Correspondence to<EMAIL_ADDRESS>
# Imaging low-mass planets within the habitable zones of nearby stars with
ground-based mid-infrared imaging

Kevin Wagner  Steward Observatory, University of Arizona, USA  NASA Hubble
Fellowship Program $-$ Sagan Fellow  NASA Nexus for Exoplanet System Science
$-$ Alien Earths Team  Steve Ertel  Steward Observatory, University of
Arizona, USA  Large Binocular Telescope Observatory, Tucson, AZ, USA  Jordan
Stone  Naval Research Laboratory, Washington, D.C., USA  Jarron Leisenring
Steward Observatory, University of Arizona, USA  Dániel Apai  Steward
Observatory, University of Arizona, USA  NASA Nexus for Exoplanet System
Science $-$ Alien Earths Team  Lunar and Planetary Laboratory, University of
Arizona, USA  Markus Kasper  European Southern Observatory, Garching, Germany
Olivier Absil  Université of Liège, Belgium  Laird Close  Steward Observatory,
University of Arizona, USA  NASA Nexus for Exoplanet System Science $-$ Alien
Earths Team  Denis Defrère  Université of Liège, Belgium  Olivier Guyon
Steward Observatory, University of Arizona, USA  Subaru Telescope, Hilo,
Hawaii, USA  Jared Males  Steward Observatory, University of Arizona, USA
NASA Nexus for Exoplanet System Science $-$ Alien Earths Team

###### Abstract

Giant exoplanets on 10-100 au orbits have been directly imaged around young
stars. The peak of the thermal emission from these warm young planets is in
the near-infrared ($\sim$1-5 µm), whereas mature, temperate exoplanets (i.e.,
those within their stars’ habitable zones) radiate primarily in the mid-
infrared (mid-IR: $\sim$10 $\mu$m). If the background noise in the mid-IR can
be mitigated, then exoplanets with low masses–including rocky exoplanets–can
potentially be imaged in very deep exposures. Here, we review the recent
results of the Breakthrough Watch/New Earths in the Alpha Centauri Region
(NEAR) program on the Very Large Telescope (VLT) in Chile. NEAR pioneered a
ground-based mid-IR observing approach designed to push the capabilities for
exoplanet imaging with a specific focus on the closest stellar system,
$\alpha$ Centauri. NEAR combined several new optical technologies–including a
mid-IR optimized coronagraph, adaptive optics system, and rapid chopping
strategy to mitigate noise from the central star and thermal background within
the habitable zone. We focus on the lessons of the VLT/NEAR campaign to
improve future instrumentation$-$specifically on strategies to improve noise
mitigation through chopping. We also present the design and commissioning of
the Large Binocular Telescope’s Exploratory Survey for Super-Earths Orbiting
Nearby Stars (LESSONS), an experiment in the Northern hemisphere that is
building on what was learned from NEAR to further push the sensitivity of mid-
IR imaging. Finally, we briefly discuss some of the possibilities that mid-IR
imaging will enable for exoplanet science.

###### keywords:

Exoplanets, Adaptive Optics, Infrared Astronomy, High-contrast Imaging,
Coronagraphic Imaging, Alpha Centauri, mid-IR Imaging, Ground-based Astronomy

##  1 INTRODUCTION

Earth-sized exoplanets have been indirectly observed through transit and
radial velocity surveys [e.g., 1, 2, 3, 4, 5, 6], while direct imaging has
enabled the discovery of young super-Jovian planets on wide orbits [e.g., 7,
8, 9, 10, 11, 12, 13]. For the nearest stars, the habitable zones overlap with
background-limited (and not contrast-limited) sensitivity regions of large
telescopes operating in the mid-infrared (mid-IR). Such wavelengths also
coincide with the thermal emission peak of temperate ($T\sim$ 300K)
exoplanets. Therefore, low-mass exoplanets can be directly imaged with
sufficient exposure time. In a three-week observing campaign on an 8-m
telescope, a sub-Neptune-sized planet could currently be imaged in the mid-IR
[14, 15, 16]. If the instrumental background can be further mitigated, then
lower-mass planets$-$including those that are potentially habitable$-$can be
directly imaged. Through subsequent (spectroscopic) observations, we can then
study their atmospheric and orbital properties [e.g., 17, 18].

Here we review the state-of-the-art of mid-IR exoplanet imaging, some of the
lessons learned so far, and some of the current work being done to continue
progress toward imaging low-mass planets around nearby stars with these
techniques. As a relatively new type of observation, much progress can be made
by understanding and limiting the instrumental artifacts and sources of noise
that were not known or not well-understood before the firsts on-sky tests.
Now, with over 100 hours of data from a high-contrast mid-IR imager on the
Very Large Telescope (VLT), and on-sky results beginning at the Large
Binocular Telescope (LBT), a significant volume of data is available. This
will enable us to understand and construct detailed systematic models, which
can in turn be used to reduce the impact of some of the dominant sources of
noise for mid-IR exoplanet imaging.

##  2 NEAR: New Earths in the Alpha Centauri Region

The recently completed New Earths in the Alpha Centauri Region (NEAR) program
explored several upgrades to the mid-IR imaging capabilities of the VLT. These
enabled establishing the first sensitivity to habitable-zone Neptune-sized
planets within the closest stellar system, $\alpha$ Centauri [15, 16]. Some of
the more significant instrumental upgrades included transferring the VLT’s
mid-IR camera, VISIR, to the unit telescope of the VLT equipped with the
observatory’s deformable secondary mirror (DSM)111The similar (deformable)
optics within the LBT are frequently referred to as “adaptive” secondary
mirrors, or ASMs., which enables performing adaptive optics without additional
warm optics [19]. VISIR was also equipped with a mid-IR optimized coronagraph
[20] and a shaped-pupil mask specifically designed to limit the diffraction
from the secondary star [21].

A crucial component of ground-based mid-IR imaging is the ability to measure
and subtract the thermal background$-$including the contributions from the
sky, telescope, and instrument$-$each of which have distinct temporal
properties. The current generation of mid-IR detectors (specifically the
Raytheon AQUARIUS Si:As arrays [22] within VISIR and NOMIC [23], a similar
camera installed at the LBT) also suffer from excess low frequency noise
(ELFN) that increases exponentially toward lower frequencies [24]. To mitigate
the impact of this systematic noise feature, NEAR pioneered $\sim$10 Hz
chopping using the DSM, which significantly reduced the background noise and
enabled a dramatic improvement in the practicality of observing low-mass
planets in realistic exposure times ($\lesssim$100 hr). Chopping also removes
other static features, such as detector bias and the glow of the annular
groove phase mask (AGPM) coronagraph [25] that can be seen in Fig. 1.

---
Figure 1:  Top: an example of a chop-subtraction image pair from VLT/NEAR.
Image B, following 0.05 sec after A, is subtracted to remove the detector
bias, thermal background, AGPM glow, and excess low frequency noise (ELFN).
Bottom: an example model of a chop-subtraction persistence residual (CSPR) of
$\alpha$ Centauri AB. This model was generated with an offset of $\sim$2% of a
pixel, or $\sim$1 mas, of the star’s position behind the coronagraph following
each chop. The effects of the coronagraph, persistence non-linearity and
temporal decay, and post-processing are not included in the model. The CSPR is
too faint to be seen in the frames above, but is easily seen in longer
integrations (see Fig. 2).

However, chopping introduces other challenges. For coronagraphic observations,
an obvious challenge is to maintain a precise positioning behind the mask
following each chop cycle.222Centering is a particular problem of focal-plane
coronagraphs. The newer pupil-plane apodized phase plate coronagraphs [26, 27]
might be particularly well suited for chopping because they are tip/tilt
invariant. NEAR utilized the Quadrant Analysis of Coronagraphic Images for
Tip-tilt Sensing (QACITS) algorithm [28, 29], which results in $\sim$4 mas
centering residuals ($\sim$0.1 pixels). This is also similar to the limit of
precision of the DSM to re-position the star (with or without a coronagraph).
Therefore, successive chops do not follow exactly the same trajectory. While
these errors are randomly distributed and relatively small compared to the
beam diameter, they introduce a major limitation to the current approach. This
is due to a separate systematic of the AQUARIUS detectors$-$a persistence
effect that becomes more pronounced for bright stars (the optimal targets) and
that occurs even when the detector is not actively recording an exposure.
This, combined with the random fractional-pixel displacements of the star with
each chop cycle, results in a residual along the chopping axis that changes in
form and intensity with each exposure pair. We refer to this feature as the
chop-subtraction persistence residual, or CSPR (see Fig. 1).

Given that typical exposure times of $\lesssim$0.1 sec are required to avoid
saturation of the detector (due to thermal background) and to ensure
correlated ELFN states, the CSPR becomes intractable to model and subtract in
post-processing in a frame-by-frame manner. Furthermore, with NEAR’s binary-
chopping strategy, in which $\alpha$ Cen A and B are alternated in position
behind the coronagraph, the persistence pattern rotates with the field of view
and is not removed by angular differential imaging (ADI: 30). Ultimately, this
limited the sensitivity of the NEAR data and the regions most impacted by this
feature were excluded from analysis (see Fig. 2, reproduced from 16). This
leads to two useful lessons for mid-IR exoplanet imaging with similar
detectors: 1) improved chopping precision will lead to a less significant CSPR
feature; and 2) chopping along a consistent path in the pupil-stabilized image
will also enable the remaining CSPR to be removed in ADI-based post-
processing.

In addition to serving as a pathfinder experiment to explore the major
challenges to mid-IR exoplanet imaging, the NEAR campaign succeeded in
establishing the first sensitivity to warm sub-Neptune-sized planets within
the habitable zone of $\alpha$ Centauri A [15, 16]. This is a siginficant
advancement for exoplanet imaging, which has been limited to studying planets
of super-Jovian masses and on orbits of $\gtrsim$10 au. The sensitivity
demonstrated by NEAR is also approaching levels sufficient to image rocky
planets (R$\lesssim$1.7 R⊕) in the habitable zones of nearby stars$-$i.e.,
worlds that might support life. Now, a path is clear to implement the lessons
brought by NEAR in existing and future mid-IR instruments to ultimately enable
us to achieve the goal of finding a second Earth.

---
Figure 2:  Results of the VLT/NEAR experiment on $\alpha$ Centauri, reproduced
from [16]. Left: Contrast vs. angular separation of injected point sources
recovered with a signal to noise ratio (SNR) $\sim$ 3 (black curve) and model
planetary contrasts for hot and cold planets (red and blue curves). Right:
image of the habitable zone of $\alpha$ Centauri, showing the CSPR feature and
also a candidate signal around $\alpha$ Centauri A, labeled as C1 (see [16]
for more details).

##  3 Building on the Lessons of NEAR with LBTI/NOMIC

---
Figure 3:  Pixel-to-pixel noise as a function of the chopping half-cycle
frequency with LBTI/NOMIC. Similar to results from the NEAR experiment with
VLT/VISIR, we find a rapidly decreasing level of noise with increasing
chopping frequency up to 2.3 Hz. The 5$-$10% increase in noise between 2.3 Hz
and 4.55 Hz is a feature not seen previously with VISIR, but that has been
seen repeatedly in multiple LBTI/NOMIC datasets.

In the Northern hemisphere, telescopes such as the LBT in Arizona, those
situated on Mauna Kea, and several other facilities present an opportunity for
continued progress to improve mid-IR exoplanet imaging instrumentation. The
LBT’s location on the high-altitude inland peak of Mount Graham experiences
average winter temperatures that are 15$-$20∘ C colder than the coastal sites
of other large telescopes, while providing similar precipitable water vapor
conditions [31]. This results in a thermal background that is $\sim$25$-$30%
lower than other sites. Furthermore, with its dual apertures, the LBT offers
an additional improvement of a factor of at least $\sqrt{2}$ for a given
amount of time (a larger factor can be gained by Fizeau imaging, see below).
Most of the necessary exoplanet imaging hardware exists within the framework
of the Large Binocular Telescope Interferometer (LBTI: 32, 33), as was
recently demonstrated by [34, 35, 36]. LBTI’s bent-Gregorian position directly
on the telescope mount is fixed with respect to the telescope pupil, which
also leads to a relatively stable thermal background. Each side of the
telescope contains a low-background adaptive secondary mirror (ASM) that can
be simultaneously imaged with NOMIC [23], whose AQUARIUS detector is nearly
identical to that within VISIR.

LBTI is a focal plane interferometer that operates by overlapping the
individual point spread functions of each 8.4-m mirror of the LBT on a single
detector. 23-m nulling and Fizeau imaging is achieved using cryogenic
pathlength correctors (pupil-mirrors) operating at kHz frequencies to correct
tip, tilt, and piston abberations in order to keep the beams overlapped and
coherent [37, 38, 39, 33]. LBTI can also be used in a classical (incoherent)
imaging mode, in which the beams are displaced to form two images of a single
star on the detector [e.g., 40, 34, 35, 36]. We have developed a method using
LBTI’s pupil-mirrors for chopping when observing in an incoherent mode.
Chopping with the pupil-mirrors does not introduce additional background nor
does it impact adaptive optics operations. The control loop required for
chopping is also much slower than the $\sim$kHz rates used for interferometry.
Chopping with the pupil-mirrors takes place in the pupil-stabilized
frame$-$enabling the remaining CSPR to be removed with ADI-based post-
processing. As of mid-2021 we have commissioned up to $\sim$4.5 Hz chopping
with the pupil-mirrors, with faster rates being limited by NOMIC’s readout
capabilities. Similar to NEAR, faster chopping (up to rates of $\sim$2 Hz)
results in exponentially lower background noise (Fig. 3). However, unlike
NEAR, most on-sky LBTI/NOMIC chopping datasets show an optimal chopping rate
around $\sim$2 Hz, with the single higher rate showing a $\sim$10% increase in
noise. The reason for this is not yet understood, but is possibly related to
the smaller number of coadded (0.04 second) frames per half-cycle position.

---
Figure 4:  An example of a 512x512 chop-subtracted image pair of Sirius from
NOMIC. A CSPR feature similar to that in the NEAR data is easily identified
adjacent to each PSF along the chopping axis. This artifact is stable with the
telescope pupil and is subtracted with ADI-based image processing. Once
double-sided operations resume at the LBT, the bottom half of the image plane
will be used to simultaneously observe with the second aperture.

The LBT offers another avenue for improvement with its dual 8.4-m apertures.
Unfortunately, its most sensitive imaging mode (in principle), Fizeau
interferometry, cannot currently be combined with chopping due to the fringe-
tracker’s narrow field of view, and in this case the ELFN quickly precludes
any gain in sensitivity. Overlapping the two beams incoherently results in an
additional $\sqrt{2}$ gain in sensitivity [41], but loses discriminating power
in the independent speckle patterns. By installing new cold field-stops, we
plan to enable background-limited imaging of the two beams simultaneously and
separately on the detector (see Fig. 4). This will enable working at smaller
separations via identification of true sources within the independent speckle
patterns (speckle-limited imaging). The cold field-stops have been designed
and fabricated specifically for this project and will be installed over Summer
2021. Once both adaptive secondary mirrors are operational (planned for late
2021), double-sided observations will enable imaging habitable-zone sub-
Neptune-sized planets in as little as $\sim$6 nights.

---
Figure 5:  Left: masses vs. orbital semi-major axes of known exoplanets from
all detection methods. The planets detectable with mid-IR high-contrast
imaging in less than six nights with a single 8.4-m telescope are shown in
blue$-$representing a distinct and complementary discovery space to currently
imaged super-Jupiters. Right: an example of a simulated Saturn-sized planet in
the habitable zone of Sirius. Such planets would be readily detectable in
1$-$2 nights of observations.

##  4 Exoplanet Science With High-Contrast Mid-IR Imaging

The primary scientific benefit of mid-IR exoplanet imaging (including exo-
Earth imaging capabilities: 16, 42, 43) will come with the next generation of
extremely large telescopes (ELTs). However, it is useful to briefly consider
the science that will be enabled by the current generation of experimental
instruments on 8-m telescopes. With high-contrast mid-IR imaging capabilities
now in both hemispheres, and others in preparation [e.g., 44, 45], it will be
possible to conduct a complete survey for habitable-zone giant planets around
nearby stars. While reflected light imaging [e.g., 46] will also enable the
study of many of these same planets, its combination with mid-IR imaging is
needed to break the degeneracy between radius and albedo. Indeed, such
observations have already started to place limits on the properties of planets
within a handful of systems [e.g., 47, 48, 49, 16].

The frequency of directly imaged giant planets is increasing toward smaller
masses and smaller separations [50, 51, 52]. Furthermore, the peak in the
occurrence of giant planets exists at $\lesssim$10 au [53, 54]. Therefore, the
probability of discovering a temperate giant planet around one of the nearby
stars is relatively high. To quantify the discovery space of a complete mid-IR
exoplanet imaging survey of nearby stars (those within $\sim$15 pc), we
simulated the $N$-band ($\sim$11 $\mu$m) contrast of habitable-zone giant
planets following [16] and assuming planetary temperatures determined by
radiative thermal equilibrium. This resulted in a list of ten stars around
which Jupiter$-$ and Saturn-sized planets between 1$-$5 au will result in a
$\geq$5-$\sigma$ detection in a single night of observations. Current
population estimates suggest that $\sim$10$-$40% of stars host a 0.3$-$13
Jupiter-mass planet between 1$-$5 au [53], which translates into an expected
population of $\sim$1$-$4 nearby habitable-zone giant planets.333While all of
these would be detectable over multiple epochs, a single observation may miss
some highly-inclined planets that are in close projected separation to their
stars.

Since the sensitivity scales roughly with $\sqrt{t}$, it will be possible to
push even deeper by observing each target for multiple nights and with
multiple instruments. Based on commissioning data, six nights of observations
with both sides of the LBTI will be sufficient for a 3-$\sigma$ detection of a
sub-Neptune-sized planet at $\sim$2$-$3 au around Sirius. Such worlds would
occupy a new range of parameter space for imaged exoplanets (see Fig. 5). As
no sub-Jovian exoplanets have been directly imaged444Transit and high-
resolution spectroscopy, however, can be an effective means for studying
close-in planets., this would open up a new class of planets for detailed
atmospheric and orbital characterization. This is the motivation behind our
on-going LBTI Exploratory Survey for Super-Earths Orbitng Nearby Stars
(LESSONS). These observations will also demonstrate the ability to combine
data from multiple nights and will test new data reduction methods for
handling orbital motion [e.g., 55, 56]. Finally, the upcoming ELTs will be
able to efficiently image super-Earths at radiative equilibrium temperatures
around several nearby stars, and possibly Earth-sized planets within the
closest system, $\alpha$ Centauri [42, 43, 16]. NEAR and LESSONS will serve as
pathfinders for these groundbreaking instruments [e.g., 57, 42].

##  5 Summary

We have reviewed the current status of mid-IR ($\sim$10 $\mu$m) exoplanet
imaging instrumentation and presented results on recent progress to expand
these capabilities. The most sensitive observations to date were taken by the
NEAR campaign, which established sensitivity to warm Neptune-sized planets
around $\alpha$ Centauri A. As a first of its kind long-stare mid-IR campaign
for exoplanet imaging, NEAR also revealed a number of promising avenues to
improve future instrumentation. The main results of NEAR can be found in [14,
15, 16]. Here, we focused on a primary challenge to NEAR’s strategy:
persistence combining with chop-subtraction to create a complicated systematic
artifact. We discussed the properties of this feature and how it can be
mitigated in the future. We introduced a new experiment, LESSONS, through
which we aim to build on the results of NEAR to continue pushing sensitivity
toward imaging low-mass planets. Finally, we discussed how mid-IR exoplanet
imaging will enable new science. It is now possible to discover and
characterize exoplanets that are roughly an order of magnitude lower in mass
than those that have so far been directly imaged. These planets will also be
at orbital separations that are much closer-in than currently imaged
exoplanets$-$enabling new studies across a broad range of physical parameters.
Ultimately, the sample capable of being targeted with current telescopes is
limited by the number of bright nearby stars. In the future, the ELTs will
enable expanding the list of target stars and will enable imaging potentially
Earth-like planets within a handful of nearby systems.

###### Acknowledgements.

NEAR was made possible by contributions from the Breakthrough Watch program,
as well as contributions from the European Southern Observatory, including
director’s discretionary time. Breakthrough Watch is managed by the
Breakthrough Initiatives, sponsored by the Breakthrough Prize Foundation. The
results reported herein benefited from collaborations and/or information
exchange within NASA’s Nexus for Exoplanet System Science (NExSS) research
coordination network sponsored by NASA’s Science Mission Directorate. K.W.
acknowledges support from NASA through the NASA Hubble Fellowship grant HST-
HF2-51472.001-A awarded by the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in Astronomy,
Incorporated, under NASA contract NAS5-26555.

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