arXiv subjects

D. Kitzmann

Publications and source records attributed to D. Kitzmann.

At least 19 recordsLinked to original sources

arXiv2026

CHEOPS photometry from 2024 reveals a reversal in the transit-timing variations of AU Mic c

We present new CHEOPS transit observations of AU Mic b and AU Mic c obtained between June and September 2024, extending the baseline of transit-timing measurements of this young planetary system. For AU Mic b, the timing signal is well established, with a semi-amplitude (10 $\pm$ 3 min) and a characteristic modulation timescale (1168 $\pm$ 20 d) consistent with previous determinations. By contrast, the new CHEOPS data show that the large transit-timing deviation of AU Mic c reported previously was not sustained. After the steadily increasing timing trend observed in 2022 and 2023, the 2024 timings returned closer to the zero point of the observed-minus-calculated diagram, indicating a reversal of the previously reported behavior. For AU Mic c, both the transit-timing semi-amplitude (46 $\pm$ 26 min) and the characteristic modulation timescale (2150 $\pm$ 110 d) remain tentative. These results highlight the importance of continued long-term monitoring of the AU Mic system.

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arXiv2026

The multi-planet system TOI-5624: Four transiting sub-Neptunes with an outer companion revealed by transit-timing variations

Following the 2022 alert of a TESS object of interest transiting TOI-5624 (a G7 V star $\sim$100 pc away), a CHEOPS campaign in 2023 detected four planetary signals at $P_b\approx3.4$, $P_c\approx7.9$, $P_d\approx13.7$, and $P_e\approx21.5$ days, later confirmed by additional TESS and CHEOPS photometry in 2024-2025. After analysing the TESS & CHEOPS photometric data, we extracted and modelled the HARPS-N & SOPHIE RV time series using two independent methodologies both within an MCMC framework. We further integrated the N-body equations of motion, while simultaneously fitting the transit times and the detrended RVs, to dynamically characterise the system. We present the discovery of four transiting sub-Neptunes with radii of $R_b=2.314\pm0.035 R_{\oplus}$, $R_c=2.474\pm0.042 R_{\oplus}$, $R_d=3.584_{-0.050}^{+0.051} R_{\oplus}$, and $R_e=3.247_{-0.043}^{+0.042} R_{\oplus}$ and masses of $M_b=9.4\pm1.4 M_{\oplus}$, $M_c=4.8\pm1.9 M_{\oplus}$, $M_d=4.9\pm2.2 M_{\oplus}$, and $M_e=8.9_{-3.0}^{+2.9} M_{\oplus}$. Our photometric analysis reveals that the outermost transiting planet TOI-5624 e shows significant TTVs. We find a robust Keplerian signal in the RV time series close to the 2:1 period commensurability with TOI-5624 e, which explains the TTV pattern exhibited by TOI-5624 e according to our dynamical analysis. We label this non-transiting planet as TOI-5624 f and find its minimum mass to be $M_f\sin{i_f}=13.0\pm3.7 M_{\oplus}$. Among the known systems hosting more than four planets, the remarkable precision with which the radii have been measured (<1.7%) and the firm assessment (>3$\sigma$) of the mass for at least three planets has been previously reached only for TRAPPIST-1. Additional photometric observations will enable a better sample of the TTV modulation and a more robust dynamical determination of the masses.

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arXiv2026

TOI-4616 b: a benchmark Earth-sized planet transiting a nearby M4 dwarf

Rocky exoplanets are particularly abundant around M-type stars. Their small radii and low luminosities provide favourable conditions for detecting transiting terrestrial planets and probing their atmospheric properties. We report the discovery and statistical validation of TOI-4616 b, an Earth-sized planet transiting a nearby mid-M dwarf observed by the Transiting Exoplanet Survey Satellite (TESS). We confirm the planetary nature of the signal and determine the system parameters by combining TESS photometry with ground-based multi-band transit observations, high-resolution imaging, and optical and near-infrared spectroscopy. The host star lies at a distance of 28.10 +(-) 0.07 pc and has a radius of 0.1889 +(-)0.0096 solar radii, a mass of 0.1881 +(-) 0.0094 solar masses, and an effective temperature of 3150 +(-) 75 K. TOI-4616 b has a radius of 1.22 Earth radii and an orbital period of 1.55 days. The planet receives an incident flux of approximately 40 times that of Earth, corresponding to an equilibrium temperature of about 525 K. This places TOI-4616 b in a regime intermediate between Earth-sized planets orbiting early M dwarfs and those around ultra-cool hosts. Statistical validation with the TRICERATOPS framework, supported by high-resolution imaging and chromatic transit constraints, yields a false-positive probability of 0.0135, below the recommended validation threshold of 0.015, confirming TOI-4616 b as a validated planet. Owing to its proximity to Earth, well-constrained stellar properties, and extensive multi-band follow-up, TOI-4616 b constitutes a valuable benchmark system for comparative studies of terrestrial planets around mid-M dwarfs and for future atmospheric investigations.

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arXiv2026

Clouds and Chemistry Across the Brown Dwarf T-Y Sequence: Insights from JWST Atmospheric Retrievals

The James Webb Space Telescope (JWST) offers exceptional spectral resolution and wavelength coverage, which are essential for studying the coldest brown dwarfs, particularly Y dwarfs. These objects are at the cold end of the sub-stellar sequence and exhibit atmospheric phenomena such as cloud formation, chemical disequilibrium, and radiative-convective coupling. We examine a curated sample of 22 late-T to Y dwarfs through Bayesian atmospheric retrieval (nested sampling) and supervised machine learning (random forests). Bayesian model comparison indicates that cloud-free models are generally favored for the hottest objects in the sample (T6-T8). Conversely, later-type dwarfs exhibit varying preferences, with both gray-cloud and cloud-free models providing comparable fits. The atmospheric parameters retrieved are consistent across the applied methodologies. Evidence of vertical mixing and disequilibrium chemistry is found in several objects; notably, the Y1 dwarf WISEPAJ1541-22 favors a gray cloud model and shows elevated abundances of both CO and CO2 compared to equilibrium chemistry calculations. As anticipated, the abundances of H2O, CH4, and NH3 increase with decreasing effective temperature over the T-Y sequence.

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arXiv2025

TESS phase curve of ultra-hot Jupiter WASP-189 b

The thermal structures of highly irradiated ultra-hot Jupiters can deviate substantially from those of cooler hot Jupiters. For planets orbiting rapidly rotating, and consequently oblate, host stars, photometric light curves provide a unique opportunity to measure the spin-orbit angle. Moreover, in systems with significant spin-orbit misalignment, the stellar oblateness can induce observable orbital precession. We wish to study the atmosphere and orbital architecture of an ultra-hot Jupiter WASP-189 b, orbiting around a hot A-type star. We use the photometric phase curves and gravity-darkened transits of WASP-189 b observed with the Transiting Exoplanet Survey Satellite (TESS), complemented with the archival observations from CHaracterising ExOPlanet Satellite (CHEOPS). We detected a phase curve signal with significant occultation depth of 203.4 (+16.2) (-16.3) ppm, while the nightside flux, -71.8 (+36.4) (-36.0) ppm, is consistent with zero at 2-sigma. We invert the phase curve signal to construct the temperature map of the planet. The map was subsequently used to estimate the Bond albedo and heat redistribution efficiency, the expected median ranges of which are found to be 0.19-0.35 and 0.09-0.41, respectively. Finally, we analysed gravity-darkened transits to find that the planet is in polar orbit with the spin-orbit angle of 89.46 (+1.08) (-1.08) deg. We found no hint of orbital precession while comparing our results with those from the literature. Our observations, together with atmospheric modelling, suggest that the dayside emission of WASP-189 b in TESS and CHEOPS bandpasses is dominated by thermal emission from an atmosphere with extremely inefficient heat transport and negligible contribution from reflected light.

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arXiv2025

Dark skies of the slightly eccentric WASP-18 b from its optical-to-infrared dayside emission

We performed a joint analysis of phase-curve observations of the ultra-hot Jupiter WASP-18 b from the visible to the mid-infrared, using data from CHEOPS, TESS and Spitzer. We aim to characterise the planetary atmosphere with a consistent view over the large wavelength range covered using GCMs and retrieval analyses, and including JWST data. We obtained new ephemerides with unprecedented precisions of 1 second and 1.4 millisecond on the time of inferior conjunction and orbital period, respectively. We computed a planetary radius of $R_p = 1.1926 \pm 0.0077 R_J$ with a precision of 0.65% (or 550 km). Based on a timing inconsistency with JWST, we discuss and confirm orbital eccentricity ($e = 0.00852 \pm 0.00091$). We also constrain the argument of periastron to $\omega = 261.9^{+1.3}_{-1.4}$ deg. We show that the large dayside emission implies the presence of magnetic drag and super-solar metallicity. We find a steep thermally inverted gradient in the planetary atmosphere, which is common for UHJs. We detected the presence of strong CO emission lines at 4.5 $\mu$m from an excess of dayside brightness in the Spitzer/IRAC/Ch2 passband. Using these models to constrain the reflected contribution in the CHEOPS passband, we derived an extremely low geometric albedo of $A_g^\text{CHEOPS} = 0.027 \pm 0.011$.

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arXiv2025

Hot Rocks Survey II: The thermal emission of TOI-1468 b reveals a hot bare rock

Terrestrial exoplanets orbiting nearby small, cool stars known as M dwarfs are well suited for atmospheric characterisation. Given the strong XUV irradiation from M dwarf host stars, orbiting exoplanets are thought to be unable to retain primordial H/He-dominated atmospheres. However, the survivability of heavier secondary atmospheres is currently unknown. The aim of the Hot Rocks Survey programme is to determine if exoplanets can retain secondary atmospheres in the presence of M dwarf hosts. Among the sample of 9 exoplanets in the programme, here we aim to determine whether TOI-1468 b has a substantial atmosphere or is consistent with a low-albedo bare rock. The James Webb Space Telescope provides an opportunity to characterise the thermal emission with MIRI at 15 $\mu$m. TOI-1468 b's occultation was observed three times. We compare our observations to atmospheric models including varying amounts of CO$_{2}$ and H$_{2}$O. The observed occultation depth for the individual visits are 239$\pm$52 ppm, 341$\pm$53 ppm and 357$\pm$52 ppm. A joint fit yields an occultation depth of 311$\pm$31 ppm. The thermal emission is mostly consistent with no atmosphere and zero Bond albedo at 1.65-$\sigma$ confidence level or a blackbody at a brightness temperature of 1024$\pm$78 K. A pure CO$_{2}$ or H$_{2}$O atmosphere with a surface pressure above 1 bar is ruled out over 3-$\sigma$. Surprisingly, TOI-1468 b presents a surface marginally hotter than expected, hinting at an additional source of energy on the planet. It could originate from a temperature inversion, induction heating or be an instrumental artifact. The results within the Hot Rocks Survey build on the legacy of studying the atmospheres of exoplanets around M dwarfs. The outcome of this survey will prove useful to the large-scale survey on M dwarfs recently approved by the STScI.

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arXiv2025

Titanium chemistry of WASP-121 b with ESPRESSO in 4-UT mode

Transit spectroscopy usually relies on the integration of one or several transits to achieve the S/N necessary to resolve spectral features. Consequently, high-S/N observations of exoplanet atmospheres are essential for disentangling the complex chemistry and dynamics beyond global trends. In this study, we combined two partial 4-UT transits of the ultrahot Jupiter WASP-121 b, observed with the ESPRESSO at the VLT in order to revisit its titanium chemistry. Through cross-correlation analysis, we achieved detections of H I, Li I, Na I, K I, Mg I, Ca I, Ti I, V I, Cr I, Mn I, Fe I, Fe II, Co I, Ni I, Ba II, Sr I, and Sr II. Additionally, narrow-band spectroscopy allowed us to resolve strong single lines, resulting in significant detections of H$\alpha$, H$\beta$, H$\gamma$, Li I, Na I, K I, Mg I, Ca II, Sr I, Sr II, and Mn I. Our most notable finding is the high-significance detection of Ti I ($\sim$ 5$\sigma$ per spectrum, and $\sim$ 19$\sigma$ stacked in the planetary rest frame). Comparison with atmospheric models reveals that Ti I is indeed depleted compared to V I. We also resolve the planetary velocity traces of both Ti I and V I, with Ti I exhibiting a significant blueshift toward the end of the transit. This suggests that Ti I primarily originates from low-latitude regions within the super-rotating jet observed in WASP-121 b. Our observations suggest limited mixing between the equatorial jet and the mid-latitudes, in contrast with model predictions from GCMs. We also report the non-detection of TiO, which we attribute to inaccuracies in the line list that could hinder its detection, even if present. Thus, the final determination of the presence of TiO must await space-based observations. We conclude that the 4-UT mode of ESPRESSO is an excellent testbed for achieving high S/N on relatively faint targets, paving the way for future observations with the ELT.

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arXiv2025

Searching for Hot Water World Candidates with CHEOPS: Refining the radii and analysing the internal structures and atmospheric lifetimes of TOI-238 b and TOI-1685 b

Studying the composition of exoplanets is one of the most promising approaches to observationally constrain planet formation and evolution processes. However, this endeavour is complicated for small exoplanets by the fact that a wide range of compositions is compatible with their bulk properties. To overcome this issue, we identify triangular regions in the mass-radius space where part of this degeneracy is lifted for close-in planets, since low-mass H/He envelopes would not be stable due to high-energy stellar irradiation. Planets in these Hot Water World triangles need to contain at least some heavier volatiles and are therefore interesting targets for atmospheric follow-up observations. We perform a demographic study to show that only few well-characterised planets in these regions are currently known and introduce our CHEOPS GTO programme aimed at identifying more of these potential hot water worlds. Here, we present CHEOPS observations for the first two targets of our programme, TOI-238 b and TOI-1685 b. Combined with TESS photometry and published RVs, we use the precise radii and masses of both planets to study their location relative to the corresponding Hot Water World triangles, perform an interior structure analysis and study the lifetimes of H/He and water-dominated atmospheres under these conditions. We find that TOI-238 b lies, at the 1-sigma level, inside the corresponding triangle. While a pure H/He atmosphere would have evaporated after 0.4-1.3 Myr, it is likely that a water-dominated atmosphere would have survived until the current age of the system, which makes TOI-238 b a promising hot water world candidate. Conversely, TOI-1685 b lies below the mass-radius model for a pure silicate planet, meaning that even though a water-dominated atmosphere would be compatible both with our internal structure and evaporation analysis, we cannot rule out the planet to be a bare core.

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arXiv2024

Time Resolved Absorption of Six Chemical Species With MAROON-X Points to Strong Drag in the Ultra Hot Jupiter TOI-1518 b

Wind dynamics play a pivotal role in governing transport processes within planetary atmospheres, influencing atmospheric chemistry, cloud formation, and the overall energy budget. Understanding the strength and patterns of winds is crucial for comprehensive insights into the physics of ultra-hot Jupiter atmospheres. This study focuses on unraveling the wind dynamics and the chemical composition in the atmosphere of the ultra-hot Jupiter TOI-1518 b. Two transit observations using the high-resolution (R{\lambda} = 85 000), optical (spectral coverage between 490 and 920 nm) spectrograph MAROON-X were obtained and analyzed to explore the chemical composition and wind dynamics using the cross-correlation techniques, global circulating models, and atmospheric retrieval. We report the detection of 14 species in the atmosphere of TOI-1518 b through cross-correlation analysis. Additionally, we measure the time-varying cross-correlation trails for 6 different species, compare them with predictions from General Circulation Models (GCM) and conclude that a strong drag is present in TOI-1518b's atmosphere. We find that the trails are species-dependent. Fe+ favors a stronger drag than Fe, which we interpret as a sign of magnetic effects being responsible for the observed strong drag. Furthermore, we show that Ca+ probes layers above the Roche lobe, leading to a qualitatively different trail than the other species. Finally, we use a retrieval analysis to characterize the abundance of the different species detected. That analysis is refined thanks to the updated planetary mass we derived from the radial-velocity detection using SOPHIE data. We measure an abundance of iron corresponding to 0.07 to 1.62 solar enrichment. The retrievals appear to be biased for the other elements, probably due to the different Kp/Vsys shifts between iron and the other elements, which we demonstrate in the case of VO.

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arXiv2024

Detection of faculae in the transit and transmission spectrum of WASP-69b

Context: Transmission spectroscopy is a powerful tool for understanding exoplanet atmospheres. At optical wavelengths, it makes it possible to infer the composition and the presence of aerosols in the atmosphere. However, unocculted stellar activity can result in contamination of atmospheric transmission spectra by introducing spurious slopes and molecular signals. Aims: We aim to characterise the atmosphere of the transiting exoplanet WASP-69b, a hot Jupiter orbiting an active K star, and characterise the host star's activity levels. Methods: We obtained three nights of spectrophotometric data with the FORS2 instrument on the VLT, covering a wavelength range of 340-1100 nm. We performed retrievals on the full spectrum with combined stellar activity and planet atmosphere models. Results: We directly detect a facula in the form of a hot spot crossing event in one of the transits and indirectly detect unocculted faculae through an apparently decreasing radius towards the blue end of the transmission spectrum. We determine a facula temperature of $\Delta T=+644^{+427}_{-263}$ K for the former and a stellar coverage fraction of around 30% with a temperature of $\Delta T=+231\pm72$ K for the latter. The planetary atmosphere is best fit with a high-altitude cloud deck at 1.4 mbar that mutes atomic and molecular features. We find indications of water and ammonia with $log(H_2O)=-2.01^{+0.54}_{-0.86}$ and $log(NH_3)=-3.4^{+0.96}_{-5.20} respectively and place 3$\sigma$ upper limits on TiO ($10^{-7.65}$) and K ($10^{-7}$). Conclusions. The simultaneous multi-wavelength observations allow us to break the size-contrast degeneracy for facula-crossings, meaning we can obtain temperatures for both the directly and indirectly detected faculae, which are consistent with each other.

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arXiv2024

JWST reveals a rapid and strong day side variability of 55 Cancri e

The nature of the close-in rocky planet 55 Cnc e is puzzling despite having been observed extensively. Its optical and infrared occultation depths show temporal variability, in addition to a phase curve variability observed in the optical. We wish to explore the possibility that the variability originates from the planet being in a 3:2 spin-orbit resonance, thus showing different sides during occultations. We proposed and were awarded Cycle 1 time at the James Webb Space Telescope (JWST) to test this hypothesis. JWST/NIRCam observed five occultations (secondary eclipses), of which four were observed within a week, of the planet simultaneously at 2.1 and 4.5 {\mu}m. While the former gives band-integrated photometry, the latter provides a spectrum between 3.9-5.0 {\mu}m. We find that the occultation depths in both bandpasses are highly variable and change between a non-detection (-5 +/- 6 ppm and 7 +/- 9 ppm) to 96 +/- 8 ppm and 119 (+34) (-19) ppm at 2.1 {\mu}m and 4.5 {\mu}m, respectively. Interestingly, the variations in both bandpasses are not correlated and do not support the 3:2 spin-orbit resonance explanation. The measured brightness temperature at 4.5 {\mu}m varies between 873-2256 K and is lower than the expected dayside temperature of bare rock with no heat re-distribution (2500 K) which is indicative of an atmosphere. Our atmospheric retrieval analysis of occultation depth spectra at 4.5 {\mu}m finds that different visits statistically favour various atmospheric scenarios including a thin outgassed CO/CO2 atmosphere and a silicate rock vapour atmosphere. Some visits even support a flat line model. The observed variability could be explained by stochastic outgassing of CO/CO2, which is also hinted by retrievals. Alternatively, the variability, observed at both 2.1 and 4.5 {\mu}m, could be the result of a circumstellar patchy dust torus generated by volcanism on the planet.

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arXiv2024

An atlas of resolved spectral features in the transmission spectrum of WASP-189 b with MAROON-X

Exoplanets in the ultra-hot Jupiter regime provide an excellent laboratory for testing the impact of stellar irradiation on the dynamics and chemical composition of gas giant atmospheres. In this study, we observed two transits of the ultra-hot Jupiter WASP-189 b with MAROON-X/Gemini-North to probe its high-altitude atmospheric layers, using strong absorption lines. We derived posterior probability distributions for the planetary and stellar parameters by calculating the stellar spectrum behind the planet at every orbital phase during the transit. This was used to correct the Rossiter-McLaughlin imprint on the transmission spectra. Using differential transmission spectroscopy, we detect strong absorption lines of Ca+, Ba+, Na, H$\alpha$, Mg, Fe, and Fe+, providing an unprecedented and detailed view of the atmospheric chemical composition. Ca+ absorption is particularly well suited for analysis through time-resolved narrow-band spectroscopy, owing to its transition lines formed in high-altitude layers. The spectral absorption lines show no significant blueshifts that would indicate high-altitude day-to-night winds, and further analysis is needed to investigate the implications for atmospheric dynamics. These high signal-to-noise observations provide a benchmark data set for testing high-resolution retrievals and the assumptions of atmospheric models. We also simulate observations of WASP-189 b with ANDES/ELT, and show that ANDES will be highly sensitive to the individual absorption lines of a myriad of elements and molecules, including TiO and CO.

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arXiv2023

Small but mighty: High-resolution spectroscopy of ultra-hot Jupiter atmospheres with compact telescopes. KELT-9 b's transmission spectrum with Wendelstein's FOCES Spectrograph

When observing transmission spectra produced by atmospheres of ultra-hot Jupiters, large telescopes are typically the instrument of choice due to the very weak signal of the planet's atmosphere. This study aims to alleviate the desire for large telescopes by illustrating that the same science is possible with smaller telescope classes. We use the cross-correlation technique to showcase the potential of the high-resolution spectrograph FOCES at Wendelstein Observatory and demonstrate its potential to resolve the atmosphere of the ultra-hot Jupiter, KELT-9 b. A performance comparison is conducted between FOCES and HARPS-N spectrographs, considering both single transit and combined observations over three nights. With FOCES, we have detected seven species in KELT-9 b's atmosphere: Ti II, Fe I, Fe II, Na I, Mg I, Na II, Cr II, Sc II. Although HARPS-N surpasses FOCES in performance, our results reveal that smaller telescope classes are capable of resolving ultra-hot Jupiter atmospheres. This broadens the scope of potential studies, allowing for investigations into phenomena like temporal variations in atmospheric signals and the atmospheric loss characteristics of these close-in planets.

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arXiv2023

Investigating the visible phase-curve variability of 55 Cnc e

55 Cnc e is an ultra-short period super-Earth transiting a Sun-like star. Previous observations in the optical range detected a time-variable flux modulation that is phased with the planetary orbital period, whose amplitude is too large to be explained by reflected light and thermal emission alone. The goal of the study is to investigate the origin of the variability and timescale of the phase-curve modulation in 55 Cnc e. To this end, we used the CHaracterising ExOPlanet Satellite (CHEOPS), whose exquisite photometric precision provides an opportunity to characterise minute changes in the phase curve from one orbit to the next. CHEOPS observed 29 individual visits of 55 Cnc e between March 2020 and February 2022. Based on these observations, we investigated the different processes that could be at the origin of the observed modulation. In particular, we built a toy model to assess whether a circumstellar torus of dust driven by radiation pressure and gravity might match the observed flux variability timescale. We find that the phase-curve amplitude and peak offset of 55 Cnc e do vary between visits. The sublimation timescales of selected dust species reveal that silicates expected in an Earth-like mantle would not survive long enough to explain the observed phase-curve modulation. We find that silicon carbide, quartz, and graphite are plausible candidates for the circumstellar torus composition because their sublimation timescales are long. The extensive CHEOPS observations confirm that the phase-curve amplitude and offset vary in time.We find that dust could provide the grey opacity source required to match the observations. However, the data at hand do not provide evidence that circumstellar material with a variable grain mass per unit area causes the observed variability. Future observations with the James Webb Space Telescope promise exciting insights into this iconic super-Earth.

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arXiv2023

The Mantis Network III: Expanding the limits of chemical searches within ultra hot-Jupiters. New detections of Ca I, V I, Ti I, Cr I, Ni I, Sr II, Ba II, and Tb II in KELT-9 b

Cross-correlation spectroscopy is an invaluable tool in the study of exoplanets. However, aliasing between spectral lines makes it vulnerable to systematic biases. This work strives to constrain the aliases of the cross-correlation function to provide increased confidence in the detections of elements in the atmospheres of ultra-hot Jupiters (UHJs) observed with high-resolution spectrographs. We use a combination of archival transit observations of the UHJ KELT-9 b obtained with the HARPS-N and CARMENES spectrographs and show that it is possible to leverage each instrument's strengths to produce robust detections at substantially reduced signal-to-noise. Aliases that become present at low signal-to-noise regimes are constrained through a linear regression model. We confirm previous detections of H I, Na I, Mg I, Ca II, Sc II, Ti II, Cr II, Fe I, and Fe II, and detect eight new species Ca I, Cr I, Ni I, Sr II, Tb II at the 5$\sigma$ level and Ti I, V I, Ba II above the 3$\sigma$ level. Ionised terbium (Tb II) has never before been seen in an exoplanet atmosphere. We further conclude that a 5$\sigma$ threshold may not provide a reliable measure of confidence when used to claim detections, unless the systematics in the cross-correlation function caused by aliases are taken into account.

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arXiv2023

Large Interferometer For Exoplanets (LIFE): IX. Assessing the Impact of Clouds on Atmospheric Retrievals at Mid-Infrared Wavelengths with a Venus-Twin Exoplanet

The Large Interferometer For Exoplanets (LIFE) initiative aims to develop a space based mid-infrared (MIR) nulling interferometer to measure the thermal emission spectra of temperate terrestrial exoplanets. We investigate how well LIFE could characterize a cloudy Venus-twin exoplanet to: (1) test our retrieval routine on a realistic non-Earth-like MIR spectrum of a known planet, (2) investigate how clouds impact retrievals, (3) refine the LIFE requirements derived in previous Earth-centered studies. We run retrievals for simulated LIFE observations of a Venus-twin exoplanet orbiting a Sun-like star located 10 pc from the observer. By assuming different models (cloudy and cloud-free) we analyze the performance as a function of the quality of the LIFE observation. This allows us to determine how well atmosphere and clouds are characterizable depending on the quality of the spectrum. Our study shows that the current minimal resolution ($R=50$) and signal-to-noise ($S/N=10$ at $11.2\mu$m) requirements for LIFE suffice to characterize the structure and composition of a Venus-like atmosphere above the cloud deck if an adequate model is chosen. However, we cannot infer cloud properties. The accuracy of the retrieved planet radius ($R_{pl}$), equilibrium temperature ($T_{eq}$), and Bond albedo ($A_B$) depend on the choice of model. Generally, a cloud-free model performs best and thus the presence of clouds cannot be inferred. This model dependence of retrieval results emphasizes the importance of developing a community-wide best-practice for atmospheric retrieval studies. If we consider higher quality spectra (especially $S/N=20$), we can infer the presence of clouds and pose first constraints on their structure.

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arXiv2023

The geometric albedo of the hot Jupiter HD 189733b measured with CHEOPS

Context. Measurements of the occultation of an exoplanet at visible wavelengths allow us to determine the reflective properties of a planetary atmosphere. The observed occultation depth can be translated into a geometric albedo. This in turn aids in characterising the structure and composition of an atmosphere by providing additional information on the wavelength-dependent reflective qualities of the aerosols in the atmosphere. Aims. Our aim is to provide a precise measurement of the geometric albedo of the gas giant HD 189733b by measuring the occultation depth in the broad optical bandpass of CHEOPS (350 - 1100 nm). Methods. We analysed 13 observations of the occultation of HD 189733b performed by CHEOPS utilising the Python package PyCHEOPS. The resulting occultation depth is then used to infer the geometric albedo accounting for the contribution of thermal emission from the planet. We also aid the analysis by refining the transit parameters combining observations made by the TESS and CHEOPS space telescopes. Results. We report the detection of an $24.7 \pm 4.5$ ppm occultation in the CHEOPS observations. This occultation depth corresponds to a geometric albedo of $0.076 \pm 0.016$. Our measurement is consistent with models assuming the atmosphere of the planet to be cloud-free at the scattering level and absorption in the CHEOPS band to be dominated by the resonant Na doublet. Taking into account previous optical-light occultation observations obtained with the Hubble Space Telescope, both measurements combined are consistent with a super-stellar Na elemental abundance in the dayside atmosphere of HD 189733b. We further constrain the planetary Bond albedo to between 0.013 and 0.42 at 3$\sigma$ confidence.

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