.  All  of  these  atmospheric  characterization  studies  have  been  helped  along  by  the discovery  of  thousands  of  transiting  exoplanets 2 with  ground-based  (e.g.  HAT,  WASP,  MEarth, Speculoos) and space-based (e.g. CoRoT, Kepler, TESS) surveys. On the population level, tantalizing hints of planetary diversity have been uncovered, and well-founded attempts are being made to tie statistical trends in atmospheric properties to underlying theories of planet formation and evolution (e.g. Sing et al. 2016; Tsiaras et al. 2018; Welbanks et al. 2019; Mans*ield et al. 2021; Goyal et al. 2021; Changeat et al. 2022; Deming et al. 2023; Brande et al. 2023; Gandhi et al. 2023).  \nIn  late  2021,  the  James  Webb  Space  Telescope  (JWST)  launched  successfully,  and  scienti*ic operations began in the summer of 2022. The telescope's large aperture and IR observing capabilities have opened the door to studies of smaller and colder planets than had previously been possible (e.g. Kempton et al. 2023; Greene et al. 2023; Zieba et al. 2023). The high signal-to-noise (S/N) spectra delivered by JWST for larger and hotter planets additionally enable analyses of processes that had remained hidden in earlier datasets such as inhomogeneous cloud formation (Feinstein et al. 2023) and  photochemistry  (Tsai  et  al.  2023).  This  new  era  of  exoplanet  characterization  with  JWST  is accompanied  by  a  windfall  of  ground-based  exoplanet  data  using  recently  commissioned highresolution spectrographs (e.g. CARMENES, ESPRESSO, CRIRES+, IGRINS, GIANO, MAROONX, etc.) that  are  providing  detailed  compositional  measurements  for  hot  and  ultra-hot  giant  planets  (e.g. Birkby 2018; Giacobbe et al. 2021; Pelletier et al. 2023; Gandhi et al. 2023). The *irst JWST exoplanet observations have already been transformative, as have studies that have detected a slew of atomic, ionic, and molecular species in hot Jupiter atmospheres from the ground. These recent results will be summarized  in  the  following  sections  along  with  the  pre-existing  context  from  two  decades  of exoplanet atmospheric characterization studies.", '1.2. Exoplanet Demographics': 'We currently know of more than 10,000 extrasolar planets and planet candidates 3 ,  most  of which orbit stars with masses ranging from 0 . 5 - 1 . 5× the mass of the Sun (for astronomers, this corresponds  to  F  through  early  M  spectral  types).  If  we  exclude  planets  detected  using  the microlensing technique (a small fraction of this total), nearly all of these planets orbit stars in our local  neighborhood 4 of  the  Milky  Way  galaxy.  This  means  that  when  we  discuss  the  properties  of extrasolar planets in subsequent sections, we are implicitly focusing on planets orbiting relatively nearby and (unless speci*ied otherwise) Sun-like stars. In this section, we provide a brief overview of this  exoplanet  population  for  the  non-expert  reader.  We  begin  by  brie*ly  summarizing  the  two detection techniques most commonly used to *ind exoplanets and their corresponding sensitivities to different kinds of planets. For readers interested in learning more about complementary microlensing and direct imaging techniques, we recommend reviews by Gaudi (2022) and Currie et al. (2023). For a more comprehensive overview of exoplanet demographics, we recommend the review by Gaudi et al. (2021).', '1.2.1. Detection Techniques': "The *irst planet orbiting a Sun-like star was detected using the radial velocity technique (Mayor & Queloz 1995). This technique relies on the fact that a star and planet will orbit around their mutual center of mass. This causes the star's spectrum to be Doppler shifted as it moves towards and then away from the observer. The semi-amplitude of this Doppler shift is largest for massive planets with short orbital periods (e.g. Fischer et al. 2014); smaller planets on more distant orbits have smaller  \nradial velocity semi-amplitudes and are correspondingly harder to detect 5 . By measuring a planet's radial velocity semi-amplitude, we can place constraints on its mass (technically M p sin( i ), where M p is the planet mass and i is the orbital inclination), orbital period, and orbital eccentricity. We can then convert this orbital period to an orbital semi-major axis using Kepler's third law.  \nOver the past decade the radial velocity technique has been overtaken by the transit technique, which is responsible for identifying most of the exoplanets known today. This technique focuses on planetary systems where the planet passes in front of its host star as seen from the Earth. During a transit, the planet will block part of the star's light. The amount of light blocked tells us the radius of the planet relative to that of the star, and the intervals between transits tell us the planet's orbital period. If we assume that the planet orbits are randomly oriented, the probability of seeing a transit P is given by P = R ∗ /a , where R ∗ is the stellar radius and a is the planet's orbital semi-major axis (Winn 2010). This means that transit surveys are biased towards close-in planets; this bias is even stronger than that of radial velocity surveys. Transit surveys also detect large planets more easily than small planets, as they block more of the star's light.  \nIt is very challenging to detect Earth analogues orbiting Sun-like stars in current radial velocity and transit surveys. Fortunately, the size of both the transit and radial velocity signals increase with decreasing stellar mass. As a result, it is signi*icantly easier to detect small planets orbiting small stars ('M dwarfs'). Small stars are also signi*icantly less luminous than the Sun, and the orbital periods corresponding  to  Earth-like  insolations  are  much  closer  in.  This  means  that  most  small (approximately 1-2 R ⊕ ) transiting planets that are amenable to atmospheric characterization orbit low-mass  stars.  This  has  important  implications  for  our  understanding  of  the  population-level properties of small rocky exoplanets.  \nAs noted in Section 1.1, the close-in gas giant exoplanets known as 'hot Jupiters' were the *irst type of exoplanet detected in orbit around nearby Sun-like stars. These planets are relatively rare, with an order-of-magnitude occurrence rate of approximately 1% for Sun-like stars (e.g., Petigura et al. 2018; Dattilo et al. 2023). Gas giant planets at intermediate orbital distances (orbital periods of ∼ 10-100 days) are often referred to as 'warm Jupiters', and have a moderately enhanced occurrence rate relative to hot Jupiters (e.g., Fernandes et al. 2019; Fulton et al. 2021). Gas giant planets at larger separations  (orbital  periods  greater  than  several  hundred  days)  are  typically  referred  to  as  'cold Jupiters'.  The most precise estimates of the occurrence rates of cold Jupiters currently come from radial velocity surveys, as there are very few transiting gas giant planets at these separations (e.g. Foreman-Mackey et al. 2016). These surveys indicate that the occurrence rate of gas giant planets  \nFigure  1. Distribution  of  con/irmed  exoplanets  in  mass-period  space.  Planets  with  spectroscopic measurements that constrain their atmospheric properties are shown as dark points, those without are shown as light points. This review article focuses on atmospheric characterization of transiting exoplanets, i.e. the dark purple pentagon symbols. Solar system planets are shown as yellow circles for context. Both the radial velocity and transit techniques are most sensitive to detecting massive planets on close-in orbits, while the direct imaging technique is most sensitive to young, self-luminous planets on relatively wide orbits. Figure adapted from Currie et al. (2023).  \n<!-- image -->  \nrises dramatically as we move farther away from the star (e.g., Fernandes et al. 2019; Fulton et al. 2021), with 14 ± 2% of Sun-like stars hosting a gas giant planet between 2 - 8 AU (Fulton et al. 2021).  \nCurrent radial velocity surveys of bright nearby stars have baselines as long as ∼ 30 years; this means that our knowledge of the occurrence rates of gas giant planets in these data sets is limited to planets with orbital semi-major axes comparable to or less than that of Saturn in our own solar system.  \nExoplanets smaller than Neptune (typically de*ined as < 4 R ⊕ or ≲ 10 M ⊕ ) are often found on close-in orbits around Sun-like stars. Such planets have an overall much higher occurrence than the gas giant planets: ∼ 50% for orbital periods of less than 100 days (just outside the orbit of Mercury in the solar system; e.g., Fulton & Petigura 2018; Hsu et al. 2019). This population is observed to have a bimodal radius distribution, with peaks at 1.3 and 2.4 R ⊕ (e.g., Fulton et al. 2017; Van Eylen et al. 2018;  Fulton  &  Petigura  2018;  Hardegree-Ullman  et  al.  2020;  Petigura  et  al.  2022).  The  smaller planets (radii between 1 . 0-1 . 7 R ⊕ ) have bulk densities consistent with Earth-like compositions (e.g. Lozovsky et al. 2018; Dai et al. 2019), and are therefore termed as 'super-Earths'. The larger planets (radii between 1 . 7-3 . 5 R ⊕ ) have lower bulk densities, consistent with the presence of modest (a few percent of the total planet mass) hydrogen- and helium-rich gas envelopes (e.g., Lozovsky et al. 2018; Lee 2019; Neil et al. 2022). These planets are therefore termed as 'sub-Neptunes', although some may also have water-rich envelopes (see discussion below). The location of the bimodal radius 'gap' moves towards smaller radii at larger orbital separations (Fulton et al. 2017; Van Eylen et al. 2018; Fulton & Petigura 2018; Hardegree-Ullman et al. 2020; Petigura et al. 2022). This suggests that the gap was carved out by either photoevaporative (e.g., Owen & Wu 2017) or core-powered (e.g., Ginzburg et al. 2018; Gupta & Schlichting 2019) mass loss 6 ,  although Lee et al. (2022) proposed that the division between the two populations might instead be largely primordial.  \nThe order-of-magnitude occurrence rates stated above apply to planets orbiting Sun-like stars (meaning F/G/K main-sequence stars, for astronomers). These values change with decreasing stellar mass; gas giant planets are a factor of 2-3 less common around low-mass stars (e.g., Montet et al. 2014;  \nBryant et al. 2023), while small planets on close-in orbits appear to be a factor of a few more common (e.g., Dressing & Charbonneau 2015; Mulders et al. 2015; Hardegree-Ullman et al. 2019; Hsu et al.  \n2020).", '1.2.3. Composition Constraints from Bulk Density Measurements': "For planets with measured masses and radii, we can obtain a constraint on their bulk densities and corresponding  bulk  compositions.  The  bulk  densities  of  gas  giant  exoplanets  are  relatively  low, indicating that they possess thick, hydrogen-dominated gas envelopes (e.g. Thorngren et al. 2016). These bulk densities can be used to place an upper limit on the abundance of hydrogen and helium relative to heavier elements in the planet's atmosphere (often referred to as the planet's 'atmospheric  \nFigure 2. Measured masses and radii for small planets orbiting small (M dwarf) stars from Luque & Pall'e (2022). This sample only includes the subset of planets with small fractional uncertainties in mass and radius. Theoretical mass-radius relations for planets with an Earth-like bulk composition (dashed brown line), half water  and  half  Earth-like  (solid  blue  line),  and  Earth-like  with  a  few  percent  hydrogen-rich  atmosphere (dashed orange line) are overplotted for comparison. The light orange shading denotes the range of possible hydrogen-rich  atmospheres  that  can  be  retained  by  these  planets  under  a  range  of  possible  starting assumptions. Figure adapted from Rogers et al. (2023) .  \n<!-- image -->  \nmetallicity' by astronomers; Thorngren et al. 2019). Exoplanets smaller than Neptune exhibit widely varying  bulk  densities,  which  re*lect  their  varying  bulk  compositions.  Rocky  super-Earths  have relatively high bulk densities, while sub-Neptunes with puffy hydrogen-rich atmospheres have much lower bulk densities (e.g. Lozovsky et al. 2018; Neil et al. 2022).  \nSmall planets with intermediate densities are more ambiguous, as their masses and radii can be equally well *it with either water-rich or hydrogen-rich envelopes (e.g., Mousis et al. 2020; Turbet et al. 2020; Aguichine et al. 2021, see Fig. 2). For lower-mass stars, which are less luminous, the water ice line is located much closer to the star. This means that even relatively close-in planets forming around low-mass stars may still be able to accrete signi*icant quantities of ice-rich solids (e.g. Kimura &  Ikoma  2022).  Although  some  of  these  'water  worlds'  may  subsequently  accrete  hydrogenrich envelopes, such envelopes are more dif*icult to retain when they orbit low-mass stars. These stars are more magnetically active than their solar counterparts, which means that they emit more high energy photons,  and  also  have  more  frequent  *lares  and  coronal  mass  ejections,  all  of  which  can  drive atmospheric out*lows (e.g., Harbach et al. 2021; Atri  &  Mogan  2021). It is  therefore  thought  that planets  with  water-dominated envelopes may be more common around low-mass stars. Luque & Pall'e  (2022)  plotted  all  of  the  currently  known  planets  orbiting  low-mass  stars  with  precisely measured  masses  and  radii  in  mass-radius  space  and  identi*ied  a  sub-population  of  low-density planets  whose  densities  appear  to  be  well-matched  by  water-rich  compositions  (for  alternative hydrogen-rich models, see Rogers et al. 2023). Upcoming observations of candidate water worlds using JWST will soon provide the *irst direct constraints on their atmospheric water content (see Section 2). These atmospheric characterization studies should provide a much clearer picture of the relative frequency of water worlds around low-mass stars.", '1.3. Observational Techniques for Atmospheric Characterization': "There are multiple complementary techniques that can be used to detect and characterize the atmospheric compositions of transiting extrasolar planets, as detailed below. All of these techniques leverage knowledge of the transiting planet's orbit to disentangle the combined (unresolved) light from the planet and its much brighter host star. This is distinct from the approach used to characterize  \ndirectly imaged planets and brown dwarfs, whose thermal emission can be spatially resolved from that of their host stars (for a recent review, see Currie et al. 2023).  \nWhen a transiting planet passes in front of its host star, it will block more of the star's light and  \ntherefore appear larger in wavelengths at which the planet's atmosphere is strongly absorbing. Conversely, the planet will appear smaller and block less of the star's light in wavelengths at which its atmosphere  is  relatively  transparent.  This  wavelength-dependent  transit  depth  is  called  a 'transmission  spectrum',  and  is  the  most  widely  used  method  for  characterizing  the  atmospheric compositions of transiting extrasolar planets. We can calculate the relative size of this wavelengthdependent change in transit depth δD tr using the following expression:  \n(1)  \nwhere R P is the planet radius, R ∗ is the stellar radius, and δR P is the wavelength-dependent change in the planet radius. The approximation holds so long as δR P ≪ R P , which is true even for hot giants with very low densities. This change can be approximated as a multiple of the atmospheric scale height H :  \n(2)  \nwhere k is the Boltzmann constant, T eq is the predicted atmospheric equilibrium temperature, µ is the atmospheric mean molecular weight, and g is the planet's surface gravity. The scaling factor s typically ranges between 1 - 5, with lower values more representative of weak absorption and/or atmospheres with  signi*icant  aerosol  opacity,  and  higher  values  more  representative  of  strong  absorption  in  a cloud-free atmosphere (e.g., Seager & Sasselov 2000; Miller-Ricci et al. 2009; Benneke & Seager 2012, 2013). For atmospheres with very high aerosol opacity, this signature may be completely obscured (see § 3).  If  we  assume  that  the  star  and  planet  both  radiate  as  blackbodies,  we  can  calculate  the planet's predicted equilibrium temperature as:  \n(3)  \nwhere T ∗ is the effective temperature of the host star, a is the planet's semi-major axis, and A B is its Bond albedo (de*ined as the fraction of incident radiation that is re*lected back to space; Seager 2010).  \nThis equation assumes that the planet's atmosphere ef*iciently redistributes heat from the day side to the night side. In the limit of no heat redistribution (i.e., instantaneous radiative equilibrium at each longitude and latitude point), the effective hemisphere-integrated dayside equilibrium temperature can be calculated by replacing the factor of   with a factor of   (Hansen 2008), and intermediate values  \nFigure 3. Schematic diagram illustrating three techniques that can be used to characterize the atmospheric properties of transiting exoplanets. Adapted from a Gigure originally created by Sara Seager (private commun.).  \n<!-- image -->  \nare possible between these two extremes 7 . As demonstrated by these expressions, the overall strength of  absorption  during  the  transit  can  vary  by  more  than  an  order  of  magnitude  when  comparing hydrogen-dominated  (low  mean  molecular  weight)  atmospheres  to  those  with  higher  mean molecular weights (e.g., water, carbon dioxide, methane). The presence of high-altitude aerosols from photochemical  hazes  or  condensate  clouds  can  also  attenuate  the  amplitude  of  gas  absorption features  by  scattering  the  stellar  photons  as  they  pass  through  the  atmosphere.  The  geometry  of transmission spectroscopy means that we are primarily sensitive to the properties of the planet's atmosphere  near  the  day-night  terminator.  This  is  particularly  relevant  when  determining  cloud  \nproperties, which can vary signi*icantly with longitude (see Section 6). Even for clear atmospheres without  signi*icant  cloud  opacity,  the  relatively  long  path  length  of  starlight  passing  through  the planet's  atmosphere  means  that  this  technique  is  primarily  sensitive  to  atmospheric  pressures between 0 . 001 - 0 . 1 bars for typical hot Jupiter atmospheres observed at near-infrared wavelengths (e.g., Fortney 2005; Sing et al. 2016).  \nIf we wait approximately half an orbit we can also observe the planet passing behind its host star (the 'secondary eclipse'). By measuring the relative decrease in light during this eclipse, we can determine the amount of light re*lected (at optical wavelengths) or emitted (at IR wavelengths) by the  planet.  If  we  assume  that  the  star  and  the  planet  both  radiate  as  blackbodies  and  take  the longwavelength  (Rayleigh-Jeans)  limit,  we  can  write  a  simple  expression  for  the  depth  of  the secondary eclipse D sec :  \n(4)  \nThe planet's emission spectrum also contains information about its atmospheric composition, as well as the average temperature as a function of pressure in its dayside atmosphere. For cloud-free hot Jupiter atmospheres observed at near-infrared wavelengths, the shorter path length of light emitted from the deeper layers of the atmosphere means that we can also potentially probe somewhat higher (a factor of a few) pressures as compared to transmission spectroscopy (e.g., Fortney 2005; Showman et al. 2009).  \nClose-in exoplanets are expected to be tidally locked, and as a result can exhibit large daynight temperature  gradients.  This  means  that  the  temperature,  chemistry,  and  cloud  properties  on  the daysides  of  these  planets  can  differ  from  those  measured  at  the  terminator  via  transmission spectroscopy. We can obtain a global view of these atmospheres by measuring changes in the planet's brightness as a function of orbital phase (the planet's 'phase curve'). By measuring the planet's phase curve  at  IR  wavelengths  where  its  spectrum  is  dominated  by  thermal  emission,  we  can  map  its emission spectrum as a function of longitude (Cowan & Agol 2008, 2011; Rauscher et al. 2018; Morris et al. 2022). These phase curves provide invaluable information about the atmospheric circulation patterns of tidally locked exoplanets; see Section 6 for more details. We can also spatially resolve the  \ndayside atmosphere using a second technique called 'eclipse mapping' (Williams et al. 2006; de Wit et al. 2012; Majeau et al. 2012). This technique utilizes the measured changes in brightness during secondary eclipse ingress and egress (de*ined as the periods when the planet is only partially occulted by the star; for de*initions of these terms see Winn 2010) to map the planet's dayside brightness as a function of longitude and latitude. This technique is complementary to phase curve observations, which  can  characterize  the  planet's  night  side  but  can  only  measure  changes  in  the  planet's atmospheric properties as a function of longitude.  \nTo  date,  most  published  observations  of  exoplanet  atmospheres  have  been  obtained  at  low spectral resolution using space telescopes (Spitzer, HST, and/or JWST). Because all of these techniques rely on measurements of very small changes in the star's brightness over multi-hour timescales, it is often dif*icult to achieve the required stability and precision using ground-based observatories. This is  because  the  properties  of  Earth's  atmosphere  also  vary  on  similar  timescales.  However,  recent advances  in  instrumentation  on  ground-based  telescopes  have  opened  up  new  venues  for atmospheric characterization at higher spectral resolution ( R > 20 , 000, where R = δλ/λ ).  At  these resolutions,  spectral  features  from  the  star,  planet,  and  Earth's  atmosphere  can  all  be  readily differentiated from one another. Crucially, the planet's spectral features are Doppler shifted by its orbital  motion,  while  those  of  the  star  and  Earth's  atmosphere  remain  approximately  constant  in wavelength over several hour timescales. This means that we can use this wavelength-dependent shift to uniquely identify the absorption features from the planet's transmission or emission spectrum. Notably, this technique is not limited to transiting exoplanets and can also be used to detect spectral features in the emission spectra of non-transiting planets. For more details see the review by Birkby (2018).", '1.4. Common Model Frameworks for Interpreting Exoplanet Spectra': 'When  *itting  transmission  and  emission  spectra,  we  must  necessarily  make  a  range  of simplifying assumptions in order to build simple parametric models that can be used in atmospheric retrieval frameworks. Although we are only sensitive to the atmospheric properties in a narrow range of pressures (typically 0 . 001 - 1 bars for hot Jupiter atmospheres observed at infrared wavelengths  \nat low to moderate spectral resolution), most retrievals typically assume that the inferred elemental abundances are representative of the bulk atmosphere (i.e., there is no net gradient in elemental abundances  over  the  range  of  pressures,  latitudes,  or  longitudes  probed).  Similarly,  *its  to transmission spectra often make the simplifying assumption that the atmosphere is isothermal, while *its to emission spectra typically utilize a simple parametric vertical temperature pro*ile with up to six free parameters (e.g., Madhusudhan & Seager 2009; Line et al. 2013). Many models assume that the atmospheric chemistry is in local thermal equilibrium, or retrieve for the abundances of individual molecules assuming a single *ixed abundance for each molecule as a function of pressure. For an overview of the exoplanet retrieval codes commonly in use and the corresponding assumptions made by each, see MacDonald & Batalha (2023). It is worth noting that the high quality of recent JWST observations of hot Jupiters has forced modelers to revisit many of these assumptions, some of which have proven to be too simple for the sensitivity of these new data sets. For more background on exoplanet atmosphere modeling, the reader is encouraged to refer to the following review articles:  \nMarley & Robinson (2015), Madhusudhan (2019), and Fortney et al. (2021).', '1.5. High-level Scienti8ic Questions': 'Our ability to characterize transiting exoplanet atmospheres is fundamentally limited by our great distance from these systems and the fact that the planet is viewed as an unresolved object, blended  with  the  light  from  its  host  star.  Despite  immense  improvements  in  remote  sensing capabilities,  it  is  safe  to  say  that  we  will  never  in  any  of  our  lifetimes  characterize  an  individual exoplanet atmosphere to the degree that we have for planets within our solar system. This is a crucial piece of context for the non-astronomer to understand when formulating a realistic vision for the types  of  questions  that  exoplanet  studies  can  address.  Yet  exoplanets  also  present  an  immense opportunity - that of studying a myriad of planetary systems at a population level. Exoplanets also provide access to types of planetary environments that do not exist in our solar system (e.g. hot Jupiters,  sub-Neptunes,  super-Earths,  and  perhaps  water-worlds).  A  simple  summary  is  that exoplanets allow for coarse measurements for many objects, whereas solar system studies provide detailed  data  on  the  outcome  of  a  single  instance  of  planet  formation.  Leveraging  both  types  of  \ninformation together equips us with a more complete view of planetary systems and the processes that give rise to them.  \nGiven this context, the types of questions that exoplanet atmosphere studies aim to address are typically those that relate to bulk properties or large-scale atmospheric structure, or those that tie a collection  of  rough  measurements  to  our  understanding  of  the  exoplanet  population  (or  a subpopulation, thereof). Below, we provide an illustrative list of major open scienti*ic questions that can be targeted through exoplanet atmospheric studies. These questions span the planet size and temperature range represented by transiting exoplanetary systems. Meaningful movement toward answering any of these questions would represent a major advance for (exo)planetary science.  \n- · Did close-in gas giant planets form in situ or migrate in from farther out in the disk?\n- · What are the large scale atmospheric dynamics for hot Jupiters, and how do they differ with respect to solar system giant planets and young, hot, directly-imaged planets on wide orbits?\n- · What are the aerosols in exoplanet atmospheres made of and how do they form?\n- · How much hydrogen and helium gas can small planets accrete, and which planets keep (or lose) their primordial hydrogen-rich atmospheres?\n- · Do water worlds exist, and if so, how common are they?\n- · How do interactions  with  magma  oceans  shape  the  observed  atmospheric  compositions  of subNeptunes and terrestrial exoplanets?\n- · What kinds of outgassed, high mean molecular weight atmospheres do terrestrial planets have, and what does that mean for their potential habitability?\n- · Which terrestrial exoplanets lose their outgassed atmospheres? What determines their total atmospheric masses?', '2. ATMOSPHERIC COMPOSITION': "- 2.1. Composition as a Signpost of Formation and Atmospheric Chemistry  \nThe atmosphere is the outermost layer of a planet and the only component of an exoplanet that can readily have its composition directly measured using remote sensing techniques 8 . We therefore rely on observations of an exoplanet's atmosphere as a window into its history and the processes that shape  its  present-day  state.  For  example,  atmospheric  observations  can  be  used  to  inform  our understanding of unseen features and processes such as surface-atmosphere interactions or interior structure. High H 2 S or SO 2 concentrations in a terrestrial habitable zone planet could be indicative of surface volcanism (Kaltenegger & Sasselov 2010); atmospheric O 2 and O 3 could signify the possible presence of surface life, especially when accompanied by disequilibrium biosignature pairs such as CH 4 (Lovelock 1965; Domagal-Goldman et al. 2014); and a water world might be distinguished from a sub-Neptune with a dry rocky interior via an elevated abundance of water in its atmosphere (e.g. Rogers & Seager 2010).  \nAs with solar system planets, the present-day state of an exoplanet atmosphere is the outcome of  its  entire  history  of  planet  formation  and  evolution.  By  measuring  an  exoplanet's  atmospheric composition, one can attempt to decode the processes that gave rise to that planet in the *irst place. On a single-planet basis, such an analysis is nearly impossible due to vast degeneracies in the range of histories that can all produce similar outcomes, in addition to fundamental uncertainties in the planet formation process and the evolution of protoplentary disks. But on a population level, we can hope to link trends in atmospheric properties to simple theories for how planets form and evolve, and anchor those theories with measurements, analogously to how our understanding of the history of our solar system stems from observations of the many bodies orbiting the Sun. Several examples for how  trends  in  exoplanet  atmospheric  observations  might  be  tied  back  to  planet  formation  and evolution theories are listed, below:  \n- · Giant Planet Mass-Metallicity Relation: Solar system giant planets exhibit a tight anticorrelation between their mass and atmospheric metallicity 9 (Figure  4,  left  panel).  A  similar  relation  is predicted to be a general outcome of planet formation via core accretion, although there may also  be  considerable  intrinsic  scatter  in  the  trend  due  to  the  stochastic  nature  of  planet formation (Fortney et al. 2013; Venturini et al. 2016).\n- · Carbon-to-Oxygen Ratios: The composition of a planet depends on its formation location relative to various snow lines in the protoplanetary disk. The abundant volatiles oxygen and carbon are expected to be especially critical to forming planets due to their roles in delivering icy materials. Measuring the C/O ratio in exoplanetary atmospheres is therefore useful for linking presentday envelope composition to the planet's birth location and the relative import of accreting solids vs. gas during envelope formation (Oberg et al.¨ 2011; Madhusudhan et al. 2014a). It has been dif*icult to measure C/O in solar system giant planets because they are all cold enough for oxygen to be sequestered out of the observable atmosphere via condensation processes (e.g. Helled & Lunine 2014). Transiting exoplanets, which are typically highly irradiated, provide an excellent  opportunity  to  directly  measure  atmospheric  C/O  without  relying  on  model extrapolations (Madhusudhan 2012).\n- · Other  Elemental  Abundance  Ratios: As  with  C/O,  measuring  elemental  abundance  ratios  of various  volatile  and/or  refractory  species  (e.g.  Si/O,  Si/C,  Fe/O,  etc.)  provides  a  tracer  for formation location and conditions (Piso et al. 2016; Lothringer et al. 2021; Cross*ield 2023; Chachan et al. 2023). Relative abundance measurements can also be used to constrain physical and chemical processes such as condensation or transport (e.g. Gibson et al. 2022; Pelletier et al. 2023).\n- · Atmospheric Composition Straddling the Sub-Neptune to Super-Earth Radius 'Gap': A strong dip in  exoplanetary  occurrence  for  planets  with  radii  ≈1.6 R ⊕ has  been  explained  as  being  the  \ndividing line between two populations of low-mass exoplanets: rocky super-Earths and gas-rich sub-Neptunes  (Fulton  et  al.  2017,  and  see  discussion  in  Section  1.2).  Theories  of photoevaporative  (Owen  &  Wu  2017)  and  core-powered  (Ginzburg  et  al.  2018;  Gupta  & Schlichting 2019) mass loss both posit that the sub-Neptunes are planets that have succeeded in  retaining  their  primordial  nebular  gas  atmospheres,  while  super-Earths  have  lost  their hydrogen entirely and have secondary high mean molecular weight atmospheres.  \n- · The Presence or Absence of Atmospheres on Terrestrial Exoplanets: In the solar system, a 'cosmic shoreline'  in  escape  velocity  and  insolation  separates  bodies  with  atmospheres  from  those without (Zahnle & Catling 2017, Figure 4, right panel). Identifying whether a similar dividing line exists for terrestrial exoplanets will help to constrain the processes by which (exo)planets retain or lose their atmospheres.  \nIn all of these cases, the trends being sought out are *irst-order to begin with, and the theories being tested are often highly simpli*ied. As statistical trends in exoplanet atmospheres data are uncovered and as the data warrant it, it is only natural that these simpler ideas will give way to more complex ones, and progress will be made toward understanding the universality of the processes that shape planetary atmospheres throughout their lifetimes.  \nAn even more direct way to constrain planet formation and evolution via exoplanet studies would be to observe exoplanets of different ages. In fact, in recent years a considerable number of exoplanets orbiting young stars (i.e. with ages ≲ 100 Myr) have been discovered (e.g. David et al. 2016; Benatti et al. 2019; David et al. 2019; Plavchan et al. 2020). Atmospheric observations of younger planets could reveal atmospheric escape or degassing processes while they are still ongoing (Zhang et al. 2022b) and might even show us what true primordial atmospheres look like. Unfortunately, the practical challenges to characterizing atmospheres of young planets are considerable. Young stars tend  to  be  quite  active.  The  resulting  stellar  variability  hinders  our  ability  to  detect  the  minute atmospheric signatures of exoplanets orbiting these stars (Cauley et al. 2018; Hirano et al. 2020; Palle et al. 2020a; Rackham et al. 2023). We are also fundamentally limited by the number of nearby young stars that are bright enough to present suf*icient SNR for atmospheric characterization studies.  \nFinally, measurements of atmospheric composition provide a direct indication of the chemical processes unfolding in a planet's atmosphere. For example, the measured abundances of molecules, atoms, and ions can be cross-checked against the predictions of thermochemical equilibrium for a given  elemental  mixture  (e.g.  Burrows  2014;  Lodders  &  Fegley  2002;  Schaefer  &  Fegley  2010). Departures  from  equilibrium  are  then  attributed  to  disequilibrium  processes  such  as  vertical  or horizontal mixing, or photochemistry (Tsai  et  al.  2023). Furthermore, the detection of any aerosol species (see Section 3) can be related back to the chemical and physical conditions that gave rise to them in the *irst place. We therefore turn to spectroscopic measurements of exoplanet atmospheric composition  as  a  powerful  tool  for  probing  the  physics,  chemistry,  and  history  of  exoplanetary environments.", '2.2. Water, Water Everywhere': "The *irst molecule to be reliably detected in a large number of exoplanet atmospheres was H 2 O. Water  has  many  vibration-rotation  absorption  bands  across  the  near-to-mid  IR,  and  oxygen  and hydrogen are cosmically abundant, making this an ideal molecule to search for. Furthermore water is stable in gas phase from ∼370 - 2200 K - at lower temperatures it condenses into clouds, droplets, or ice; and at higher temperatures it thermally dissociates. Fortunately, most transiting exoplanets have temperatures within the range in which gas-phase H 2 O is the expectation.  \nThe  search  for  H 2 O  in  transiting  exoplanet  atmospheres  from  space  was  enabled  by  the installation of the Wide Field Camera 3 (WFC3) instrument on board HST during its 2009 servicing mission. WFC3 carries a grism centered on the strong 1.4 µ m water absorption band with suf*icient spectral  resolution  to  resolve  the  shape  of  the  band,  and  a  novel  spatial  scanning  procedure  was developed  to  spread  the  exoplanetary  spectrum  across  many  detector  pixels  so  as  to  minimize concerns about detector systematics (McCullough & MacKenty 2012). The *irst detection of the 1.4 µ m water feature in a giant planet atmosphere with the WFC3 spatial scanning mode was made by Deming et al. (2013), and many more soon followed (e.g. Wakeford et al. 2013; Kreidberg et al. 2014b; Sing et al. 2016; Fu et al. 2017; Tsiaras et al. 2018; Changeat et al. 2022, etc; Figure 5).  \n<!-- image -->  \nFigure 4. Examples of statistical comparative planetology approaches (Bean et al. 2017) to constrain planet formation and evolution processes via ensemble observations of exoplanet atmospheres. Left: Atmospheric metallicity vs. mass for solar system planets (black symbols) and for exoplanets that have detections of carbonand/or  oxygen-bearing  species  using  JWST  (red  symbols).  Overlaid  are  the  predictions  from  population synthesis models from Fortney et al. (2013) showing a rise and then a plateau in metallicity as planetary mass decreases (gray dots). The solar system giant planets are observed to follow a very tight mass-metallicity correlation (dashed line), with the caveat that oxygen is undetected in the atmospheres of these planets, as it is sequestered in condensates below the photosphere. Figure adapted from MansGield et al. (2018). Right: The 'cosmic  shoreline'  (Zahnle  &  Catling  2017)  is  denoted  (yellow  diagonal  band),  which  is  an  observed delineation in escape speed and insolation between solar system bodies that do and do not possess gaseous atmospheres (toward the upper left and toward the lower right of the plot, respectively). Transiting exoplanets that will be observed in Cycles 1 and 2 of JWST are over-plotted in this same parameter space. Symbol size denotes the expected S/N of a single transit or eclipse observation using the methods of Kempton et al. (2018). The letters b-h denote the planets in the TRAPPIST-1 system, which are all slated for JWST observations, and the terrestrial solar system planets are shown for reference. By identifying which terrestrial exoplanets possess atmospheres and whether they are bounded by the same 'shoreline' as for the solar system, astronomers can constrain the processes by which planets lose or retain their atmospheres. Figure courtesy of Jegug Ih.  \n<!-- image -->  \nThe  ease  with  which  the  1.4 µ m  water  feature  became  detectable  with  HST  turned  this absorption band into a powerful diagnostic for the chemistry of exoplanet atmospheres. Compared to the baseline expectation of solar composition, a weaker than expected water feature in transmission can  be  attributed  to  a  low  water  abundance,  a  high  mean  molecular  weight  atmosphere,  or  an obscuring cloud deck that mutes the underlying spectral features (Miller-Ricci et al. 2009; Benneke & Seager 2013). In thermal emission, the strength of the water feature depends on the H 2 O abundance and on the temperature gradient in the planet's atmosphere. Subtle differences in the shape of a spectrum  resulting  from  each  of  these  various  scenarios  can  potentially  be  disentangled  with  \nsuf*iciently high S/N and broad enough wavelength coverage (Benneke & Seager 2012). By combining WFC3 measurements and longer wavelength observations with Spitzer's IRAC instrument (Figure 5), one can furthermore obtain constraints on a planet's metallicity and C/O ratio, by assuming that the atmosphere resides in a state of thermochemical equilibrium (e.g. Wakeford et al. 2018; Zhang et al. 2020). However, without simultaneous detection of the major carbon- and oxygen-bearing species in an  atmosphere,  these  two  properties  remain  degenerate  with  one  another.  To  quantify  water abundances  and  their  associated  uncertainties,  novel  retrieval  techniques  have  ultimately  been brought  to  bear  on  WFC3  transmission  and  emission  spectra  (e.g.  Kreidberg  et  al.  2014b; Madhusudhan et al. 2014b; Line et al. 2016).  \nUltra-hot  Jupiters,  which  have  equilibrium  temperatures  in  excess  of  ∼2000  K,  have  presented  a particularly interesting case for interpreting H 2 O detections. Weak or absent H 2 O features in dayside thermal emission spectra were noted for multiple ultra-hot Jupiters, and several hypothesis were posed to explain these observations (Evans et al. 2017; Sheppard et al. 2017; Kreidberg et al. 2018). Initially,  retrievals  were  run  that  indicated  either  very  low  metallicities  or  very  high  C/O  ratios (Sheppard et al. 2017; Pinhas et al. 2019; Gandhi et al. 2020b). The former reduces the abundances of all  'metal'-bearing  species,  and  the  latter  reduces  the  H 2 O  abundance  by  tying  up  nearly  all atmospheric oxygen in the CO molecule. Either of these abundance patterns would be surprising though,  especially  since  slightly  cooler  hot  Jupiters  are  not  observed  to  have  similarly  weak  H 2 O features (Mans*ield et al. 2021). A more natural explanation was posed by Arcangeli et al. (2018) and Parmentier et al. (2018) who pointed out that thermal dissociation of H 2 O at temperatures in excess of ∼2200 K coupled with the onset of continuum opacity from the hydrogen anion (H -) around the same  temperature  provided  a  high-quality  *it  to  available  data  without  resorting  to  elemental abundance patterns that differed dramatically from the planets' host stars. Still, the precision and wavelength coverage from HST and Spitzer alone were not suf*icient to unambiguously resolve the question of why the ultra-hot planets have muted water features.  \nOn the other end of the planetary 'spectrum', sub-Neptunes and super-Earths have also been observed to have muted or absent water features in transmission. In this case, the interpretation is  \nFigure 5. Transmission spectra for various hot Jupiters plotted in units of the planet's atmospheric scale height. Data are shown from the HST STIS and WFC3 instruments, and Spitzer IRAC channel 1 and 2 (3.6 and 4.5 µ m, respectively), as indicated. The WFC3 data cover a strong water band at 1.4 µ m, and STIS covers absorption lines from Na and K. The two Spitzer IRAC photometric channels are sensitive to CH 4 , CO, and CO 2 , although the lack of spectroscopic information over this wavelength range makes it dif/icult to fully constrain the atmospheric carbon chemistry. Muted spectral features and strongly sloping optical and near-IR spectra for the planets plotted toward the bottom of the /igure are attributed to aerosol obscuration. Figure adapted from Sing et al. (2016).  \n<!-- image -->  \ndifferent because these smaller planets can have high mean molecular weight atmospheres, and they also  tend  to  be  colder  planets,  which  makes  them  potentially  amenable  to  aerosol  formation. (Aerosols are not generally considered to be a major atmospheric constituent for ultra-hot planets  \nbecause  we  do  not  know  of  any  cloud  or  haze  species  that  can  form  and  persist  at  such  high temperatures.) The spectra of rocky super-Earths will be discussed in more detail in Section 2.5. A key science question that astronomers aimed to address with the initial atmospheric observations of sub-Neptunes  was  to  break  degeneracies  between  'mini-Neptune'  and  water  world  scenarios  by measuring  the  atmospheric  composition  and  ascertaining  whether  it  was  hydrogen-  or  waterdominated  (Miller-Ricci  et  al.  2009;  Miller-Ricci  &  Fortney  2010;  Rogers  &  Seager  2010). Unfortunately,  degeneracies  between  aerosols  and  high  mean  molecular  weight  made  such distinctions extremely challenging with available instruments prior to the launch of JWST (e.g. Bean et al. 2010; Berta et al. 2012; Knutson et al. 2014; Guo et al. 2020; Mikal-Evans et al. 2021, 2023a). Perhaps the most famous among sub-Neptunes is the planet GJ 1214b, which was observed to have a staggeringly  *lat  transmission  spectra  with  12  stacked  transits  with  the  HST+WFC3  instrument (Kreidberg et al. 2014a). The spectrum is so featureless that the only viable interpretation is a very thick and high-altitude layer of clouds or haze (see Section 3), which obscures any direct indications of the atmosphere below.  \nOne challenge to interpreting any claimed detections of water in low-mass exoplanets is that many such planets that are amenable to atmospheric characterization necessarily orbit low-mass Mdwarf stars. Small host stars are required to produce large transit depths and therefore suf*iciently high S/N transmission spectra. But low-mass stars also have water in their own spectra due to their correspondingly  low  temperatures.  What's  worse,  the  water  is  not  expected  to  be  uniformly distributed throughout the stellar atmosphere and instead to preferentially lie in cooler star-spot regions.  The  result  is  that  H 2 O  features  can  be  spuriously  imprinted  on  transmission  spectra  for planets orbiting M-dwarfs that do not originate in the planetary atmosphere but actually in the star itself  (e.g.  Deming  &  Sheppard  2017;  Rackham  et  al.  2018;  Zhang  et  al.  2018;  Lim  et  al.  2023). Techniques for mitigating stellar contamination in the transmission spectra of these systems is an ongoing area of research.  \nDetection of H 2 O from the ground has also been enabled via high resolution spectroscopy. The *irst  such  measurement  was  made  by  Birkby  et  al.  (2013)  using  the  high-resolution  CRIRES  \nspectrograph on the Very Large Telescope (VLT) to capture water absorption lines in the dayside emission spectrum of the hot Jupiter HD189733b. As the observing techniques have matured and more  near-IR  high  resolution  spectrographs  have  come  online,  the  rate  of  ground-based  water detections  has  accelerated  (see  Figure  6).  One  particular  advantage  of  high-resolution  water detections  over  space-based  measurements  with  HST+WFC3  is  that  the  former  are  often simultaneously sensitive to oxygen- and carbon-bearing molecules, enabling direct constraints on the atmospheric C/O ratio (Pelletier et al. 2021; Line et al. 2021; Brogi et al. 2023). Such measurements have indicated C/O values for various hot Jupiters ranging from near-solar (the solar value is 0.55) to super-solar  values  near  1.  With  signi*icant  scatter  in  the  results  to-date,  the  implications  for  hot Jupiter formation are murky, but the picture should solidify in the coming years with many more direct  C/O  measurements  enabled  by  JWST.  Ground-based  measurements  of  H 2 O  have  been attempted for sub-Neptunes and super-Earths as well, typically at lower spectral resolution, but todate all have resulted in nondetections (e.g. Bean et al. 2010, 2011; Ca'ceres et al. 2014; DiamondLowe et al. 2018, 2020a,b).", '2.3. Refractory Species in Hot Jupiter Atmospheres': "Hot  and  ultra-hot  Jupiters  have  high  enough  temperatures  that  most  refractory  species  are rendered in the gas phase, and some can even be ionized via thermal or non-thermal processes. This is advantageous for exoplanet studies because it means that many elements that would otherwise be sequestered deep within a colder giant planet like Jupiter or Saturn are accessible to direct detection. Measured  abundance  patterns  can  then  be  compared  to  theories  of  planet  formation  or  used  to identify various chemical processes, as discussed in Section 1.5. Another goal of refractory species detections in hot Jupiter atmospheres has been to identify the optical and UV absorbers that drive thermal  inversions  in  these  planets  (see  Section  5  for  more  details).  TiO  and  VO  were  initially proposed as likely species to drive stratospheric inversions in hot Jupiters (Fortney et al. 2008). More recently Lothringer et al. (2018) pointed out that a whole host of atomic metals, metal hydrides, and  \noxides should be in the gas phase in ultra-hot planets and would serve as even stronger optical and UV opacity sources.  \nMotivated by goals of measuring refractory abundances and identifying key optical absorbers, a large and increasing number of chemical species have been detected in hot Jupiter atmospheres in recent years. This has mostly been made possible by ground-based high-resolution spectrographs that observe at optical wavelengths, as well as the STIS instrument and WFC3/UVIS instrument mode aboard HST. Space-based observations with STIS and WFC3/UVIS jointly provide broad near-UV to optical wavelength coverage (∼ 200-1000 nm) but only at relatively low spectral resolution, which presents a challenge for uniquely identify chemical species. For example, with low-resolution optical spectra, it has often been dif*icult to disentangle the causes of slopes in transmission spectra, which can be attributed to some combination of aerosol scattering (see Section 3), stellar activity, or optical absorbers (e.g. Pont et al. 2008; McCullough et al. 2014; Evans et al. 2018). At near-UV wavelengths certain ultrahot planets have been observed to have sharply increased transit depths, consistent with the  presence  of  SiO,  SH,  Mg,  and/or  Fe,  which  would  serve  to  drive  thermal  inversions  or  act  as condensate cloud precursors (Evans et al. 2018; Fu et al. 2021; Lothringer et al. 2022). Individual strong lines due to atomic (e.g. Na, K; Sing et al. 2016) and ionic (e.g. Fe + and Mg + ; Sing et al. 2019) species have been easier to uniquely identify, albeit the line pro*iles are typically not fully resolved by HST, resulting in degenerate interpretations of abundances vs. broadening mechanisms. Sodium and potassium in particular have been identi*ied in a large number of hot Jupiter spectra with STIS (see Figure 5). In some planets just one of these two species is detected, whereas others produce clear detections of both. Identifying abundance patterns in Na and K vs. fundamental parameters such as equilibrium temperature has so far been elusive.  \nThe  optical  opacity  'bumps'  that  have  been  observed  with  HST  can  be  fully  resolved  via highresolution spectroscopy in order to uniquely identify the species present. To date, well over a dozen elements and 37 individual molecular, atomic, and ionic species have been identi*ied in hot Jupiter atmospheres with high-resolution techniques, spanning a broad portion of the periodic table (e.g. Wyttenbach et al. 2015; Hoeijmakers et al. 2018; Ehrenreich et al. 2020; Tabernero et al. 2021;  \nKesseli et al. 2022; Langeveld et al. 2022; Pelletier et al. 2023; Flagg et al. 2023, Figure 6). Of these species, iron and sodium have so far proven the most readily detectable in a large number of hot and ultrahot atmospheres due to their especially strong and unique optical opacity patterns.  \nMuch of the focus of high-resolution studies initially was on the detection of individual species. Papers reporting detection signi*icances have recently been giving way to those that quantify relative and/or absolute abundances via high-resolution retrieval techniques (e.g. Gibson et al. 2020; Pelletier et al. 2021; Maguire et al. 2023; Kasper et al. 2023; Gandhi et al. 2023). Such studies have revealed a range of solar and non-solar abundance patterns. For instance, in a retrieval study of six high S/N ultrahot Jupiters, Gandhi et al. (2023) found iron abundances to be well-matched to the planets' host stars. However, other refractories such as Mg, Ni, and Cr presented more variable abundance patterns; and several species such as Na, Ti, and Ca were found to be uniformly under-abundant relative to stellar, implying some sort of depletion process such as condensation or ionization. In a detailed study of the ultrahot Jupiter WASP-76b, which measured abundances of 14 individual refractory species, Pelletier et al. (2023) similarly found abundances broadly consistent with solar (and stellar), with some notable exceptions. Elements with high condensation temperatures were found to be depleted, potentially  implying  condensation  cold-trapping  on  the  planet's  night  side,  whereas  Ni  was  overabundant, perhaps indicating that WASP-76b accreted a differentiated planetary core during its late stages of formation. Studies such as these highlight the power of systematic investigations of gasphase refractory elements in hot Jupiters to reveal the physics, chemistry, and history of these planets' atmospheres.", '2.4. The JWST Landscape': "The *irst JWST spectrum of a transiting exoplanet was released on July 12, 2022 as part of a handful of 'early release observations' (EROs) meant to demonstrate to the public the power of the newly commissioned space telescope 10 (Pontoppidan et al. 2022). The ∼1300 K hot Jupiter WASP96b  \nFigure 6. Current state of detections of ions, atoms, and molecules with high-resolution spectroscopy, as of summer 2023. The signi/icance of each claimed detection is indicated by the symbol color. The embedded histogram shows the number of high-resolution spectroscopy atmospheric characterization papers published per year, revealing a steep acceleration. The uptick corresponds to multiple new instruments coming online as well as the maturation of the observing technique. Figure courtesy of Arjun Savel.  \n<!-- image -->  \nwas targeted with the NIRISS instrument (Figure 7). The resulting spectrum spanning 0.6 - 2.8 µ m had exactly the intended effect. It revealed a full rainbow of water features along with evidence for obscuring aerosols, and beyond that it gave the astronomical community a small taste of what was to come from exoplanet studies in the JWST era.  \nJust  a  month  and  a  half  later,  the  *irst  peer-reviewed  scienti*ic  exoplanet  result  from  JWST revealed  the  striking  *irst-time  discovery  of  CO 2 in  an  exoplanet  atmosphere  (JWST  Transiting Exoplanet Community Early Release Science Team et al. 2023, Figure 7). Carbon dioxide, which had previously been out of reach for spectroscopic studies due to the wavelength coverage of available instruments,  was  detected  at  a  staggering  signi*icance  of  26 σ .  Chemically,  the  CO 2 molecule  is especially interesting because it serves as a metallicity indicator in hot hydrogen-rich atmospheres (Fortney  et  al.  2010).  The  strong  CO 2 absorption  feature  identi*ied  in  the  hot  Jupiter  WASP-39  b  \nHD  \nindicates that the planet has ∼10× enhanced metallicity relative to its host star. The planet's high metallicity and low mass (relative to Jupiter), intriguingly place it right along the solar system giant planet mass-metallicity relation (Constantinou et al. 2023, Figure 4).  \nFigure 7. A selection of transmission spectra analyzed to-date from JWST. Wavelength coverage of the various instrument modes used for transmission spectroscopy are indicated above. Atomic and molecular opacity sources that have been identi/ied in the planets shown are indicated below. Several other planets that have been observed with JWST but do not readily reveal any identi/iable  absorbers  are  not  shown.  Comparing against Figure 5, one can see the bene/its of the expanded wavelength coverage and improved precision of JWST relative to HST and Spitzer for exoplanet atmospheric characterization. ( Figure data from Fu et al. (2022), Radica et al. (2023), Carter et al. (submitted), Xue et al. (submitted), and Fu et al. (in prep.). )  \n<!-- image -->  \nFurther studies of WASP-39b by the The JWST Transiting Exoplanet Community (JTEC) Early Release Science (ERS) program have since produced a full panchromatic transmission spectrum of the planet from 0 . 6-5 . 2 µ m (JWST Transiting Exoplanet Community Early Release Science Team et al. 2023; Ahrer et al. 2023; Rustamkulov et al. 2023; Alderson et al. 2023; Feinstein et al. 2023, Figure 7). Spectral features from H 2 O, SO 2 (Tsai et al. 2023), and CO (Grant et al. 2023; EsparzaBorges et al. 2023) have been identi*ied, in addition to the aforementioned CO 2 ,  as  well as signatures of patchy aerosol coverage. The discovery of SO 2 at 4.05 µ m is especially intriguing because this molecule was not predicted in observable amounts by any chemical equilibrium models. Instead, it is believed to be the byproduct of photochemical alteration of the atmosphere (Polman et al. 2023; Tsai et al. 2023). The strength of the observed feature is well-matched by hot Jupiter photochemistry models, and it is now anticipated  that  SO 2 might  appear  in  many  JWST  hot  Jupiter  observations.  The  discovery  of photochemically derived species opens the door to a whole host disequilibrium chemistry studies, which will be an exciting new arena for JWST.  \nAnother  possible  hint  of  disequilibrium  chemistry  comes  from  attempts  to  detect  methane, which  is  expected  to  be  abundant  in  hydrogen-rich  atmospheres  below  ∼1000  K.  Exoplanet atmosphere observations prior to the launch of JWST already hinted at a 'missing methane' problem, with cooler planets not showing obvious signs of methane absorption in HST or Spitzer data (e.g. Stevenson et al. 2010; Kreidberg et al. 2018; Benneke et al. 2019a), although some ground-based detections had been reported (Guilluy et al. 2019, 2022; Giacobbe et al. 2021; Carleo et al. 2022). Methane should be readily observable with JWST, as it has multiple strong absorption bands over the 1 - 8 µ m wavelength range. However, the molecule is notably absent from the transmission spectrum of the ∼850 K planet HAT-P-18b with JWST's NIRISS instrument (Fu et al. 2022). Recently, methane was *inally detected by JWST in yet colder planets: the ∼825 K 'warm' Jupiter WASP-80b (Bell et al. 2023a) and the ∼360 K sub-Neptune K2-18b (Madhusudhan et al. 2023). In the latter case, the JWST measurement resolves previous ambiguity from HST+WFC3 observations as to which gas had been detected,  H 2 O  or  CH 4 (Benneke  et  al.  2019b;  Tsiaras  et  al.  2019;  B'ezard  et  al.  2022).  The accompanying detection of CO 2 and non-detection of water vapor in K2-18b, also perhaps indicates  \nthe presence of a liquid water ocean below the planet's thick atmosphere (Madhusudhan et al. 2023). Further JWST observations will map out the parameter space over which methane exists in hydrogenrich planetary atmospheres and will hopefully hint at the underlying mechanisms behind the missing methane problem such as hot planetary interiors coupled with ef*icient vertical mixing (Fortney et al. 2020), horizontal quenching (Cooper & Showman 2006; Zamyatina et al. 2023), or photochemistry (Line et al. 2011; Miller-Ricci Kempton et al. 2012).  \nIt is still early days for JWST, and the observatory has just begun to reveal its prowess in characterizing exoplanet atmospheric composition. Along with metallicities, the reliable measurement of C/O ratios in exoplanet atmospheres has been highly anticipated, enabled by the broad wavelength coverage of the JWST instruments. For example, the 0.6 - 12 µ m wavelength range covered by the JWST exoplanet instrument  suite  spans  spectral  features  from  H 2 O,  CH 4 ,  CO 2 ,  and  CO,  which  allows  for  direct measurement of the atmospheric C/O ratio under the assumption of thermochemical equilibrium (e.g. Batalha  &  Line  2017).  The  *irst  contstraints  on  metallicities  and  C/O  ratios  reveal  the  diverse outcomes of planet formation processes (Figures 4 and 8). Derived metallicities in hot Jupiters range from sub-solar (August et al. 2023) to highly super-solar (Bean et al. 2023). Whereas WASP-39b is found to lie directly along the solar system mass-metallicity relation, several other planets do not, implying either that this is not a universal correlation for giant planets, or that there is considerable scatter in the underlying trend. Measurements of C/O ratios for hot Jupiters have have also recovered a range of values from sub-solar (JWST Transiting Exoplanet Community Early Release Science Team et al. 2023; Coulombe et al. 2023; August et al. 2023), to solar (Radica et al. 2023), to super-solar (Bean et al. 2023). Published hot Jupiter studies with JWST are still in the small number statistics regime. However, planned observations of several dozen such planets with JWST in its *irst two years establishing whether abundance patterns align with speci*ic theories of giant planet formation.  \nFigure 8. Metallicity  vs.  C/O  ratio  for  all  planets  with  measurements  of  both  carbon-  and  oxygen-bearing species from JWST or high-resolution ground-based spectrographs as of the writing of this article. Formation scenarios consistent with different combinations of metallicity and C/O are a summary of work described in Oberg et al.¨ (2011), Madhusudhan et al. (2017), Booth et al. (2017), and Reggiani et al. (2022). ( Figure data from Bean et al. (2023), Brogi et al. (2023), August et al. (2023), Bell et al. (2023a), Xue et al. (submitted.), Fu et al. (in prep.), Welbanks et al. (in prep.), Pelletier et al. (in prep.), and MansGield et al. (in prep.). )  \n<!-- image -->  \nAnother arena in which JWST has already made its mark is to resolve previous ambiguity over the atmospheric composition of ultrahot Jupiters (see Section 2.2). Whereas with HST alone it had been  challenging  to  determine  the  cause  of  weakened  H 2 O  features  in  ultrahot  Jupiter  emission spectra, the wavelength coverage and precision of JWST data for the planet WASP-18b has allowed for a robust measurement of the planet's underlying atmospheric composition (Coulombe et al. 2023). The NIRISS secondary eclipse spectrum of WASP-18b clearly shows evidence for weakened, but still signi*icant, H 2 O features in emission. The detailed shape of the spectrum is best-*it by models with near-solar metallicity, sub-solar C/O, H -continuum opacity, and water depleted in the observable atmosphere  via  thermal  dissociation.  This  composition  is  in  line  with  'vanilla'  predictions  of  an  \nunaltered  nebular  gas  atmosphere  in  thermochemical  equilibrium  and  rules  out  more  exotic scenarios. Attempts to characterize even smaller exoplanets with JWST are also just beginning in earnest.  \nTerrestrial planets will be discussed in more detail below, but the *irst JWST investigations of subNeptunes  are  also  taking  shape.  Following  on  years  of  ambiguous  characterization  of  subNeptunes that have produced degenerate interpretations of atmospheric composition and aerosols (see Sections 2.2 and 3), the *irst phase curve observation of a sub-Neptune (the planet GJ 1214 b) has revealed clear evidence that the planet has a high mean molecular weight atmosphere (Kempton et  al.  2023).  The  planet's  dayside  and  nightside  thermal  emission  spectra  additionally  show spectroscopic signs of H 2 O and perhaps CH 4 (the two are partially degenerate with one another over the mid-IR wavelength range observed). The derived composition of the planet is consistent with GJ 1214b  either  being  a  water  world  or  'gas  dwarf',  i.e.  an  initially  hydrogen-rich  planet  that  has experienced  considerable  loss  of  lighter  elements  throughout  its  lifetime.  Approximately  20 additional sub-Neptunes are scheduled for transmission spectrum observations with JWST during Cycles 1 and 2 11 , opening the door to further compositional characterization of this intriguing class of planets, so long as spectral features are not entirely obscured by aerosols.", '2.5. The Challenge of Terrestrial Planets': "Terrestrial planets are especially challenging targets for atmospheric characterization due to their  small  sizes  and  also  the  expectation  that  their  atmospheres  will  typically  have  high  mean molecular weight, which reduces the size of spectral features observed in transmission (see Section 1.3 and also a review article by Wordsworth & Kreidberg (2022) for more background on terrestrial exoplanets). The *irst attempts to measure the atmospheric composition of rocky exoplanets typically resulted  in  non-detections  of  spectral  features,  which  in  turn  could  rule  out  cloud-free  hydrogen  \ndominated atmospheres but left open a wide range of plausible atmospheric compositions and cloud properties (e.g. de Wit et al. 2016, 2018; Diamond-Lowe et al. 2018, 2020b, 2023; Mugnai et al. 2021; Libby-Roberts et al. 2022). To this day, there have not yet been any robust detections of atmospheric species  in  terrestrial  exoplanets.  The  small  number  of  works  that  have  claimed  the  detection  of atmospheric gases for rocky planets via transmission spectroscopy have been called into question or have not been reproduced (e.g. Southworth et al. 2017; Swain et al. 2021).  \nAn alternative approach to characterizing rocky planet atmospheres was *irst demonstrated by Kreidberg et al. (2019) and Cross*ield et al. (2022). The former measured the phase curve of the terrestrial exoplanet LHS 3844b, and the latter observed the secondary eclipse of GJ 1252 b, both with the Spitzer Space Telescope. The goal of both observations was to determine whether the planet in question has a thick atmosphere or is an airless barren rock. The technique is discussed in more detail in Section 5.4, but brie*ly, the premise is that, for tidally-locked exoplanets (as is expected to be the case  for  these  and  most  other  terrestrial  planet  atmospheric  characterization  targets  orbiting  Mdwarfs), an atmosphere serves to transport heat away from the planet's hot dayside to its colder nightside  (Seager  &  Deming  2009;  Koll  2022).  In  both  cases,  the  planets'  high  observed  dayside temperatures  were  found  to  be  consistent  with  the  lack  of  a  substantial  atmosphere,  although Earththickness  1-bar  atmospheres  could  not  be  ruled  out.  The  inferred  limits  on  atmospheric thickness are also composition-dependent due to the differing abilities of various gases to absorb light and transport heat, governed by their wavelength-dependent opacities (Whittaker et al. 2022; Ih et al. 2023; Lincowski et al. 2023). With Spitzer, such dayside thermal emission measurements were only possible for the few hottest and most favorable targets, but JWST has much expanded capabilities in this arena (Koll et al. 2019).  \nThe large aperture and IR observing capabilities of JWST have long promised to extend the parameter space of observable exoplanet atmospheres to terrestrial planets (e.g. Deming et al. 2009; Beichman  et  al.  2014;  Batalha  et  al.  2015).  Nearly  30  such  planets  (i.e.  rocky  super-Earths  to subEarths) are already approved for observation in Cycle 1 and 2 of JWST operations. The list of planned observations includes all 7 of the planets in the TRAPPIST-1 system, as well as numerous  \nterrestrial  planets  orbiting  earlier  (larger,  warmer,  and  more  massive)  M  stars,  and  a  handful  of ultrashort-period (USP) rocky planets with periods shorter than 1 day orbiting G, K, and M stars. The TRAPPIST-1 system is of particular interest for habitability studies aiming to identify biosignature gases because the late (i.e. small and cool) M-dwarf host star brings the habitable zone to very short orbital periods and produces large transit depths and thus atmospheric signal sizes (e.g. Barstow & Irwin 2016; Krissansen-Totton  et  al.  2018; Lustig-Yaeger  et  al.  2019). TRAPPIST-1 e, f, and g are all potentially  habitable  environments,  and  are  largely  seen  as  the  best  prospects  for  characterizing potentially habitable worlds within the next decade (Gillon et al. 2017).  \nThe  *irst  year  of  JWST  data  has  so  far  been  marked  by  more  non-detections  of  terrestrial atmospheres, but now with the vastly improved capabilities of the new facility, such measurements are  more  meaningfully  constraining.  Transmission  spectra  to-date  are  consistent  with  *lat  lines (LustigYaeger et al. 2023), or with the possibility that spectral features are caused by the host star and not the planet (Moran et al. 2023; Lim et al. 2023). Thermal emission studies of terrestrial planets (so far limited to TRAPPIST-1 b and c) have been more revealing. As with previous Spitzer thermal emission  studies,  the  goal  with  these  observations  has  been  to  measure  the  planets'  dayside temperatures and infer the presence or lack of an atmosphere. For both planets, the measured dayside temperatures are again consistent with no atmosphere being present (Greene et al. 2023; Zieba et al. 2023). The mid-IR capabilities of JWST has allowed for these measurements to be made at much longer wavelengths than Spitzer could access. By observing at 15 microns in the center of an expected strong  CO 2 absorption  band,  the  JWST  measurements  can  rule  out  very  thin  atmospheres  (for TRAPPIST-1 b down to even Mars thickness), under the assumption that CO 2 would be a dominant gas in any moderately-irradiated terrestrial environment (Ih et al. 2023; Lincowski et al. 2023).  \nStill, a key promise of JWST is to deliver spectra of smaller and cooler exoplanets than what was previously possible with HST. In light of the *lat-line spectra and dayside thermal emission results, a question that has supplanted the characterization of rocky exoplanets has been to identify whether such planets possess atmospheres at all. Figure 4 (right panel) shows that Cycle 1 and 2 JWST targets cover the parameter space of planets that would and would not be expected to host atmospheres,  \nbased on solar system considerations. If some of the less-irradiated and/or higher surface gravity terrestrial exoplanets are found to have atmospheres, multiple modeling studies have shown JWST's capabilities to spectroscopically characterize such environments under optimal conditions of minimal cloud obscuration, large scale heights, and stacking multiple transits to improve S/N (e.g. Barstow & Irwin 2016; Morley et al. 2017; Batalha et al. 2018; Krissansen-Totton et al. 2018; Lustig-Yaeger et al. 2019; Fauchez et al. 2019; Suissa et al. 2020; Pidhorodetska et al. 2020). For the subset of rocky planets  without  atmospheres,  mid-IR  emission  spectroscopy  measurements  with  JWST  offer  an exciting opportunity to characterize their surface compositions for the *irst time (e.g. Hu et al. 2012; Whittaker et al. 2022; Ih et al. 2023).", '3.1. Terminology and Background': "Aerosols in this work are de*ined to be any kind of particle suspended in a gaseous atmosphere, regardless  of  their  composition  or  formation  pathway 12 .  Aerosols  can  be  broken  up  into  subcategories: clouds are de*ined as solid particles or liquid droplets formed by condensation processes, hazes are involatile particles produced by chemical (and often photochemical) processes, and dust is made  up  of  solid  particles  suspended  in  an  atmosphere  that  originated  elsewhere  (e.g.  particles kicked up from the surface or those that originated from a meteorite breaking up as it entered a planet's atmosphere). These are all process-based de*initions. If the formation mechanism for the particles in question is unknown, we revert to the blanket term 'aerosol'. We warn the reader that some published papers in the exoplanetary literature employ the term 'haze' when referring to small particles ( ≲ 1 µ m) and 'clouds' when referring to larger particles, but we prefer the process-based de*initions for the physical insight they bring.  \nAll solar system planets and moons with signi*icant atmospheres have some sort of aerosol layer. For example, Earth has water clouds, surface dust, and technology-derived haze (i.e. smog). Venus has  \nsulfuric acid clouds and haze. Titan has clouds and haze formed from organic compounds. It stands to reason that exoplanets too should have aerosol layers and that these will be a fundamental component of their atmospheres, governing energy balance, thermal structures, and observed spectra; as is the case for the solar system planets. As we will see, there is indeed plentiful observational evidence for exoplanet aerosols. For a more detailed review of aerosols in exoplanet atmospheres, we refer the reader to a recent article by Gao et al. (2021).  \nThe  observational  signatures  of  clouds  and  hazes  in  exoplanet  transmission  spectra  are primarily  muted  (or  absent)  spectral  features  or  strong  blue-ward  spectral  slopes  at  optical wavelengths 13 . These come about due to the propensity of the aerosol particles to scatter or absorb starlight. Rayleigh-like scattering slopes arise for small particles, whereas *latter spectra result when particle sizes are larger or the clouds are very thick. The aerosol species themselves also have their own spectral signatures, but these are typically weak features with wavelength-dependent shapes that depend on particle size distribution (e.g. Wakeford & Sing 2015). This results in degeneracies among spectra associated with distinct aerosol species, making the aerosol composition dif*icult to uniquely constrain spectroscopically. In thermal emission, the signatures of aeorols tend to be even more subtle. Aerosol layers can impact the thermal structure of an atmosphere (for example, causing thermal  inversions  in  the  case  of  very  absorptive  clouds  or  hazes,  thus  altering  the  shape  of  the planet's  emission  spectrum;  Arney  et  al.  2016;  Morley  et  al.  2015;  Lavvas  &  Arfaux  2021),  and optically thick aerosols can mute spectral features; but these effects are not uniquely attributable to clouds and therefore can be challenging to interpret. The result is that one can often tell from an observation that aerosols are present, and inferences can be made about the vertical distribution of the clouds or haze, but concluding anything robustly about the aerosol composition on a planet-byplanet basis is exceedingly dif*icult. Forward models are useful to motivate which types of aerosols are  consistent  with  a  speci*ic  observation,  providing  probabilistic  arguments  on  the  aerosol composition. This approach can be especially powerful at the population level.  \nOftentimes in the exoplanet literature, aerosols are treated as a 'nuisance' parameter, due to their impact of hindering the detection the underlying gaseous atmosphere. Modeling the complex microphysics  and  chemical  processes  that  lead  to  aerosol  formation  is  a  challenging  task,  so parameterized studies that reduce the aerosols to as few de*ining properties as possible are common (e.g. Ackerman & Marley 2001; Benneke & Seager 2012). Yet it is only by understanding the aeorols themselves, including their composition, formation, and optical properties, that we can gain a holistic picture of the planetary atmosphere in question. Studies of exoplanet aerosols additionally provide us with unique laboratories for probing cloud and haze formation in conditions that are not accessible within the solar system.", '3.2. Hot Jupiter Aerosols': "For hydrogen-rich atmospheres hotter than ∼1000 K, the types of clouds that are able to form due to condensation processes are those that are more commonly thought of as refractory species. For example, based on chemical equilibrium calculations one would expect clouds of Fe, Ni, Al 2 O 3 , Mg 2 SiO 4 , TiO 2 , and MnS for a solar-composition gas mixture (e.g. Burrows & Sharp 1999; Mbarek & Kempton 2016; Woitke et al. 2018; Kitzmann et al. 2023). Because such clouds are expected to only form at very high temperatures, and they incorporate trace species, it can be tempting to ignore the impacts of aerosols on hot Jupiter studies. Yet it was shown early on that transmission spectroscopy geometry, speci*ically the oblique geometric path taken by stellar photons through the exoplanetary atmosphere on their way to the observer, can result in cloud optical depths considerably in excess of unity, even for trace species (Fortney 2005). Obscuration by clouds was one explanation immediately put forth for the weaker than expected sodium absorption signal seen in the very *irst exoplanet transmission  observation  (Charbonneau  et  al.  2002).  These  early  studies  indicated  that  cloud modeling would need to be an integral component of interpreting hot Jupiter atmospheric observations.  \nThe  presence  of  aerosol  layers  has  been  inferred  from  multiple  hot  Jupiter  transmission spectroscopy studies, starting with the benchmark planet HD 189733b (Pont et al. 2008). For that planet, a strong spectral slope over optical wavelengths accompanied by non-detections of sodium  \nand  potassium,  which  should  have  been  present  under  clear  atmosphere  conditions,  constituted strong evidence for cloud or haze obscuration. However, later work showed that the optical slope could equivalently be the signature of unocculted starspots on the surface of the planet's active host star, leading to an ambiguity in how to interpret the observational result. Since then, many other hot Jupiters have revealed optical spectral slopes and/or muted spectral features over IR wavelengths, indicating  that  aeorosol  coverage  is  a  likely  culprit  across  the  population  (e.g.  Sing  et  al.  2016; Wakeford et al. 2017, and see Figure 5). Other observational indications of clouds in hot Jupiters come from optical and IR phase curve observations, which will be discussed further in Section 6.2.  \nFigure 9. Clear and cloudy atmosphere model tracks compared with transmission spectroscopy measurements of the 1.4 µ m water feature amplitude as a function of planetary equilibrium temperature for transiting gas giant  planets.  Hot  Jupiter  transmission  spectra  for  planets  colder  than  ∼2100  K  generally  have  weaker absorption features than what is predicted for clear solar-composition gas mixtures. The cloud microphysics models shown here provide a good overall /it to the observed trend. In addition to cloud condensation, the models include the formation of hydrocarbon hazes, which increasingly dominate at equilibrium temperatures below 950 K. The clouds are primarily formed from Mg 2 SiO 4 , with minor contributions from Al 2 O 3 and TiO 2 . Some variation in the degree of aerosol coverage is expected based on surface gravity, metallicity, and C/O ratio, which is likely driving the intrinsic scatter in the observed data points. Figure from Gao et al. (2020).  \n<!-- image -->  \nOn a population level, the aerosol composition and formation mechanism can become more apparent.  By  comparing  the  strength  of  transmission  spectral  features  to  a  suite  of  aerosol microphysics forward models, Gao et al. (2020) argued that the muted spectral features for transiting  \ngas  giant  planets  are  primarily  caused  by  silicate  clouds  for  planets  hotter  than  950  K  and hydrocarbon  hazes  for  cooler  planets  (Figure  9).  Their  argument  hinges  on  the  relatively  high abundances of  Si,  Mg,  and  C,  their  three  main  aerosol-forming  species,  and  that  other  species  of comparable  abundance  (e.g.  Fe  and  Na)  don't  readily  form  clouds  due  to  high  nucleation  energy barriers inhibiting particle formation.  \nJWST  is  expected  to  improve  our  understanding  of  hot  Jupiter  aerosols  by  providing higherprecision spectra and broader wavelength coverage, allowing for degeneracies to be broken between aerosol coverage and high mean molecular weight or non-solar abundance patterns (Batalha & Line 2017). Already this enhanced precision has led to the *inding of partial cloud coverage of the terminator of WASP-39b due to subtle departures in the shape of its transmission spectrum from a fullyclouded planet (Feinstein et al. 2023). The access to longer wavelengths with JWST also presents the opportunity to directly measure spectral features from aerosols (e.g. Wakeford & Sing 2015), such as  the  silicate  features  that  have  been  observed  in  mid-IR  spectra  of  brown  dwarfs  and  directlyimaged  giant  planets  (e.g.  Burningham  et  al.  2021;  Miles  et  al.  2023).  As  for  high-resolution spectroscopy studies, the signatures of aerosols are dif*icult to distinguish because high resolution data  processing  techniques  typically  remove  the  spectral  continuum,  which  is  where  most  of  the aerosol information is contained (e.g. Snellen et al. 2010; de Kok et al. 2013). An advantage of highresolution studies though is that the sharply peaked cores of spectral lines tend to extend above cloud decks, resulting in an ability to measure gas-phase composition even for aerosol enshrouded planets (Kempton et al. 2014; Hood et al. 2020; Gandhi et al. 2020a).  \nAnother  particularly  promising  avenue  for  further  constraining  aerosol  composition  in  hot Jupiter atmospheres is by using 3-D diagnostics to determine where on the planet (i.e. as a function of longitude and/or latitude) the aerosols are located. The aerosol spatial distribution and the physical conditions derived at those locations (e.g. temperature, UV irradiation, wind speeds) can then be directly linked to a proposed aerosol formation mechanism and composition. Such analyses can be accomplished  on  high  S/N  spectra  and  phase  curves  from  JWST  or  high-resolution  spectra  from ground-based telescopes (e.g. Kempton et al. 2017; Ehrenreich et al. 2020; Espinoza & Jones  \n2021; Parmentier et al. 2021; Roman et al. 2021). We discuss 3-D diagnostics for aerosols further in Section 6.2.", '3.3. Aerosols in Sub-Neptunes and Super-Earths': "Because they tend to orbit smaller stars and thus have cooler temperatures, transiting planets smaller than Neptune are especially likely to host aerosol layers. This was heavily implied by the *irst investigations of sub-Neptune exoplanets, which returned featureless transmission spectra (e.g. Bean et al. 2010; Berta et al. 2012; Knutson et al. 2014). To date, most *lat and muted sub-Neptune and super-Earth transmission spectra are consistent with either aerosols or high mean molecular weight atmospheres, leading to degenerate interpretation. However, in the case of the planet GJ 1214b, the data  were  obtained  at  high  enough  precision  by  stacking  multiple  transits  with  HST  that  the degeneracy  could  be  broken,  and  a  thick  aerosol  layer  remains  as  the  only  viable  explanation (Kreidberg et al. 2014a). JWST should similarly provide the precision to break the aerosol vs. mean molecular weight degeneracy in a single transit for many sub-Neptunes, allowing for improved aerosol characterization for smaller planets.  \nFigure 10. Strength of the 1.4 µ m water feature in units of scale heights for transmission spectra of planets 2-6 R ⊕ in  size.  The  blue  parabola  is  a  best-/it  second  order  polynomial  trend,  implying  a  minimum  in  the strength of transmission spectral features around an equilibrium temperature of 600 K, perhaps indicating the conditions for maximal aerosol coverage. Left: Colored lines indicate clear and cloudy models from Morley et al. (2015). The dotted and dashed contours are for 100× and 300× solar metallicity, respectively. Right: Colored lines indicate clear and hazy models from Morley et al. (2015). The hazy models include soot hazes only. The dotted and dashed contours are for 1% haze precursor conversion into soots while the dashed contours are 10%. Figure courtesy of Yoni Brande.  \n<!-- image -->  \nEven without an unambiguous detection of aerosols on a planet-by-planet basis, the ubiquity of *lattened transmission spectra and indications that the *latness correlates with planetary equilibrium temperature (Cross*ield & Kreidberg 2017; Libby-Roberts et al. 2020; Gao et al. 2021) point to aerosol coverage being a de*ining characteristic of sub-Neptune exoplanets. Hints that the aerosol coverage may clear at high temperatures ( ≳ 900 K) and lower temperatures ( ≲ 400 K) provide hints as to the dominant particle composition and formation pathway (Figure 10).  \nAs  for  the  aerosol  formation  mechanism,  it  is  hypothesized  that  hydrocarbon-based  hazes readily form in hydrogen-rich sub-Neptune exoplanets below a temperature of ∼850 K (e.g. Morley et al. 2015; Kawashima & Ikoma 2019). Under such conditions, methane is expected to be plentiful in chemical equilibrium. Methane is readily photolyzed by the UV radiation from the host star, producing a rich collection of higher-order hydrocarbons that can continue to polymerize and ultimately form large involatile haze particles (e.g. Miller-Ricci Kempton et al. 2012; Morley et al. 2013; Kawashima & Ikoma 2018; Lavvas et al. 2019). This is analagous to how we believe hydrocarbon 'tholin' haze forms in Titan's atmosphere. The propensity for hazes to form under sub-Neptune conditions is supported by lab work, in which an ensemble of gases are irradiated by a UV or plasma energy source, and the resulting solid particles are collected and analyzed (Horst et al. 2018; He et al. 2018). Interestingly, lab studies are also able to form hazes in gas mixtures without methane (He et al. 2020), indicating that haze formation in exoplanet atmospheres may come about via diverse chemical pathways that have yet to be characterized.  \nCandidates for condensate clouds in sub-Neptunes include sul*ides (ZnS, Na 2 S), sulfates (K 2 SO 4 ),  salts  (KCl),  and  graphite (Miller-Ricci Kempton et al. 2012; Morley et al. 2013; Mbarek & Kempton 2016). These are the expected equilibrium condensates over the temperature range of most sub-Neptunes studied to date, although some of these species may not form clouds due to their high nucleation-limited energy barriers (Gao et al. 2020). Arguments for haze being dominant over clouds in sub-Neptune atmospheres also hinge on how thick and high up the aerosols layers must be in order to fully *latten transmission spectra, especially in the case of the well-studied planet GJ 1214b. It is  \ndif*icult  to  build  models  in  which  low-abundance  species  such  as  ZnS  or  KCl  are  able  to  provide suf*icient opacity to match existing observational data (Morley et al. 2013, 2015).  \nRecent  JWST  phase  curve  observations  of  GJ  1214b  have  thrown  another  surprise  into  our evolving  understanding  of  sub-Neptune  exoplanets.  The  very  high  measured  Bond  albedo  of  the planet based on its global thermal emission ( A B ≈ 0 . 5) implies that the planet's aerosol layer is highly re*lective (Kempton et al. 2023). This is in tension with our understanding of hydrocarbon hazes (e.g. soots and tholins), which are primarily believed to be dark and absorptive (Khare et al. 1984; Morley et al. 2013, 2015). Additional lab and theoretical work is urgently needed to understand how such re*lective and abundant aerosols are formed in sub-Neptune environments. Some possibilities are a more  re*lective  type  of  hydrocarbon  or  darker  particles  coated  in  high-albedo  condensates  (e.g. Lavvas et al. 2019). Upcoming JWST observations should shed light on whether sub-Neptune aerosols share  a  universal  set  of  properties  and  whether  transitions  from  clear  to  aerosol-enshrouded conditions occur at expected levels of insolation.", '4. ATMOSPHERIC MASS LOSS': "Transiting exoplanets are particularly vulnerable to atmospheric mass loss as a result of their close-in orbits. The smallest and most highly irradiated exoplanets may lose their entire atmosphere (see Section 1.2), while the population of close-in gas giant planets appears to be minimally altered by  atmospheric  mass  loss  (e.g.  Vissapragada  et  al.  2022;  Lampo'n  et  al.  2023).  For  giant  planets, atmospheric out*lows  are  driven  by  high  energy  irradiation  from  the  host  star,  which  causes  the uppermost layers of the planet's atmosphere to expand until they become unbound; this process is often referred to as 'photoevaporation' (for a comprehensive review of theoretical work on this topic, see Owen 2019). For smaller planets, core-powered mass loss (Ginzburg et al. 2018) might also play an important role. For a more detailed discussion of current constraints on these processes from the measured radius-period distribution of sub-Neptune-sized exoplanets, see Rogers et al. (2021) and Owen et al. (2023).  \nWe can directly observe the atmospheric out*lows of transiting planets by measuring the depth of the transit in strong atomic absorption lines. The large planet-star radius ratios of transiting gas giant  exoplanets  make  them  particularly  favorable  targets  for  these  observations.  The  *irst atmospheric out*lows from close-in gas giant planets were detected by measuring the strength of the absorption in the Lyman α line of hydrogen (Vidal-Madjar et al. 2003; Lecavelier Des Etangs et al. 2010). This line is located in the UV, and can therefore only be accessed by space telescopes like HST. Because the core of this line is masked by absorption from the interstellar medium, these observations are only sensitive to absorption in the line wings and are limited to relatively nearby (distances of ∼ 100 pc or less) stars. This absorption corresponds to the higher velocity components of the out*low, which are located farther from the planet (e.g., Owen et al. 2023). To date, there are seven planets whose out*lows have been measured in this line; see Fig. 11 for a visualization of where these planets are located in mass-period space.  \nFigure 11. Orbital period versus mass distribution for the current sample of planets with measured mass loss rates ( > 3 σ signi/icance; for a complete list see review by Dos Santos 2022) using either Lyman α (blue /illed circles; we also include a detection of AU Micb by Rockcliffe et al. 2023), metastable helium (red /illed circles, we also include detections for HAT-P-67b, TOI-1268b, TOI-1420b, and TOI-2134b from GullySantiago et al. 2023; P'erez Gonz'alez et al. 2023; Zhang et al. 2023a), or both (blue circles with red /ill). The size of the points is scaled according to the host star's brightness in J band (infrared), which depends on the star's distance and mass; brighter stars (smaller J magnitudes) are generally located closer to the Earth and/or have larger masses. The full sample of con/irmed planets is shown as grey circles for comparison. There is a de/icit of sub-Saturnsized planets on close-in orbits; this region is called the 'Neptune desert', and its approximate boundaries as de/ined in Mazeh et al. (2016) are shown as black dashed lines. Figure courtesy of M. Saidel .  \n<!-- image -->  \nRecent theoretical (Oklopˇci'c & Hirata 2018) and observational work (Spake et al. 2018; Nortmann et  al.  2018)  revealed  that  atmospheric  out*lows  could  also  be  detected  using  metastable  helium absorption  at  1083  nm.  Unlike  Lyman α ,  this  line  can  be  readily  observed  using  high  resolution spectrographs on ground-based telescopes. Because we can measure absorption in the line core, this line provides a complementary tool to probe the lower velocity components of the out*low, which are located closer to the planet. To date, atmospheric out*lows have been measured for 20 planets using this line (see Fig. 11).  \nOut*lows have also been detected in the optical H α , H β , and H γ lines (e.g., Jensen et al. 2012; Yan & Henning 2018; Casasayas-Barris et al. 2019; Wyttenbach et al. 2020), as well as UV lines of other atomic species (e.g., Vidal-Madjar et al. 2004; Sing et al. 2019; Dos Santos et al. 2023). Some of the  \nFigure  12. Measurement  of  hot  Jupiter  HAT-P-32  b's  1083  nm  metastable  helium  absorption  signal  as  a function of orbital phase from Zhang et al. (2023c). The unusually extended nature of this planet's out/low is distinct  from  that  of  most  other  hot  Jupiters  with  published  helium  detections,  which  tend  to  have  more narrowly con/ined out/lows. Left, upper panel: Helium line equivalent width (EW) for HAT-P-32 b as a function of orbital phase. Solid circles indicate data taken in conjunction with a transit event, while open circles indicate data taken as part of a stellar monitoring program. The phased data are divided into /ive sections marked by gray shading and colored accordingly. The period where the planet is transiting the star is shown with dark gray shading. Results from a 3D hydrodynamic model are overplotted as a solid gray line. Left, lower panel: Equivalent width values after subtracting the average stellar spectrum. Right: Slice through the orbital plane of a 3D hydrodynamic simulation of a system with properties similar to that of HAT-P-32. The out/lowing gas expands into long tails that lead and trail the planet's orbit, resulting in strong helium absorption before and after the transit. Approximate viewing angles for each colored time segment are shown with colored labels. The logarithmic gas density distribution is indicated using the color bar on the right. Figure from Zhang et al. (2023c).  \n<!-- image -->  \nrefractory atomic species detected in optical high spectral resolution data sets (see Section 2.3) likely also probe unbound regions of the atmosphere, but more detailed models are needed in order to interpret these absorption signals (Linssen & Oklopčić 2023). By combining the information from multiple  lines  together,  we  can  obtain  a  more  detailed  picture  of  the  overall  structure  and thermodynamics of the out*low (Lampo'n et al. 2021; Yan et al. 2022a; Huang et al. 2023; Linssen & Oklopčić 2023).  \nThe magnitude of the atmospheric absorption signal during transit can be converted into a mass loss rate by modeling the out*low as a spherically symmetric isothermal Parker wind (Oklopčić & Hirata 2018; Lampo'n et al. 2020; Dos Santos et al. 2022; Linssen et al. 2022). If the out*low is not spherical but instead is sculpted into a comet-like tail by the stellar wind, we would expect to see an extended absorption signal after the end of the transit egress (e.g., Ehrenreich et al. 2015; Lavie et al. 2017; Kirk et al. 2020; Spake et al. 2021). If there is out*lowing material orbiting just ahead of the planet, we may also see absorption prior to the planet's ingress, or even absorption extending many hours before and/or after the planet transit (Zhang et al. 2023c; Gully-Santiago et al. 2023, see Fig. 12). The time-dependent absorption signal, as well as its spectroscopically resolved velocity structure, therefore  provide  us  with  important  information  about  the  three-dimensional  structure  of  the atmospheric out*low (e.g. Wang  &  Dai  2021a,b; MacLeod & Oklopčić 2022). In addition to stellar winds,  these  out*low  geometries  may  also  be  shaped  by  the  planetary  and  stellar  magnetic  *ield geometries (Owen & Adams 2014; Schreyer et al. 2023; Fossati et al. 2023).  \nIt is more challenging to detect atmospheric out*lows from sub-Neptune-sized planets. There are currently only three sub-Neptune-sized planets orbiting mature ( > 1 Gyr) stars with published detections (Bourrier et al. 2018; Ninan et al. 2020; Palle et al. 2020b; Orell-Miquel et al. 2022; Zhang et al. 2023a), one of which is disputed (GJ 1214b; see discussion in Spake et al. 2022). Fortunately, these out*lows are more easily observable if we focus on the subset of small planets orbiting young stars. Young stars are more active and have enhanced high energy *luxes (e.g., Johnstone et al. 2021; King & Wheatley 2021), while young planets have radii that are still in*lated by leftover heat from their formation. As a result, young sub-Neptunes are expected to have enhanced mass loss rates as  \ncompared to their more evolved counterparts. Observations of young transiting sub-Neptunes have revealed the presence of atmospheric out*lows in both Lyman α (Zhang et al. 2022c) and metastable helium (Zhang et al. 2022a, 2023b; Orell-Miquel et al. 2023). These observations can be used to test the predictions of atmospheric mass loss models seeking to explain the origin of the bimodal radius distribution of small close-in planets (see Section 1.2).", '5.1. The Physics of Thermal Inversions': "Measuring the thermal structure of exoplanet atmospheres provides critical insight into how energy is transported and deposited in planetary envelopes. In the solar system, for example, we know that Earth has a stratospheric thermal inversion due to UV/optical absorption by the O 3 molecule. Venus has a lower equilibrium temperature than the Earth, despite receiving nearly twice as much energy from the Sun as the Earth does, due to its high Bond albedo. Mercury has a scalding hot dayside and a frigid nightside because it lacks a thick atmosphere to transport heat. All of these types of processes can be assessed by measuring the dayside temperatures and vertical temperature gradients in exoplanet atmospheres.  \nThermal inversions in particular have been an interesting phenomenon accessed via dayside thermal emission spectra. Planetary atmospheres that are strongly absorbing at the wavelengths at which their host stars puts out most of their energy will experience heating at the location where the stellar energy is deposited. To ensure global energy balance, this heating comes at a cost of cooling regions deeper in the atmosphere, thus creating a thermal inversion in which temperature increases with altitude, peaking around the region where the starlight is absorbed (i.e. the τ ∼ 1 surface, where τ is the optical depth). Spectroscopically, thermal inversions are identi*ied by observing spectral lines in emission, as opposed to absorption lines, which are seen when temperature decreases outwardly. The shape of spectral features relative to the surrounding continuum is therefore used to assess the temperature gradient in the observable portion of an exoplanet atmosphere via thermal emission measurements.  By  detecting  a  thermal  inversion  and  simultaneously  measuring  atmospheric composition,  astronomers  can  also  attempt  to  infer  which  absorber(s)  are  responsible  for  the  \nupperatmosphere heating. As discussed in Section 2.3, TiO, VO, and a variety of refractory species have been proposed as optical and UV absorbers that can generate thermal inversions in hot Jupiters (e.g. Fortney et al. 2008; Lothringer et al. 2018). Other opacity sources such as hazes, clouds, or even water vapor  for  planets  orbiting  M-dwarfs  have  been  proposed  to  similarly  drive  thermal  inversions  in cooler planets (e.g. Morley et al. 2015; Arney et al. 2016; Malik et al. 2019; Lavvas & Arfaux 2021; Roman et al. 2021). Because thermal inversions are generated by absorption of incident stellar energy, they are primarily expected to be a dayside phenomenon, although ef*icient horizontal heat exchange can cause them to persist away from the sub-stellar point and even around to a planet's nightside (e.g. Komacek et al. 2022).", '5.2. The Hot-to-Ultrahot Jupiter Transition': "Forward models of hot Jupiter emission spectra have long predicted a transition  in  dayside thermal structure from planets with inversions to those without, as a function of decreasing planetary temperature. The thermal inversions would be driven by gas-phase optical and UV absorbers that condense  out  of  the  atmosphere  at  lower  temperatures,  thus  rendering  the  atmosphere  more transparent to stellar irradiation (and therefore producing un-inverted temperature pro*iles) at lower equilibrium temperatures (e.g. Hubeny et al. 2003). Fortney et al. (2008) initially proposed that TiO and  VO  should  be  the  key  drivers  of  thermal  inversions,  resulting  in  a  transition  to  inverted temperature pro*iles around a planetary equilibrium temperature of 1500 K. Evidence of thermal inversions  from  secondary  eclipse  spectra  probing  planets  around  this  cutoff  temperature  with Spitzer  observations  was  initially  mixed  (e.g.  Richardson  et  al.  2007;  Charbonneau  et  al.  2008; Knutson et al. 2008, 2009; Deming et al. 2011; Todorov et al.  2010, 2012, 2013; Baskin et al. 2013; Diamond-Lowe et al. 2014). Ultimately, improved spectroscopic investigations with the HST+WFC3 instrument  and  high  resolution  ground-based  spectrographs  clearly  demonstrated  un-inverted temperature  pro*iles  in  various  hot  Jupiters  around  and  above  the  predicted  1500  K  cutoff temperature, via spectral features appearing in absorption (Birkby et al. 2013; Kreidberg et al. 2014b; Schwarz et al. 2015; Line et al. 2016, 2021). This was accompanied by failures to de*initively detect gas-phase TiO and VO in transmission spectra of some of the same planets, implying removal via  \nnightside condensation coldtrapping or some other disequilibrium chemistry process, or perhaps a more mundane explanation such as inaccurate line lists (e.g. D'esert et al. 2008; Hoeijmakers et al. 2015).  \nEven  hotter  planets  were  ultimately  required  to  produce  de*initive  evidence  for  thermal inversions.  'Ultrahot'  Jupiters,  as  discussed  in  Section  2.2  are  those  that  are  so  hot  that  water dissociates in their atmospheres, and various refractory elements (not just Ti and V) are predicted to be in the gas phase (Parmentier et al. 2018; Lothringer et al. 2018). In these planets, temperature inversions are predicted to be helped along by gas-phase metals and oxides such as Fe, Mg, SiO, etc. Formally the cutoff between hot and ultrahot Jupiters occurs around T eq = 2200 K. The *irst ultrahot Jupiter to produce a clear detection of a dayside thermal inversion was WASP-121 b (Evans et al. 2017). The 1.4 µ m water feature in this planet's secondary eclipse spectrum appears in emission, although the feature is quite subtle. Other ultrahot Jupiters, as mentioned in Section 2.2, produced nearly  featureless  secondary  eclipse  spectra  across  the  WFC3  bandpass,  leading  to  ambiguous interpretation  as  to  whether  water  was  simply  absent  from  these  atmospheres  or  the  dayside temperature  pro*iles  were  isothermal,  thus  masking  any  spectral  features  (Sheppard  et  al.  2017; Mans*ield et al. 2018; Kreidberg et al. 2018; Mans*ield et al. 2021).  \nThe picture of a transition to ultrahot planets with thermal inversions becomes clearer with population-level studies. When looking at WFC3 thermal emission spectra vs. the planets' measured dayside temperatures, Mans*ield et al. (2021) identify a clear trend from un-inverted temperature pro*iles at lower dayside temperatures, to inverted pro*iles at dayside temperatures above ∼2500 K (Figure 13). This is in line with predictions from forward models, although such models still predict the transition to thermal inversions to occur at somewhat lower temperatures. Interestingly, both the models and the data reveal a shift back to featureless spectra with WFC3 at even higher dayside temperatures  ( T day ≳ 3000  K),  corresponding  to  full  removal  of  atmospheric  H 2 O  via  thermal dissociation. Another population-level prediction is that ultrahot planets orbiting earlier-type (i.e. hotter)  host  stars  should  produce  even  larger  thermal  inversions  because  the  peak  of  the  stellar spectral  energy  distribution  (SED)  aligns  particularly  well  with  the  expected  UV/optical  opacity  \nsources in the planets' atmospheres. This prediction has played out in secondary eclipse observations of the ultrahot Jupiter KELT-20b, which orbits a hot A-type host star. For this planet, the 1.4 µ m H 2 O feature appears strongly in emission, much more so than for comparably irradiated planets orbiting later-type G stars (Fu et al. 2022).  \nRecent  JWST  and  high-resolution  emission  spectroscopy  studies  have  solidi*ied  our understanding  of  the  hot-to-ultrahot  Jupiter  transition  by  providing  increased  precision  and wavelength coverage. For example, JWST has the power to resolve the subtle shape of spectral features that were previously hidden in the noise of HST observations. In the case of the the ultrahot Jupiter WASP-18  b,  the  planet's  emission  spectrum,  which  was  nearly  featureless  in  HST  observations (Sheppard et al. 2017; Arcangeli et al. 2018) is now revealed by JWST to contain very subtle water features in emission, thus con*irming the presence of a thermal inversion (Coulombe et al. 2023). In contrast, the cooler 'normal' hot Jupiter HD 149026b shows spectral features in absorption, including clear detections of H 2 O and a strong CO 2 feature implying high metallicity (Bean et al. 2023). With high-resolution observations from the ground, the emission spectra of various ultrahot Jupiters also provide clear indications of emission lines, demonstrating inverted thermal structures (e.g. Kasper et al.  2021; Yan  et  al.  2022b, 2023; Brogi et al. 2023; van Sluijs et al. 2023). In at least one case, the detection of a thermal inversion is (*inally) accompanied by a high-con*idence detection of TiO in the transmission spectrum of the same planet (Yan et al. 2020; Prinoth et al. 2022).  \nIn summary, with newer and better data and population-level studies, astronomers are now *inding that  the  dayside  thermal  structures  of  hot  and  ultrahot  Jupiters  appear  to  align  with  the  basic predictions of forward models, albeit with a transition to inverted temperature pro*iles occurring at somewhat  higher  equilibrium  temperatures  than  what  is  predicted  for  solar  composition atmospheres. The forward models assume thermochemical equilibrium and 1-D radiative-convective energy balance, with gas-phase metals and metal oxides serving as strong optical and UV absorbers at high temperatures. Additional new frontiers that will be opened with JWST in the near term include studies of the thermal structures of even colder giant planets. At lower temperatures, dayside clouds or even haze might play a primary role in mediating the deposition of stellar energy in the planets'  \natmospheres. Hints of  such  effects  already  exist  in  the  WFC3  emission  spectra  of  the  coolest  hot Jupiters investigated to-date (Crouzet et al. 2014; Mans*ield et al. 2021). JWST will also enable more detailed studies of the 3-D structure of giant planet daysides, which will be discussed in more detail in Section 6.  \nFigure 13. Brightness temperature vs. wavelength for hot Jupiter secondary eclipse spectra observed by HST with  the  WFC3  instrument.  Brightness  temperature  is  a  measure  of  the  approximate  temperature  of  the photosphere at the wavelength being observed. The 1.4 µ m water band is indicated by the gray shaded region. For less irradiated planets (toward the bottom right of the plot), the 1.4 µ m water band appears in absorption, indicating atmospheres with non-interted temperature pro/iles. For hotter planets, the absorption features disappear, and in some cases (e.g. WASP-76b, WASP-121b, WASP-12b), the 1.4 µ m water band subtly inverts into emission, indicating a possible thermal inversion. When compared against forward models of hot Jupiter emission spectra, these observations align well with predictions that thermal inversions occur for planets hotter than ∼2000 K, and water dissociation reduces the abundance of H 2 O in the dayside atmosphere. Figure adapted from MansGield et al. (2021).  \n<!-- image -->", '5.3. Hot Jupiter Albedos': "Efforts to measure the re*lected light from hot Jupiters at optical wavelengths began soon after the discovery of such planets, in order to constrain their albedos. These studies initially resulted in non-detections and upper limits, some of which were quite constraining (e.g. Charbonneau et al. 1999; Rowe et al. 2008; Winn et al. 2008). It was quickly realized that the implied low albedos were in line with the predictions from radiative transfer models for such planets. In the absence of dayside clouds, strong optical absorption lines such as those from Na, K, TiO, etc. absorb out much of the incident stellar  radiation,  while  the  only  source  of  re*lected  light  is  Rayleigh  scattering  from  the  gaseous atmosphere (Seager et al. 2000; Burrows et al. 2008). For cooler planets, in which re*lective dayside clouds are expected, geometric albedos should be higher (e.g. Cahoy et al. 2010; Adams et al. 2022), but the general trend of shallower secondary eclipses with lower levels of insolation typically makes it more challenging to detect such signals.  \nSpace telescopes such as CoRoT, Kepler, and TESS were ultimately able to detect the optical secondary  eclipses  of  a  number  of  hot  and  ultrahot  Jupiters,  although  the  broad  photometric bandpasses of these facilities has meant that it is typically not possible to fully disentangle the relative contributions of thermal emission vs. scattered light, resulting in model-dependent albedo inferences (e.g.  Alonso  et  al.  2009;  Christiansen  et  al.  2010;  Demory  et  al.  2011).  A  compilation  of  optical secondary eclipse measurements for 21 planets with CoRoT, Kepler, and TESS reveals that most such detections  have  been  made  at  less  than  3σ con*idence,  with  inferred  geometric  albedos  ranging between 0 and ∼0.3 (Wong et al. 2020). One notable exception is the planet Kepler-7b, which has an inferred albedo of ∼ 0 . 25-0 . 35, measured at high con*idence (Demory et al. 2011; Wong et al. 2020). This  is  consistent  with  the  planet's  (relatively)  low  dayside  temperature  of  ∼1000  K  and  the expectation that such conditions are conducive to the formation of re*lective clouds. More recently, the  European  CHEOPS  satellite  has  demonstrated  its  ability  to  produce  well-constrained measurements  of  hot  Jupiter  geometric  albedos  (Brandeker  et  al.  2022;  Krenn  et  al.  2023).  The inferred values for the hot Jupiters HD 209458b and HD 189733b from CHEOPS lightcurves are 0 . 096 ± 0 . 016 and 0 . 076 ± 0 . 016. These albedos are far lower than for any solar system planets but in line  \nwith models of hot Jupiters having cloud-free dayside atmospheres. In summary, hot Jupiters are dark, but cooler giant planets may be more re*lective.", '5.4. The Dayside Temperatures of Sub-Neptunes and Super-Earths': "Detecting  the  thermal  emission  from  smaller  and  typically  cooler  sub-Neptunes  and superEarths is a much more technically challenging endeavor than for hot Jupiters. Because of this, such studies have mostly been limited to simply detecting a secondary eclipse and measuring an associated brightness temperature, as opposed to full spectroscopic characterization. Once measured, the dayside temperature of the planet can then be used to obtain a combined constraint on both daynight  heat  redistribution  and  albedo.  All  tidally-locked  planets  have  a  maximum  dayside temperature that can be achieved if the planet's only energy source is the irradiation from its host star:  \n. (5)  \nThis  is  simply  Equation  3  taken  in  the  limit  of  no  day-night  heat  redistribution  (instantaneous reradiation)  and  zero  albedo.  Lower  measured  dayside  temperatures  are  indicative  of  either  a re*lective planet or considerable day-night heat transport (or some combination thereof; Koll et al. 2019; Mans*ield et al. 2019; Koll 2022).  \nTo date there have only been successful thermal emission detections for two sub-Neptunes: the planets TOI-824b (Roy et al. 2022) and GJ 1214b (Kempton et al. 2023). The former is a hot dense sub-Neptune, whereas the latter is a cooler planet that was already known to have a thick aerosol layer from transmission spectroscopy measurements (see Section 3.3). The dayside temperature of TOI-824b is consistent with its T max ,  whereas GJ 1214b is signi*icantly colder. For sub-Neptunes, a maximally hot dayside, implying poor day-night heat redistribution, requires a high mean molecular weight atmosphere. This result comes from 3-D general circulation models, which demonstrate that heat transport ef*iciency decreases as a function of increasing mean molecular weight (e.g. Kataria et al.  2014;  Charnay  et  al.  2015;  Zhang  &  Showman  2017).  Hydrogen-rich,  solar-composition subNeptune atmospheres are predicted to transport heat very ef*iciently, resulting in cooler daysides  \nand nearly homogeneous global temperatures. Conversely, GJ 1214b's dayside temperature is colder than even its zero-albedo temperature in the limit of fully ef*icient day-night heat transport, meaning the planet must have a non-zero albedo. This interpretation is con*irmed by a full-orbit phase curve with JWST that is best *it by a high mean molecular weight atmosphere coupled with the presence of highly re*lective aerosols (see Sections 3.3 and 6).  \nGJ 1214b is also the only planet smaller than Neptune to have spectral features identi*ied in its dayside thermal emission spectrum. Subtle departures from a blackbody shape imply the presence of geseous water in this planet's atmosphere and a non-inverted temperature pro*ile (Kempton et al. 2023).  Interestingly,  for  planets  orbiting  M-dwarf  host  stars,  water  vapor  can  actually  serve  as  a source of thermal inversions (Malik et al. 2019; Selsis et al. 2023). This is because its strong near-IR opacity ef*iciently absorbs stellar light, which in this case peaks at red to near-IR wavelengths. The predicted thermal inversions are fairly weak and high up in the planets' atmospheres though, making their observable consequences negligible for low-resolution spectroscopy with JWST.  \nRocky  planet  thermal  emission  measurements  with  Spitzer  and  more  recently  JWST  have focused on measuring dayside temperatures (as well as phase curves in certain cases) to constrain the presence or absence of an atmosphere. Rocky planets without atmospheres have no mechanism by which to transport heat to their nighsides (Seager & Deming 2009; Koll et al. 2019; Koll 2022). Furthermore, many kinds of rocks that are known to form planetary surfaces in the solar system are very dark 14 (Hu et al. 2012; Mans*ield et al. 2019). It therefore can be concluded that a terrestrial planet with a maximally hot dayside temperature is unlikely to have an atmosphere, whereas colder dayside temperatures imply the presence of an atmosphere. Several terrestrial planets to-date have been  subjected  to  this  'secondary  eclipse  test'  to  measure  their  dayside  temperatures,  with  the conclusion in the majority of cases being to rule out the presence of thick atmospheres to varying degrees of con*idence (Kreidberg et al. 2019; Cross*ield et al. 2022; Whittaker et al. 2022; Greene et al.  2023;  Ih  et  al.  2023,  see  Section  2.5).  The  coldest  terrestrial  planet  yet  observed  in  secondary  \neclipse is TRAPPIST-1c. For that planet, its dayside temperature is only ∼2σ consistent with its T max value (Zieba et al. 2023). In this case, the presence of a thick atmosphere is not de*initively ruled out, implying that perhaps less irradiated planets are more likely to retain their atmospheres, even if they orbit active M-dwarf stars. Further measurements of rocky planet secondary eclipses with JWST will continue to map out the parameter space of which planets do and do not possess atmospheres, with many such observations already planned for Cycle 2.", '6. THREE-DIMENSIONAL ATMOSPHERIC STRUCTURE': 'Close-in exoplanets are expected to be tidally locked, with permanent day and night sides. As a result, they can exhibit relatively large day-night temperature gradients, along with corresponding gradients in their atmospheric chemistries and cloud properties. Importantly, tidally locked planets will have relatively slow rotation periods (on the order of days) compared to the gas giant planets in the solar system. This means that the typical length scales for their atmospheric circulation patterns will be much larger (∼ hemisphere-scale) than those of planets like Earth, Jupiter, or Saturn. For a review of the fundamental principles and relevant dynamical regimes for atmospheric circulation on close-in gas giant planets, we recommend Showman et al. (2010) and Showman et al. (2020).', '6.1. Fundamentals of Day-Night Heat Transport on Hot Jupiters': "There  is  a  considerable  body  of  observational  constraints  on  the  atmospheric  circulation patterns of hot Jupiters. During its sixteen years of operation, the Spitzer Space telescope measured broadband infrared secondary eclipse depths for more than a hundred close-in gas giant exoplanets (e.g., Baxter et al. 2020; Wallack et al. 2021; Deming et al. 2023). It also measured broadband infrared phase curves for several dozen gas giant exoplanets (e.g. Bell et al. 2021; May et al. 2022). There are only  a  few  planets  with  spectroscopic  phase  curves  measured  with  HST  (Stevenson  et  al.  2014; Kreidberg et al. 2018; Arcangeli et al. 2019, 2021; Mikal-Evans et al. 2022) and (more recently) JWST (MikalEvans et al.  2023b;  Kempton et  al.  2023;  Bell  et  al.  2023b).  Lastly,  there  are  currently  two published secondary eclipse maps of the dayside atmospheres of these planets, one from Spitzer (Majeau et al. 2012; de Wit et al. 2012) and one from JWST (Coulombe et al. 2023).  \nThere  are  several  big-picture  takeaways  that  have  emerged  from  the  current  body  of observations.  First,  both  models  (Perez-Becker  &  Showman  2013;  Komacek  &  Showman  2016; Komacek et al. 2017) and observations (e.g., Wallack et al.  2021; Bell et al. 2021; May et al. 2022; Deming et al. 2023) agree that the most highly irradiated gas giant exoplanets have a lower day-night heat  redistribution  ef*iciency  (de*ined  as  the  fraction  of  energy  incident  on  the  dayside  that  is transported  to  the  night  side  by  atmospheric  winds)  than  their  more  moderately  irradiated counterparts. This means that the most highly irradiated gas giant exoplanets have relatively large day-night  temperature  contrasts,  while  their  cooler  counterparts  tend  to  have  more  uniform temperature distributions (see Figure 14).  \nThese same data also indicate that most close-in gas giant exoplanets have a super-rotating (eastward) equatorial band of wind that transports energy from the day side to the night side, in good agreement with predictions from general circulation models (see Figure 15 and review by Showman et al. 2020). This is readily apparent in infrared Spitzer phase curve observations (Bell et al. 2021; May  et  al.  2022),  which  show  that  the  hottest  region  on  the  day  side  is  shifted  eastward  of  the substellar point for most hot Jupiters (this corresponds to a phase curve that peaks just before the secondary eclipse). There are several notable exceptions to this trend, which we discuss in more detail later in this section. We can also see the effects of atmospheric circulation in high resolution emission and  transmission  spectroscopy,  where  we  can  directly  measure  the  Doppler  shift  induced  by  the planet's atmospheric winds. This can manifest as either a net shift in the lines for a single coherent *low direction, or an overall broadening of the lines for observations that integrate over multiple *low directions (e.g. Miller-Ricci Kempton & Rauscher 2012; Showman et al. 2013; Beltz et al. 2021, 2022). Doppler  shifts  due  to  atmospheric  winds  have  been  seen  in  high  resolution  transmission  \nspectroscopy, which probes the day-night terminator region (e.g. Snellen et al. 2010;  \nFigure 14. Dayside brightness temperatures for the sample of hot Jupiters with measured eclipse depths in the 3.6 and 4.5 µ m bands with Spitzer as a function of their predicted equilibrium temperatures. Planets with relatively  inef/icient  day-night  recirculation  will  lie  closer  to  the  red  dashed  line  (maximum  dayside temperature assuming zero recirculation and zero albedo), while planets with relatively ef/icicent day-night circulation will lie closer to the black dashed line (complete day-night recirculation of energy, zero albedo). The subset  of  hot  Jupiters  whose  dayside  albedos  are  enhanced  by  re/lective  silicate  clouds  (equilibrium temperatures near 1500 K) can also lie below the black line, as indicated by the blue dashed line. Planets with black circles have spectral slopes that are inconsistent with that of a blackbody across the 3.6 to 4.5 µ m band, indicating the presence of strong molecular features. Figure from Wallack et al. (2021) .  \n<!-- image -->  \nTequ  \nLouden & Wheatley 2015; Brogi et al. 2016; Flowers et al. 2019; Seidel et al. 2021; Kesseli et al. 2022;  \nPai Asnodkar et al. 2022; Gandhi et al. 2022), and in emission spectroscopy, which integrates over the dayside atmosphere (e.g. Yan et al. 2023; Lesjak et al. 2023).", '6.2. Complications from Clouds, Chemical Gradients, and Magnetic Fields': "The non-uniform temperature distributions in the atmospheres of close-in gas giant planets also have important implications for their condensate cloud properties. Clouds that can condense in one region of the atmosphere may not be able to condense in other regions; this can lead to hemispheresized  cloudy  and  clear  regions  in  the  atmospheres  of  these  planets  (Figure  15,  and  for  model predictions of the irradiation-dependent cloud distributions, see Parmentier et al. 2018, 2021; Roman  \net al. 2021). We can see empirical evidence for patchy clouds in the optical phase curves of close-in planets, which exhibit localized regions of high albedo due to the presence of re*lective silicate clouds (Demory et al. 2013). When viewed in transmission, the properties of clouds and/or hazes are also expected to differ between the dawn and dusk terminators. This effect will cause the shape of the transit ingress (when the planet is entering the disk of the star) to differ from that of the transit egress (when the planet is exiting the disk of the star), and should be detectable with JWST (Kempton et al. 2017; Powell et al. 2019; Espinoza & Jones 2021; Steinrueck et al. 2021; Carone et al. 2023). Close-in gas giant planets also appear to have surprisingly uniform nightside temperatures, and it has been suggested that this may be due to the presence of nightside clouds (Keating et al. 2019; Gao & Powell 2021). If con*irmed by JWST, this would have important consequences for atmospheric circulation patterns on hot Jupiters, as the presence of these clouds can inhibit radiative cooling on the planet's night side, resulting in a globally hotter atmosphere and a reduced offset for the dayside hot spot (Parmentier et al. 2021; Roman et al. 2021).  \nThese day-night temperature gradients can also lead to chemical gradients between the dayside and nightside atmospheres. For the most highly irradiated hot Jupiters, current observations suggest that refractory species may condense on the night side, even when the dayside atmosphere is hot enough for them to remain in gas phase (Lothringer et al. 2022; Pelletier et al. 2023). High resolution transmsission spectroscopy has also been used to argue for gradients in composition between the dawn and dusk terminators (Ehrenreich et al. 2020; Mikal-Evans et al. 2022; Prinoth et al. 2022, 2023; Gandhi et al. 2022). These chemical gradients can complicate efforts to measure wind speeds using high resolution spectroscopy (e.g. Wardenier et al. 2023; Savel et al. 2023). Recent studies have also explored the role that the dissociation of H 2 on the day side and its subsequent recombination on the night side might play in day-night energy transport in these highly irradiated atmospheres (Bell & Cowan 2018; Tan & Komacek 2019; Mans*ield et al. 2020; Roth et al. 2021; Changeat 2022).  \nTirr = 2500 K  \nTirr = 3500 K  \nFigure 15. Properties of hot Jupiters with three different incident /lux levels from general circulation models with and without radiatively active clouds included. The top two rows show the temperature distribution at the  top  of  the  atmosphere  (1  mbar),  and  the  middle  two  rows  show  the  temperature  distribution  slightly deeper down, at the approximate level of the infrared photosphere. The bottom panel shows how the presence of clouds alters the level of the photosphere by increasing the atmospheric opacity; more cloudy regions have lower photospheric pressures, meaning that we do not see as deep into these cloudy regions. The approximate photospheric pressure for the clear atmosphere (60 mbar) is indicated by a red line on the color bar. Figure from Roman et al. (2021) .  \n<!-- image -->  \nThere is also emerging evidence suggesting that atmospheric *low patterns on the most highly irradiated hot Jupiters may be altered by magnetic effects. At these temperatures, the atmosphere consists of a mixture of neutral and ionized species. If the planet has a strong magnetic *ield, this can lead to magnetically induced drag and correspondingly weakened day-night energy transport (Perna et al. 2010; Menou 2012). Observationally, this would have the effect of moving the hot spot on the day side closer to the substellar point. Recent observations of the ultra-hot Jupiter WASP-18 b with JWST indicate that its relatively small dayside hot spot offset is best matched by circulation models  \nwith enhanced drag due to MHD effects (Coulombe et al. 2023). Other ultra-hot Jupiters also appear to have similarly small hot spot offsets in their Spitzer phase curves (Bell et al. 2021; May et al. 2022). As the magnetic *ield increasingly dominates the atmospheric *low patterns, it may cause the location of the dayside hot spot to vary from orbit to orbit, perhaps even shifting it westward of the substellar point (i.e. opposite of the predicted wind direction for a neutral atmosphere; Rogers 2017; Hindle et al.  2021a,b).  This  may  explain  the  westward  and/or  time-varying  hot  spot  offsets  of  several  hot Jupiters (e.g.  Dang  et  al.  2018;  Bell  et  al.  2019).  Several  planets  also  appear  to  have  time-varying optical phase curves (Armstrong et al. 2016; Jackson et al. 2019a,b), although in some cases these variations may be the result of stellar and/or instrumental variability (Lally & Vanderburg 2022).", '6.3. Circulation Patterns of Sub-Neptune-Sized Planets': "There are currently only a few sub-Neptune-sized planets with phase curve observations. As discussed in Section 5.4, for rocky planets these phase curves can be used to infer the presence or absence  of  a  thick  atmosphere  based  on  the  observed  day-night  temperature  gradient  (Seager  & Deming 2009). For planets with thick atmospheres, the shape of the phase curve can also be used to constrain the planet's atmospheric composition. A recent JWST observation of the midIR phase curve of the sub-Neptune GJ 1214 b by Kempton et al. (2023) indicates that this planet likely possesses a high mean molecular weight atmosphere with highly re*lective clouds or hazes. Spitzer phase curves of hot rocky super-Earths K2-141 b and LHS 3844 b indicate that these planets have large day-night temperature  gradients,  suggesting  that  their  atmospheres  must  be  relatively  tenuous,  if  they possesses one at all (Kreidberg et al. 2019; Zieba et al. 2022). Although the Spitzer IR phase curve of the super-Earth 55 Cnc e initially appeared to require the presence of a thick atmosphere (Demory et al. 2016b), a subsequent re-analysis of these data resulted in a larger daynight temperature gradient more in line with those observed for other hot rocky super-Earths (Mercier et al. 2022). Puzzlingly, this planet also appears to have a time-varying infrared *lux from its dayside (Demory et al. 2016a), along with a variable optical phase curve (Meier Vald'es et al. 2023). This may indicate the presence of a tenuous, time-varying outgassed high mean molecular weight atmosphere (Heng 2023). JWST  \nwill soon observe phase curves for multiple additional rocky exoplanets and sub-Neptunes, expanding our understanding of their atmospheric properties.", '7. CONCLUSIONS': "With the launch of JWST and the advent of high-resolution spectrographs on large groundbased telescopes,  the  exoplanet  atmospheres  *ield  has  entered  a  new  era.  As  detailed  in  the  previous sections,  we  are  now  reliably  measuring  chemical  abundances  and  abundance  ratios,  global temperature *ields, wind speeds, and atmospheric escape rates. At the time of writing this article we are just over one year into the JWST mission, and we are already seeing results that are fundamentally shifting our understanding of exoplanet atmospheres. Some early takeaways include the diversity of chemical inventories in giant planet atmospheres and an apparent lack of atmospheres on at least some rocky planets orbiting M-dwarfs. At the same time, a multitude of results from ground-based high-resolution spectroscopy are revealing the richness of (ultra)hot Jupiter chemistry.  \nWith these new observational results come new scienti*ic questions. It is already clear that at this improved level of measurement precision, the 3-D nature of exoplanet atmospheres will need to be carefully taken into account to not bias any scienti*ic conclusions. This challenge is accompanied by  new  opportunities  to  directly  infer  properties  of  3-D  circulation  and  weather  in  exoplanet atmospheres. Measurements of individual exoplanets' spectra are also telling a story about how those planets formed and evolved, but backing out the correct narrative is a truly challenging endeavor, which  can  be  helped  along  somewhat  by  population-level  investigations.  The  characterization  of smaller exoplanets is one of the key promises of the JWST mission, but new questions have arisen about what subset of such planets even host atmospheres at all and how to disentangle the signatures of stellar  activity  from  atmospheric  absorption.  Investigations  of  sub-Neptunes  aimed  at distinguishing those with primordial atmospheres from a potential population of water worlds must still contend with the confounding in*luence of aerosols on spectroscopic observations. Along the way, the properties of the aerosols themselves are presenting their own surprises.  \nThe pace of new observational results from JWST and ground-based high-resolution studies is only accelerating. We are in a regime in which our state of knowledge of exoplanet atmospheres in each successive year expands substantially. As such, this review article serves as a snapshot in time of the state of exoplanet observations following the *irst year of JWST science. We anticipate that some of the open questions presented in this article will be resolved in the near term, whereas others will take future generations of telescopes and scientists to fully answer. What we can surely say is that exoplanet atmospheres have yet to reveal all of their surprises to us.  \nE.M.R.K. and H.A.K. would like to acknowledge Jacob Bean, Yoni Brande, Thayne Currie, Peter Gao, Jegug Ih, Megan Mans*ield, James Rogers, Michael Roman, Morgan Saidel, Arjun Savel, , Nicole Wallack, and Zhoujian Zhang who graciously contributed *igures to this review. H.A.K. is also grateful to the Woods Hole Geophysical Fluid Dynamics Program, which provided a thoughtful and interactive venue for developing ideas incorporated into several sections of this review. E.M.R.K. would like to thank Jacob Bean and Tad Komacek for insightful discussions while developing this manuscript.", '10.3847/1538-3881/abb67b': 'D. 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