L 9859 d is a SuperEarth planet orbiting an Mtype star. We performed retrievals on the transmission spectrum of L 9859 d obtained using NIRSpec G395H during a single transit from JWST Cycle 1 GTO 1224. The wavelength range of this spectrum allows us to detect the presence of several atmospheric species. We found that the spectrum is consistent with a high mean molecular weight atmosphere. The atmospheric spectrum indicates the possible presence of the sulfurbearing species HSUB2SUBS and SOSUB2SUB which could hint at active volcanism on this planet if verified by future observations. We also tested for signs of stellar contamination in the spectrum and found signs of unocculted faculae on the star. The tentative signs of an atmosphere on L 9859 d presented in this work from just one transit bodes well for possible molecular detections in the future particularly as it is one of the best targets among small exoplanets for atmospheric characterization using JWST.
['Exoplanet atmospheres', 'Super Earths', 'Transmission spectroscopy', '487', '1655', '2133', 'Astrophysics - Earth and Planetary Astrophysics']
Atmospheric Retrievals Suggest the Presence of a Secondary Atmosphere and Possible Sulfur Species on L9859 d from JWST Nirspec G395H Transmission Spectroscopy
{'No Header': "4  \nAtmospheric retrievals suggest the presence of a secondary atmosphere and possible sulfur species on L98-59d from JWST NIRSpec G395H transmission spectroscopy  \nAgnibha Banerjee , 1, ∗ Joanna K. Barstow , 1 Am'elie Gressier , 2 N'estor Espinoza , 2 David K. Sing , 3 Natalie H. Allen , 3 Stephan M. Birkmann , 4 Ryan C. Challener , 5 Nicolas Crouzet , 6 Carole A. Haswell , 1 Nikole K. Lewis , 7 Stephen R. Lewis , 1 and Jingxuan Yang 8  \n1 School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK  \n2  \nSpace Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 3 William H. Miller III Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA European Space Agency, European Space Astronomy Centre, Camino Bajo del Castillo s/n, E-28692 Villanueva de la Ca˜nada, Madrid, Spain  \n5 Department of Astronomy, Cornell University, 122 Sciences Drive, Ithaca, NY 14853, USA  \n6 Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands  \n7 Department of Astronomy and Carl Sagan Institute, Cornell University, 122 Sciences Drive, Ithaca, NY 14853, USA  \n8 Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Oxford OX1 3PU, UK", 'ABSTRACT': 'L98-59d is a Super-Earth planet orbiting an M-type star. We performed retrievals on the transmission spectrum of L 98-59 d obtained using NIRSpec G395H during a single transit, from JWST Cycle 1 GTO 1224. The wavelength range of this spectrum allows us to detect the presence of several atmospheric species. We found that the spectrum is consistent with a high mean molecular weight atmosphere. The atmospheric spectrum indicates the possible presence of the sulfur-bearing species H 2 S and SO 2 , which could hint at active volcanism on this planet if verified by future observations. We also tested for signs of stellar contamination in the spectrum, and found signs of unocculted faculae on the star. The tentative signs of an atmosphere on L 98-59 d presented in this work from just one transit bodes well for possible molecular detections in the future, particularly as it is one of the best targets among small exoplanets for atmospheric characterization using JWST.', '1. INTRODUCTION': "The era of detection and characterization of atmospheres around rocky exoplanets is now here. LustigYaeger et al. (2023); Lim et al. (2023); Moran et al. (2023); May et al. (2023); Kirk et al. (2024); Damiano et al. (2024) have already used the immense capabilities of JWST to measure the transmission spectra of the atmospheres of rocky exoplanets. However, statistically significant detectable absorption features have been scarce - owing to the low atmospheric scale heights of rocky planets, possible cloud cover, and stellar contamination from the M-dwarf host stars (Moran et al. 2023).  \nHere, we conducted atmospheric retrievals on the transmission spectrum of L 98-59 d (Kostov et al. 2019; Cloutier et al. 2019). The L98-59 system consists of two more rocky transiting planets and another nontransiting planet (Demangeon et al. 2021). Previous  \nattempts to measure the transmission spectra between 1.1 - 1.7 µ m of planet b (Damiano et al. 2022), planet c (Barclay et al. 2023), and planet d (Zhou et al. 2023) using HST WFC3 have not ruled out the possibility of high mean molecular weight or cloudy atmospheres. Zhou et al. (2023) ruled out cloud-free Hydrogen and Helium atmospheres for planets c and d. However, they could not exclude the possibility of primary cloudy/hazy or water-rich atmospheres.  \nL98-59d is an ideal target for atmospheric studies due to its high Transmission Spectroscopy Metric (Kempton et al. 2018) value. It may also sustain volcanism on its surface (Seligman et al. 2024), providing a viable source for a secondary atmosphere or a mixed primary and secondary, or hybrid (Tian & Heng 2024) atmosphere.  \nL98-59d, with a radius of 1.52 R ⊕ , a mass of 1.94 M ⊕ , and an equilibrium temperature of 416 K is inconsistent with a pure rocky (Earth-like) composition and is potentially too small to be explained by H-He gas accretion from the protoplanetary disk (Luque & Pall'e 2022). All three planets in the L 98-59 system receive  \nsubstantial X-ray and extreme-ultraviolet flux, leading to rapid water loss that significantly affects their developing climates and atmospheres (Fromont et al. 2024). While there is no solid claim of an atmosphere on L 9859 c, planet d is closer to the cosmic shoreline and more likely to possess an atmosphere. L 98-59 d's density categorizes it as a 'Water-world' according to Luque & Pall'e (2022), suggesting a composition of rock and water ice in roughly equal proportions by mass. Seligman et al. (2024) state that it may be possible to detect at atmosphere on L98-59d with 3-5 transits using NIRSpec/G395H, that could contain some amount of SO 2 .  \nWe analyzed a 2.9 - 5.2 µ m JWST transmission spectrum obtained using NIRSpec (Birkmann et al. 2022; Ferruit et al. 2022) G395H. We performed several sets of atmospheric retrievals with varying assumptions on this spectrum to infer the atmospheric composition of L98-59d. An atmospheric retrieval solves the inverse problem of going from a measured spectrum to the range of atmospheric properties consistent with the observations. This is done by comparing the observed spectrum with large numbers of generated spectra to explore the parameter space and find the best fitting combinations of model atmosphere properties (Barstow et al. 2017; Tsiaras et al. 2018; Grant et al. 2023; Beatty et al. 2024; Benneke et al. 2024; Holmberg & Madhusudhan 2024; Hu et al. 2024).  \nWe used the transmission spectrum obtained for the JWST GTO 1224 program (PI: Birkmann) using NIRSpec G395H for 1 transit of L98-59d on 25 June 2023. The observations were reduced using two different pipelines transitspectroscopy (Espinoza 2022) and FIREFLy (Rustamkulov et al. 2022, 2023), to obtain transmission spectra by binning the transit depths to a resolution of R ∼ 100. The reduction methodology is discussed in detail in an accompanying paper from the GTO 1224 collaboration (Gressier et al. 2024), hereafter Paper I. The transitspectroscopy reduction is used for the majority of the retrievals in this letter. For completeness, we also ran a retrieval on the FIREFly reduction to explore the influence of data reduction on the retrieved parameters.  \nIn Section 2 we describe the retrieval setup and the priors used for each parameter. In Section 3 we define the atmospheric models used for retrievals and discuss the results from the suite of retrievals that we have performed. In Section 4 we consider the significance of the presence of sulfur species in this atmosphere, compare results from the two independent data reductions presented in Paper I, and discuss the potential for future observations.", '2.1. Retrieval Setup with NEMESISPY': 'NEMESIS (Non-linear optimal Estimator for Multivariate spectral analySIS) (Irwin et al. 2008) is a versatile retrieval tool widely employed in the study of planetary atmospheres, both within our Solar System and beyond (Barstow et al. 2016; Irwin et al. 2018; Braude et al. 2020; Barstow 2020; Irwin et al. 2020, 2021; Sing et al. 2024). It utilizes a fast correlated-k method to solve the radiative transfer equation involving multiple absorbing gases. In this work, we used the Python adaptation of the Fortran-based NEMESIS, NEMESISPY (Yang et al. 2023, 2024b,a).  \nWe employed the PyMultiNest nested sampling solver (Buchner et al. 2014; Feroz & Hobson 2008; Feroz et al. 2009, 2019) to explore potential solutions. Nested Sampling (Skilling 2004, 2006) is a computational method for estimating the marginal likelihood of a model and performing Bayesian parameter estimation. It sequentially samples the prior distribution by generating live points, and gradually fills the parameter space with points of increasing likelihood. In our study, we configured it with 2000 live points and an evidence tolerance value of 0.5.  \nFor the gas opacities, we used k tables (R=1000) from ExoMol (Tennyson et al. 2016; Chubb et al. 2021) encompassing the following molecular species: H 2 O (Polyansky et al. 2018), CO 2 (Yurchenko et al. 2020), CO(Li et al. 2015), NH 3 (Coles et al. 2019), PH 3 (SousaSilva et al. 2014), CH 4 (Yurchenko et al. 2017), SO 2 (Underwood et al. 2016), and H 2 S (Azzam et al. 2016). These k tables are then channel-averaged to match the resolution of the data prior to the retrieval process (Irwin et al. 2020). We evaluate Rayleigh scattering cross sections using data from Allen (1976). Additionally, we incorporate collision-induced continuum absorption arising from H 2 -H 2 (Borysow & Frommhold 1989; Borysow et al. 1989, 2001; Borysow 2002; Rothman et al. 2013), and N 2 -N 2 (Lafferty et al. 1996).', '2.2. Atmospheric Model': "In our setup, the atmosphere was divided into 100 equal intervals in log-pressure, starting from 10 1 atm and extending to 10 -7 atm. To remain agnostic of the atmosphere's background composition, we used Centered Log-Ratio (CLR) (Benneke & Seager 2012a) priors for the gas abundances. The CLR parameterization treats all chemical species in the model equally, enabling any of them to be the dominant atmospheric species. This allows for a flexible exploration of possible background compositions. The CLR parameterization is a new addition to NEMESISPY , implemented in a manner similar to POSEIDON (MacDonald & Madhusudhan 2017;  \nMacDonald 2023). Our implementation of CLR priors is non-uniform in the CLR space, and thus avoids biases in results as described in Damiano & Hu (2021). In the log volume mixing ratio or log(VMR) space, this manifests as a distribution peaked in the high values, and a flat tail towards the low values. We use 10 -12 as the lower limit for the volume mixing ratios.  \nClouds were represented using an opaque cloud top ( P top ), with the prior for its logarithm, log( P top ) defined as U ( -7 , 1) atm, and power-law scattering due to hazes above the cloud layer (MacDonald & Madhusudhan 2017), with the prior for power defined as U ( -8 , 4), where U ( a, b ) is a uniform distribution with a and b as the upper and lower bounds. We also include a Rayleigh enhancement parameter named Hazemult, with priors defined as U ( -5 , 5). We used an isothermal Temperature-Pressure Profile, unless otherwise specified. The prior for the isothermal temperature was defined as U (311 , 625) K. This choice is further explained in Section 2.3. The ratio of the radius of the planet at 10 atm to the white-light transit radius of the planet was left as a free parameter ( f ref ) with the prior set as U (0 . 7 , 1 . 3). Following Alderson et al. (2023) and May et al. (2023), we allowed for an offset between the NRS1 and NRS2 wavelength ranges. The prior distribution for this offset in ppm is set as N (0 , 40), where N ( µ, σ ) is a normal distribution with a mean of µ and standard deviation of σ . The values of stellar effective temperature, stellar radius, and metallicity used were 3415 K, 0.303 R ⊙ , and -0.46 respectively (Demangeon et al. 2021).  \nWe represented stellar inhomogeneities using three parameters: fractional coverage f het , the difference between photospheric temperature and heterogeneity temperature ∆ T het , and photospheric temperature T phot . We followed the prescription used in POSEIDON (MacDonald & Madhusudhan 2017; Rathcke et al. 2021; MacDonald 2023), by using pysynphot (STScI Development Team 2013) to sample a grid of PHOENIX (Husser et al. 2013) models, and representing unocculted spots/faculae using stellar models with the same metallicity and log(g) values but a cooler/hotter temperature covering a fraction of the star. 1 The temperature of the heterogeneity is then defined as the sum of ∆ T het and T phot . This is a simplified representation of stellar inhomogeneities and does not consider time-variable stellar activity. The priors for T phot , ∆ T het , and f het are de-  \nas N (3415 , 150) K, U ( -500 , 1000) K, and U (0 , 0 . 5) respectively.", '2.3. Constraining the Temperature': 'The magnitude of the features in a transmission spectrum is proportional to the vertical extent, and therefore the scale height, of the atmosphere. The scale height is directly proportional to atmospheric temperature and inversely proportional to the mean molecular weight of the atmosphere - creating a degeneracy between temperature and mean molecular weight. One way to mitigate this degeneracy is to specify physically motivated prior bounds on the temperature. Thus, we restricted the temperature priors to within 100K of the theoretical limits of day-side temperature ( T d ) of a planet considering both extremes of circulation, 411K to 525K from Cowan & Agol (2011), giving a prior bound of 311K to 625K.  \nT d = T 0 (1 -A B ) 1 / 4 ( 2 3 -5 12 ε ) 1 / 4 (1)  \nwhere ε denotes the circulation efficiency from 0 to 1, A B is the planetary albedo, and T 0 is  \nT 0 = T ∗ ( R ∗ a ) 1 / 2 (2)  \nwhere T ∗ is the stellar effective temperature, R ∗ is the stellar radius, and a is the star-planet distance. The day-side temperature is used as a conservative estimate, as the terminator temperature that we are sensitive to in transmission spectroscopy should be lower than that.', '3.1. Main Retrievals': "As transmission spectra of planets around M-dwarfs have been plagued by stellar contamination (Lim et al. 2023; Moran et al. 2023), for our main retrieval, we included stellar contamination. As no evidence of spot crossings was seen in the light curves, here we only modeled unocculted inhomogeneites. As as additional test, we checked whether a scenario with a planetary atmosphere but no stellar inhomogeneities can reproduce the spectrum. For this case, the priors for the parameters related to the planets atmosphere were kept the same, but the stellar parameters were removed.  \nWe verified if a bare rock planet can still produce this spectrum if stellar inhomogeneities are present. For this scenario, we modeled the planet's contribution using only two parameters: the reference radius and the offset between NRS1 and NRS2. The priors for the stellar contribution parameters were left unchanged. We  \nalso tried a similar scenario with the planet's contribution only, essentially checking if a flat line with an offset between NRS1 and NRS2 can explain the spectrum. For each of our retrievals, we calculated a Bayesian logevidence value and compared it to this case. The values of sigma are computed using the difference in log evidence (Trotta 2008; Benneke & Seager 2012b).", '3.2. Results from Main Retrievals': "The main retrieval with stellar inhomogeneities and a planetary atmosphere favors an atmosphere with high H 2 S and SO 2 abundances, with log(VMR) of -0 . 62 +0 . 61 -6 . 73 and -2 . 35 +2 . 05 -5 . 88 respectively. It also suggests the presence of unocculted stellar faculae, as ∆ T het is positive with a retrieved value of 324 +295 -187 K. The results from all of the retrievals are listed in Table 1.  \nThe retrieval with only stellar inhomogeneities failed to provide a good fit to the observed spectrum, but resulted in similar posteriors for the stellar parameters. It is worth noting that this scenario fails to reproduce the feature at about 4 µ m which can be attributed to SO 2 . This confirms the results of Paper I that stellar activity alone cannot reproduce the observed spectrum. The scenario with the planet's contribution only, the flat line with an offset, provided a much worse fit to the spectrum - resulting in a reduced chi-squared value of only 1.86.  \nThe fit with stellar inhomogeneities and an atmosphere was preferred at 2.24 σ to the fit with only stellar inhomogeneities, and at 3.50 σ to the fit with only a flat line and offset. The fit with only a planetary atmosphere also favors an atmosphere with high abundances of H 2 S and SO 2 .  \nThe individual detection significances of H 2 S and SO 2 , calculated by removing each gas in turn from the main retrieval with stellar inhomogeneities and planetary atmosphere are about 2 σ . Thus, these can only be interpreted as weak detections.", '3.3. Alternate Retrieval Setups': 'In addition to the main retrievals, we also considered some alternate retrieval setups to test whether our choice of parameterization affects the inferences. The representation and priors for all parameters are as described in Section 2.2, unless otherwise stated.', '3.3.1. Equilibrium Chemistry Retrieval': 'We performed a retrieval enforcing equilibrium chemistry using NEMESISPY (Yang et al. 2023, 2024b,a), coupled with the FastChem (Stock et al. 2018, 2022) chemical solver. We parameterized elemental abundances using three parameters: Metallicity (Z/Z ⊙ ), Carbon to  \nOxygen ratio (C/O), and Sulfur to Oxygen ratio (S/O). The prior distributions for these parameters were set as U (0 . 1 , 1000), U (0 . 1 , 2 . 0), U (0 . 1 , 2 . 0) respectively. This retrieval struggled to produce a good fit to the data, and was not preferred to the baseline model of having only stellar inhomogeneities. This indicates that the atmosphere is probably not in equilibrium. The retrieved model and selected posterior distributions for some parameters are shown in Figure 4.', '3.3.2. No H 2 S/SO 2': 'In an alternate version of the main retrieval, we removed H 2 S and SO 2 from the active gases. This retrieval also struggled to produce a good fit to the data, and was not preferred to the baseline model having only stellar inhomogeneities. This result is further evidence to the tentative presence of these sulfur species in the atmosphere. The retrieved model and selected posterior distributions for some parameters are shown in Figure 4.', '3.3.3. Nitrogen Background': 'In another alternate version of the main retrieval, we assumed that the planet has a nitrogen dominated atmosphere as nitrogen is another gas that only has continuum features in this wavelength range. Thus, we fixed the background to N 2 , and used log-uniform priors for the trace gases, with priors for log(VMR) set to U ( -12 , -0 . 1). This also resulted in significant H 2 S and SO 2 abundances. This indicates that the choice of CLR priors does not bias our inferences of the atmosphere.', '3.3.4. TP Retrieval': 'In this alternate version of the main retrieval, we tested the application of a non-isothermal TemperaturePressure profile using the parameterization of Madhusudhan & Seager (2009). We found that this made a negligible difference to the quality of the fit, and the retrieved profile was close to isothermal, indicating that a non-isothermal profile is not statistically favored for this spectrum. This is probably as the spectrum probes a relatively narrow pressure range. This retrieval also resulted in high H 2 S and SO 2 abundances.  \nWehave also performed retrievals in which the isothermal temperature was allowed to vary between 100 K and 1000 K. In these, the retrieved temperatures were either much higher than the calculated bounds, when the atmospheric mean molecular weight was high, or much lower, for a low mean molecular weight atmosphere. The actual solution could possibly lie in between these extremes, and without informative prior bounds, the molecular weight and temperature are highly degenerate. These retrievals also recovered high H 2 S and SO 2  \nFigure 1. The retrieved spectra for the main retrievals: Stellar + Atmosphere scenario is plotted in red, Atmosphere Only in blue, and Stellar Only in green. The 1 σ and 2 σ credible intervals are plotted in dark and light shades of the corresponding colors respectively. The NRS2 points are shifted by the retrieved offset, and the unshifted NRS2 points are shown in gray. Underneath the spectrum, in the first row from left to right, the retrieved posterior distributions for log(H 2 S), log(SO 2 ), and Temperature are shown. In the second row, from left to right, the retrieved posterior distributions for f het , T phot , and ∆ T het are shown.  \n<!-- image -->  \nabundances. Thus, this degeneracy does not affect the possible presence of sulfur species, which is discussed in Section 4.1.', '4.1. Possible Presence of Sulfur Species': "The primary contributors to the retrieved spectrum across all the retrieval setups we tested are invariably H 2 S and SO 2 , which are both prominent sulfur species. To test their significance, we removed them from the  \nlist of active species and performed another retrieval, otherwise identical to the main retrieval as described in Section 3.3.2. This produced a much worse fit to the spectrum than the model with the sulfur species. By comparing the log evidences for these two scenarios, we obtain a combined detection significance of 2.32 σ for H 2 S and SO 2 .  \nPrevious studies have suggested that L 98-59 d is a planet which is being tidally heated (Seligman et al. 2024). It has a non-zero eccentricity of 0 . 074 +0 . 057 -0 . 046 (Demangeon et al. 2021) and the planets b, c and d have orbital resonances close to 2:4:7 (Seligman et al. 2024).  \nTable 1. Priors and retrieved values of atmospheric parameters from a selection of retrievals  \nNote -Only 2 σ upper bounds are specified for the gas abundances that are not constrained. The detection significance σ is computed by comparing the Bayesian Log Evidence of each model with the Stellar Only case for each reduction. For the FIREFLy retrieval, the baseline Stellar Only ln(Z) is 470.80.  \nTidal stresses could lead to volcanic outgassing, similar to the volcanic activity on Jupiter's moon Io, and could be a possible source of H 2 S and SO 2 in the atmosphere. Recent modeling studies have also suggested volcanic or outgassed origins of H 2 S and SO 2 (Claringbold et al. 2023; Tsai et al. 2024) in exoplanet atmospheres.  \nWe find that the abundances of common atmospheric species such as H 2 O and CO 2 are unconstrained with long tails in the posterior distribution, and we only report upper bounds for their abundances. The possible presence of H 2 S also supports the absence of CO 2 and H 2 O, as H 2 S features in the spectrum would not be visible if they were present (Janssen et al. 2023). As H 2 O is required in the photochemical production of SO 2 , as shown in Tsai et al. (2023), photochemistry is unlikely to be the source of SO 2 in the atmosphere of L 98-59 d. While the possible presence of H 2 S and SO 2 in the absence of H 2 O and CO 2 is unlikely in equilibrium conditions (Janssen et al. 2023), they might exist on an intensely volcanic planet.  \nWe also do not detect the presence of any significant clouds or haze scattering. If H 2 S and SO 2 are present, Jordan et al. (2021) found that these should survive above the cloud layer in the atmospheres of planets around M-dwarfs, where they could be detectable.", '4.2. Comparison between Reductions': 'L98-59d has a high impact parameter (Demangeon et al. 2021) and the transit chord crosses near to the limb of the host star from our perspective, thus making it difficult to pin down the limb darkening coefficients Paper I.  \nTo check the effect of data reduction methods on our retrievals, we performed all the same retrievals on another reduction of the spectrum using FIREFLy . Here, we present a retrieval identical to our main retrieval, using this reduction instead. While there are some differences in the retrieved parameters, the high H 2 S and SO 2 abundances are still present - with log(VMR) of -4 . 07 +3 . 93 -5 . 11 and -2 . 04 +1 . 94 -6 . 03 respectively. We performed  \na fit with a baseline stellar inhomogeneities only for this reduction as well. This atmospheric model is preferred at 1.67 σ to the equivalent baseline fit. The retrieved model and selected posterior distributions are shown in Figure 5.', '4.3. Comparison between Retrieval codes': 'Previous studies have compared the results obtained using different retrieval codes for the same spectrum (Barstow et al. 2020; Taylor et al. 2023). Subtle differences in the forward models used to perform retrievals can lead to significant differences in retrieved parameters. To test if our inferences are being impacted by the choice of retrieval codes, we performed a retrieval with a similar setup as our main retrieval using Taurex3 (AlRefaie et al. 2021). This retrieval, presented in Paper I, also produced similarly high abundances of H 2 S and SO 2 .', '4.4. Future Observations': 'Figure 2 shows the best fit model extended to between 0.6 - 12 µ m. This shows H 2 S and SO 2 features in other regions of the spectrum, accessible to JWST instruments NIRISS, NIRSpec and MIRI. Observations in the 0.6 5.0 µ m range can possibly confirm or refute the signatures of these sulfur species. An accepted JWST proposal in Cycle 1: GTO 1201 (PI: Lafreniere) includes 1 transit of L 98-59 d with NIRISS SOSS. An accepted JWST proposal in Cycle 2: GO 4098 (PI: Benneke) also includes 1 transit of L 98-59 d with NIRSpec G395H and 1 transit with NIRISS SOSS. Apart from these, MIRI observations can particularly help pin down the large SO 2 features in the 7-10 µ m range.', '5. CONCLUSION': 'We have presented a range of retrieval analyses for the NIRSpec/G395H spectrum of L98-59d. Our analyses favor an atmosphere with substantial amounts of sulfur species H 2 S and SO 2 , and an atmospheric temperature higher than the equilibrium temperature. We  \nalso find evidence of unocculted faculae on the star. The rocky planets in the Solar System all have different atmospheric compositions, and the study of such atmospheres in exoplanetary systems could unlock a rich diversity of unexplored chemistries.  \nOur retrievals do not constrain the abundances of other spectrally active gases such as H 2 O and CO 2 , and we only report upper bounds. The bare rock with stellar inhomogeneities scenario struggles to reproduce the observed spectral features. Several H 2 S and SO 2 features exist in other wavelength regions covered by JWST, as shown in Figure 2. The two scenarios of stellar inhomogeneity with an atmosphere and only stellar inhomogeneity also differ in other wavelength regions. Follow up observations in these wavelengths can confirm or refute the evidence of H 2 S and SO 2 , and distinguish between a planetary atmosphere and stellar inhomogeneities.', '6. ACKNOWLEDGEMENTS': 'AB is supported by a PhD studentship funded by STFC and The Open University. AB thanks Dr Ryan J. MacDonald for discussions on the implementation of the CLR priors used in this work. JKB is supported by UKRI via an STFC Ernest Rutherford Fellowship (ST/T004479/1). CAH is supported by grants ST/T000295/1 and ST/X001164/1 from STFC. SRL thanks STFC for funding under grant ST/X001180/1 and UKSA under grant ST/W002949/1. The JWST data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute. The specific observations analyzed can be accessed via 10.17909/nrxscx46. This work used the DiRAC Data Intensive service (DIaL2 / DIaL3) at the University of Leicester, managed by the University of Leicester Research Computing Service on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). The DiRAC service at Leicester was funded by BEIS, UKRI and STFC capital funding and STFC operations grants. DiRAC is part of the UKRI Digital Research Infrastructure. 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The full corner plots for all the retrievals mentioned in this paper can be found at:https://github.com/riobanerjee/ supplement L9859d.  \nFigure 3. The corner plot showing retrieved posterior distributions for each parameter for the main retrieval. The plots on the diagonal show the histograms of retrieved parameters and the inner plots show the pairwise correlations between the parameters. Large abundances for H 2 S and SO 2 can be seen, with upper bounds for all other active gases.  \n<!-- image -->', 'B. ALTERNATE RETRIEVAL PLOT': 'Here, we show the spectral fits and posteriors for selected parameters for two of the alternate retrievals: equilibrium chemistry and the retrieval without H 2 S and SO 2 included.  \nFigure 4. The retrieved spectra for two the alternate retrievals are shown: The Stellar + Atmosphere scenario with H 2 S and SO 2 removed is plotted in red and the equilibrium chemistry scenario is plotted in blue. The 1 σ and 2 σ credible intervals are plotted in dark and light shades of the corresponding colors respectively. The NRS2 points are shifted by the retrieved offset, and the unshifted NRS2 points are shown in gray. Underneath the spectrum, in the first row from left to right, the retrieved posterior distributions for f het , T phot , ∆ T het , and offset are shown. In the second row, from left to right, the retrieved posterior distribution for Temperature, Metallicity, S:O ratio, and C:O ratio are shown.  \n<!-- image -->', 'C. COMPARISON BETWEEN REDUCTIONS PLOT': 'Here, we show the spectral fits and posteriors for selected parameters for retrievals using two different reductions: transitspectroscopy and FIREFLy .  \nFigure 5. The retrieved spectra for the retrievals using two different reductions are shown: transitspectroscopy is plotted in red and FIREFLy is plotted in blue. The 1 σ and 2 σ credible intervals are plotted in dark and light shades of the corresponding colors respectively. The NRS2 points are shifted by the retrieved offset, and the unshifted NRS2 points are shown in gray. Underneath the spectrum, in the first row from left to right, the retrieved posterior distributions for log(H 2 S), log(SO 2 ), and Temperature are shown. In the second row, from left to right, the retrieved posterior distributions for f het , T phot , and ∆ T het are shown.  \n<!-- image -->'}