arXiv subjects

I. Ribas

Publications and source records attributed to I. Ribas.

At least 37 records · Page 2Linked to original sources

arXiv2025

Transit Timing Variations in HIP 41378: CHEOPS and TESS confirm a non-transiting sixth planet in the system

In multiple-planet systems, gravitational interactions of exoplanets could lead to transit timing variations (TTVs), whose amplitude becomes significantly enhanced when planets are in or near mean-motion resonances (MMRs). In cases where both TTVs and radial velocity (RV) measurements are available, combined analysis can break degeneracies and provide robust planetary and system characterization, even detecting non-transiting planets. In this context, HIP 41378 hosts five confirmed transiting planets with periods ranging from 15 to over 542 days, providing a unique dynamical laboratory for investigating wide multi-planet systems analogous to the Solar System. In this study, we present an intensive space-based photometric follow-up of HIP 41378, combining 15 new CHEOPS observations with eight TESS sectors, alongside data from K2, Spitzer, HST, and HARPS. We dynamically modeled the TTVs and RV signals of the two inner sub-Neptunes via N-body integration. These planets, HIP 41378 b ($P_{b}$ = 15.57 days) and HIP 41378 c ($P_{c}$ = 31.71 days), are close to ($Δ\sim1.8$ %) a 2:1 period commensurability. We report a clear detection of TTVs with amplitudes of 20 mins for planet b and greater than 3 hrs for planet c. We dynamically confirm the planetary nature of HIP 41378 g, a non-transiting planet with a period of about 64 days and a mass of about 7 $M_{\oplus}$, close to a 2:1 commensurability with planet c, suggesting a possible MMR chain in the inner system. Our precise determination of the masses, eccentricities, and radii of HIP 41378 b and c enabled us to investigate their possible volatile-rich compositions. Finally, by leveraging on the last TESS sectors we constrained the period of HIP 41378 d to three possible aliases ($P_{d} =$ 278, 371, and 1113 days) suggesting that the system could be placed in a double quasi resonant chain, highlighting its complex dynamical architecture.

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arXiv2025

The CARMENES search for exoplanets around M dwarfs : Understanding the wavelength dependence of radial velocity measurements

Context. Current exoplanet surveys are focused on detecting small exoplanets orbiting in the liquid-water habitable zones of their host stars. Despite the recent significant advancements in instrumental developments, the current limitation in detecting these exoplanets is the intrinsic variability of the host star itself. Aims. Our aim is to use the full CARMENES guaranteed time observations (GTO) data set spanning more than 8 years of observations of over 350 stars to investigate the wavelength dependence of high-precision radial velocities (RV), as stellar activity features should exhibit a wavelength dependence while the RV variation due to an orbiting planet will be wavelength independent. Methods. We use the chromatic index (CRX) to quantify the slope of the measured RVs as a function of logarithmic wavelength. We investigate the dependence of the CRX in the full CARMENES GTO sample on 24 stellar activity indices in the visible and near-infrared channels of the CARMENES spectrograph and each star's stellar parameters. We also present an updated convective turnover time scaling for the calculation of the stellar Rossby number for M dwarfs. Results. Our results show that approximately 17\% of GTO stars show a strong or a moderate correlation between CRX and RV. We can improve the measured RVs by a factor of up to nearly 4 in rms by subtracting the RV predicted by the CRX-RV correlation from the measured RVs. Mid M dwarfs with moderate rotational velocities, moderate CRX-gradients and quasi-stable activity features have the best rms improvement factors. Conclusions. We conclude that the CRX is a powerful diagnostic in mitigation of stellar activity and the search for low mass rocky planets.

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arXiv2025

A planetary system with a sub-Neptune planet in the habitable zone of TOI-2093

Aims. We aim to confirm and measure the mass of the transiting planet candidate around the K5V star TOI-2093, previously announced by the Transiting Exoplanet Survey Satellite (TESS) project. Methods. We combined photometric data from 32 sectors between 2019 and 2024 with 86 radial velocity measurements obtained with the CARMENES spectrograph over a period of 2.4 years, along with a series of ground-based, broadband photometric monitoring campaigns to characterize the host star and the transiting planet candidate, as well as to search for additional planets in the system. Our data indicate that TOI-2093 is a main-sequence star located at a distance of 83 pc, with solar metallicity, and a rotation period of 43.8 +- 1.8 d. Results. We have confirmed the planetary nature of the TESS transiting planet candidate, named TOI-2093 c, through the detection of its Keplerian signal in the spectroscopic data. We measured a planetary radius of 2.30 +- 0.12 Rearth, a Neptune-like mass of 15.8 +- 3.7 Mearth, and an orbital period of 53.81149 +- 0.00017 d. This makes TOI-2093 c the smallest exoplanet known in the habitable zone of a main-sequence FGK star. Given its size and relatively high density, TOI-2093 c belongs to a class of planets with no analog in the Solar System. In addition, the CARMENES data revealed the presence of a second planet candidate with a minimum mass of 10.6 +- 2.5 Mearth and an orbital period of 12.836 +- 0.021 d. This inner planet, which we designated TOI-2093 b, shows no detectable photometric transit in the TESS light curves. The orbital planes of the two planets are misaligned by more than 1.6 deg despite the near 4:1 mean-motion resonance of their orbital periods.

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arXiv2025

Improved characterization of the TOI-2141 system: a dense sub-Neptune with non-transiting inner and outer companions

We aim to refine the fundamental parameters of the TOI-2141 planetary system, which includes a transiting sub-Neptune orbiting a Sun-like star in a relatively long orbit of 18.26 days, by combining new photometric and spectroscopic observations. We analyze new space-based photometry from TESS and CHEOPS as well as 61 radial velocity measurements from HARPS-N. We perform individual and joint photometric and RV analyses using several modeling tools within a Bayesian model comparison framework. We refine the radius and mass of the transiting planet TOI-2141 b to 3.15 $\pm$ 0.04 $R_\oplus$ and 20.1 $\pm$ 1.6 $M_\oplus$, respectively, five and two times more precise than the previously reported values. Our radial velocity analysis reveals two additional non-transiting companions with orbital periods of 5.46 and 60.45 days. Despite the innermost planet's high geometric transit probability, we find no evidence for transits in the photometric data. The bulk properties of TOI-2141 b suggest a significant volatile envelope atop an Earth-like core, with modeling indicating a hydrogen-rich atmosphere that may have experienced mild photoevaporation over the system's history. Planets b and c must exhibit a modest mutual inclination of at least 2.4 degrees.

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arXiv2025

The CARMENES search for exoplanets around M dwarfs. Revisiting the GJ 317, GJ 463, and GJ 3512 systems and two newly discovered planets orbiting GJ 9773 and GJ 508.2

Surveys for exoplanets indicate that the occurrence rate of gas giant planets orbiting late-type stars in orbits with periods shorter than 1000 days is lower than in the case of Sun-like stars. This is in agreement with planet formation models based on the core or pebble accretion paradigm. The CARMENES exoplanet survey has been conducting radial-velocity observations of several targets that show long-period trends or modulations that are consistent with the presence of giant planets at large orbital separations. We present an analysis of five such systems that were monitored with the CARMENES spectrograph, as well as with the IRD spectrograph. In addition, we used archival data to improve the orbital parameters of the planetary systems. We improve the parameters of three previously known planets orbiting the M dwarfs GJ 317, GJ 463, and GJ 3512. We also determine the orbital parameters and minimum mass of the planet GJ 3512 c, for which only lower limits had been given previously. Furthermore, we present the discovery of two new giant planets orbiting the stars GJ 9733 and GJ 508.2, although for the second one only lower limits to the orbital properties can be determined. The new planet discoveries add to the short list of known giant planets orbiting M-dwarf stars with subsolar metallicity at long orbital periods above 2000 days. These results reveal that giant planets appear to form more frequently in wide orbits than in close-in orbits around low-mass and lower metallicity stars.

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arXiv2025

Discovery of a transiting hot water-world candidate orbiting Ross 176 with TESS and CARMENES

The case of Ross 176 is a late K-type star that hosts a promising water-world candidate planet. The star has a radius of $R_*$=0.569$\pm$0.020$R_{\odot}$ and a mass of $M_{\star}$ = 0.577 $\pm$ 0.024 $M_{\odot}$. We constrained the planetary mass using spectroscopic data from CARMENES, an instrument that has already played a major role in confirming the planetary nature of the transit signal detected by TESS. We used Gaussian Processes (GP) to improve the analysis because the host star has a relatively strong activity that affects the radial velocity dataset. In addition, we applied a GP to the TESS light curves to reduce the correlated noise in the detrended dataset. The stellar activity indicators show a strong signal that is related to the stellar rotation period of $\sim$ 32 days. This stellar activity signal was also confirmed on the TESS light curves. Ross 176b is an inner hot transiting planet with a low-eccentricity orbit of $e = 0.25 \pm 0.04$, an orbital period of $P \sim 5$ days, and an equilibrium temperature of $T_{eq}\sim 682K$. With a radius of $R_p = 1.84\pm0.08R_{\oplus}$ (4% precision), a mass of $M_p = 4.57^{+0.89}_{-0.93} M_{\oplus}$ (20% precision), and a mean density of $ρ_p = 4.03^{+0.49}_{-0.81} g cm^{-3}$, the composition of Ross 176b might be consistent with a water-world scenario. Moreover, Ross 176b is a promising target for atmospheric characterization, which might lead to more information on the existence, formation and composition of water worlds. This detection increases the sample of planets orbiting K-type stars. This sample is valuable for investigating the valley of planets with small radii around this type of star. This study also shows that the dual detection of space- and ground-based telescopes is efficient for confirm new planets.

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arXiv2025

The CHEOPS view of HD 95338b: refined transit parameters, and a search for exomoons

Despite the ever-increasing number of known exoplanets, no uncontested detections have been made of their satellites, known as exomoons. The quest to find exomoons is at the forefront of exoplanetary sciences. Certain space-born instruments are thought to be suitable for this purpose. We show the progress made with the CHaracterizing ExOPlanets Satellite (CHEOPS) in this field using the HD 95338 planetary system. We present a novel methodology as an important step in the quest to find exomoons. We utilize ground-based spectroscopic data in combination with Gaia observations to obtain precise stellar parameters. These are then used as input in the analysis of the planetary transits observed by CHEOPS and the Transiting Exoplanet Survey Satellite (TESS). In addition, we search for the signs of satellites primarily in the form of additional transits in the Hill sphere of the eccentric Neptune-sized planet HD 95338b in a sequential approach based on four CHEOPS visits. We also briefly explore the transit timing variations of the planet. We present refined stellar and planetary parameters, narrowing down the uncertainty on the planet-to-star radius ratio by a factor of $10$. We also pin down the ephemeris of HD 95338b. Using injection/retrieval tests, we show that a $5 σ$ detection of an exomoon would be possible at $R_{\rm Moon} = 0.8$~$R_\oplus$ with the methodology presented here. We exclude the transit of an exomoon in the system with $R_{\rm Moon} \approx 0.6$~$R_\oplus$ at the $1σ$ level. The algorithm used for finding the transit-like event can be used as a baseline for other similar targets, observed by CHEOPS or other missions.

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arXiv2025

Enhancing the detection of low-energy M dwarf flares: Wavelet-based denoising of CHEOPS data

Stellar flares are powerful bursts of electromagnetic radiation triggered by magnetic reconnection in the chromosphere of stars, occurring frequently and intensely on active M dwarfs. While missions like TESS and Kepler have studied regular and super-flares, their detection of flares with energies below $10^{30}$ erg remains incomplete. Extending flare studies to include these low-energy events could enhance flare formation models and provide insight into their impacts on exoplanetary atmospheres. This study investigates CHEOPS's capacity to detect low-energy flares in M dwarf light curves. Using its high photometric precision and observing cadence, along with a tailored wavelet-based denoising algorithm, we aim to improve detection completeness and refine flare statistics for low-energy events. We conducted a flare injection and recovery to optimise denoising parameters, applied it to CHEOPS light curves to maximise detection rates, and used a flare breakdown algorithm to analyse complex structures. We recovered 291 flares with energies ranging from $3.7\times10^{26}$ to $8.9\times10^{30}$ erg across 62 M dwarfs, with $\sim$42% exhibiting complex, multi-peaked structures. The denoising improved flare recovery by $\sim$35%, although it marginally extended the lower boundary of detectable energies. For the full sample, the power-law index $α$ was $1.99\pm0.10$, but a log-normal distribution fitted better, suggesting multiple possible flare formation scenarios. While CHEOPS's observing mode is not ideal for large-scale surveys, it captures weaker flares than TESS or Kepler, expanding the observed energy range. Wavelet-based denoising enhances low-energy event recovery, enabling exploration of the micro-flaring regime. Expanding low-energy flare observations could refine flare generation models and improve the understanding of their role in star-planet interactions.

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arXiv2025

The KELT-7b atmospheric thermal-inversion conundrum revisited with CHEOPS, TESS, and additional data

Ultrahot Jupiters are predicted to show inverted temperature-pressure (T-P) profiles in the presence of optical absorbers such as TiO and VO. An inverted T-P profile of KELT-7b was recently detected, in line with these predictions, but such diagnoses are known to be model-dependent. We used CHEOPS, TESS, and literature data to characterize the atmosphere of KELT-7b, reassess its T-P profile, measure its albedo, and search for distortions in its CHEOPS transit light curve due to stellar rotation. We jointly fitted CHEOPS and TESS data to measure the occultation depths and modeled CHEOPS transits including gravity darkening. Emission and transmission retrievals were performed, and the albedo was calculated in the CHEOPS and TESS passbands. Thermochemical-equilibrium retrievals yield a non-inverted T-P profile, while free-chemistry retrievals yield an inverted profile with likely unphysical TiO/VO abundances. A 3D GCM supports a TiO-driven inversion. We report a low geometric albedo of $A_\mathrm{g} = 0.05 \pm 0.06$, consistent with inefficient heat redistribution and supported by a GCM with magnetic drag. CHEOPS data provide no constraint on the sky-projected orbital obliquity. Retrieval results strongly depend on the chemical framework. Free-chemistry fits are better but risk unphysical solutions for ultrahot Jupiters. We applied a coherent stellar variability correction to CHEOPS and TESS data; future observations would benefit from similar treatment.

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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 $ω= 261.9^{+1.3}_{-1.4}$ 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 $μ$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

Retrieving day- and nightside atmospheric properties of the ultra-hot Jupiter TOI-2109b. Detection of Fe and CO emission lines and evidence for inefficient heat transport

The ultra-hot Jupiter (UHJ) TOI-2109b marks the lower edge of the equilibrium temperature gap between 3500 K and 4500 K, an unexplored thermal regime that separates KELT-9b, the hottest planet yet discovered, from all other currently known gas giants. To study the structure of TOI-2109b's atmosphere, we obtained high-resolution emission spectra of both the planetary day- and nightsides with CARMENES and CRIRES$^+$. By applying the cross-correlation technique, we identified the emission signatures of Fe I and CO, as well as a thermal inversion layer in the dayside atmosphere; no significant H$_2$O signal was detected from the dayside. None of the analyzed species were detectable from the nightside atmosphere. We applied a Bayesian retrieval framework that combines high-resolution spectroscopy with photometric measurements to constrain the dayside atmospheric parameters and derive upper limits for the nightside hemisphere. The dayside thermal inversion extends from 3200 K to 4600 K, with an atmospheric metallicity consistent with that of the host star (0.36 dex). Only weak constraints could be placed on the C/O ratio ($>$ 0.15). The retrieved spectral line broadening is consistent with tidally locked rotation, indicating the absence of strong dynamical processes. An upper temperature limit of 2400 K and a maximum atmospheric temperature gradient of 700 K/log bar could be derived for the nightside. Comparison of the retrieved dayside T-p profile with theoretical models, the absence of strong atmospheric dynamics, and significant differences in the thermal constraints between the day- and nightside hemispheres suggest a limited heat transport efficiency across the planetary atmosphere. Overall, our results place TOI-2109b in a transitional regime between the UHJs below the thermal gap, which show both CO and H$_2$O emission lines, and KELT-9b, where molecular features are largely absent.

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arXiv2025

The CARMENES search for exoplanets around M dwarfs. Cluster analysis of signals from spectral activity indicators to search for shared periods

A multitude of spectral activity indicators are routinely computed nowadays from the spectra generated as part of planet-hunting radial velocity surveys. Searching for shared periods among them can help to robustly identify astrophysical quantities of interest, such as the stellar rotation period. However, this identification can be complicated due to the fact that many different peaks occurring in the periodograms. This is especially true in the presence of aliasing and spurious signals caused by environmental influences affecting the instrument. Our goal is to test a clustering algorithm to find signals with the same periodicity, (i.e. with the stellar rotation period) in the periodograms of a large number of activity indicators. On this basis, we have looked to evaluate the correlations between activity indicators and fundamental stellar parameters. We used generalised Lomb-Scargle periodograms to find periodic signals in 24 activity indicators, spanning the VIS and NIR channels of the CARMENES spectrograph. Common periods were subsequently determined by a machine learning algorithm for density-based spatial clustering of applications with noise (DBSCAN). The clustering analysis of the signals apparent in the spectral activity indicators is a powerful tool for the detection of stellar rotation periods. It is straightforward to implement and can be easily automated, so that large data sets can be analysed. For a sample of 136 stars, we were able to recover the stellar rotation period in a total of 59 cases, including 3 with a previously unknown rotation period. In addition, we analysed spurious signals frequently occurring at the period of one year and its integer fractions, concluding that they are likely aliases of one underlying signal. Furthermore, we reproduced the results of several previous studies on the relationships between activity indicators and the stellar characteristics.

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arXiv2025

The CARMENES search for exoplanets around M dwarfs. Occurrence rates of Earth-like planets around very low-mass stars

Aims: Previous estimates of planet occurrence rates in the CARMENES survey indicated increased numbers of planets on short orbits for M dwarfs with masses below 0.34\,M$_\odot$. Here we focused on the lowest-mass stars in the survey, comprising 15 inactive targets with masses under 0.16\,M$_\odot$. Methods: To correct for detection biases, we determined detection sensitivity maps for individual targets and the entire sample. Using Monte Carlo simulations, we estimated planet occurrence rates for orbital periods of 1\,d to 100\,d and minimum masses from 0.5\,M$_\oplus$ to 10\,M$_\oplus$. Results: The radial velocity (RV) data from CARMENES reveal four new planets around three stars in our sample, namely G~268--110\,b, G~261--6\,b, and G~192--15\,b and c. All three b planets have minimum masses of 1.03--1.52\,M$_\oplus$ and orbital periods of 1.43--5.45\,d, while G~192--15\,c is a 14.3\,M$_\oplus$ planet on a wide, eccentric orbit with $P \approx 1218$\,d and $e \approx 0.68$. Our occurrence rates suggest considerable dependencies with respect to stellar masses. For planets below 3\,M$_\oplus$ we found rates consistent with one planet per star across all investigated periods, but the rates decrease almost by an order of magnitude for larger planet masses up to 10\,M$_\oplus$. Compared to previous studies, low-mass stars tend to harbor more planets with $P <10$\,d. We also demonstrate that synthetic planet populations based on the standard core accretion scenario predict slightly more massive planets on wider orbits than observed. Conclusions: Our findings confirm that planet occurrence rates vary with stellar masses even among M dwarfs, as we found more planets with lower masses and on shorter orbits in our subsample of very low-mass stars compared to more massive M dwarfs. Therefore, we emphasize the need for additional differentiation in future studies.

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arXiv2025

Coronal and chromospheric activity of Teegarden's star

Teegarden's star is a late-type M-dwarf planet host, typically showing only rather low levels of activity. In this paper we present an extensive characterisation of this activity at photospheric, chromospheric, and coronal levels. We specifically investigated TESS observations of Teegarden's star, which showed two very large flares with an estimated flare fluence between 10$^{29}$ and 10$^{32}$\,erg comparable to the largest solar flares. We furthermore analysed nearly 300 CARMENES spectra and 11 ESPRESSO spectra covering all the usually used chromospheric lines in the optical from the \ion{Ca}{ii} H \& K lines at 3930\,Å\, to the \ion{He}{i} infrared triplet at 10830\,Å. These lines show different behaviour: The \ion{He}{i} infrared triplet is the only one absent in all spectra, some lines show up only during flares, and others are always present and highly variable. Specifically, the H$α$ line is more or less filled in during quiescence; however, the higher Balmer lines are still observed in emission. Many chromospheric lines show a correlation with H$α$ variability, which, in addition to stochastic behaviour, also shows systematic behaviour on different timescales including the rotation period. Moreover, we found several flares and also report hints of an erupting prominence, which may have led to a coronal mass ejection. Finally, we present X-ray observations of Teegarden's star (i.e. a discovery pointing obtained with the \emph{Chandra} observatory) and an extensive study with the \emph{XMM-Newton} observatory; when these two large flares were observed, one of them showed clear signatures of the Neupert effect, suggesting the production of hard X-rays in the system.

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arXiv2025

HADES RV Programme with HARPS-N at TNG. XVI. A super-Earth in the habitable zone of the GJ 3998 multi-planet system

The low masses of M dwarfs create attractive opportunities for exoplanet radial-velocity (RV) detections. These stars, however, exhibit strong stellar activity that may attenuate or mimic planetary signals. We present a velocimetric analysis on one such M dwarf, GJ 3998 ($d=18.2\,\text{pc}$), with two published short-period super-Earths: GJ 3998 b and GJ 3998 c. We use additional data from the HARPS-N spectrograph to confirm these two planets and to look for more. We carry out joint modelling of: (i) RV planetary signals, (ii) stellar rotation in RV and activity indicators through Gaussian processes, (iii) long-term trends in RV and activity indicators. We constrain the rotational period of GJ 3998 to $P_\text{rot}=30.2\pm 0.3\,\text{d}$ and discover long-term sinusoidal imprints in RV and FWHM of period $P_\text{cyc}=316^{+14}_{-8}\,\text{d}$. We confirm GJ 3998 b and GJ 3998 c, and detect a third planet: GJ 3998 d, whose signal had been previously attributed to stellar activity. GJ 3998 d has an orbital period of $41.78\pm 0.05\,\text{d}$, a minimum mass of $6.07^{+1.00}_{-0.96}\,\text{M}_\oplus$ and a mean insolation flux of $1.2^{+0.3}_{-0.2}\,Φ_\oplus$. This makes it one of the few known planets receiving Earth-like insolation flux.

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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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arXiv2025

Connection between planetary He I $λ$10830 Å absorption and extreme-ultraviolet emission of planet-host stars

Context. The detection of the He I 10830 A triplet in exoplanet atmospheres has opened a new window for probing planetary properties, including atmospheric escape. Unlike Lyman alpha, the triplet is less affected by ISM absorption. Sufficient XUV stellar irradiation may trigger the formation of the He I triplet via photoionization and posterior recombination processes in the planet atmospheres. Only a weak trend between stellar XUV and the planetary He I strength has been observed so far. Aims. We aim to confirm this mechanism for producing the He I absorption in exoplanetary atmospheres by examining a sample of planetary systems. Methods. We obtained homogeneous measurements of the planetary He I line EW and consistently computed the stellar XUV ionizing irradiation. We first derived new coronal models for the planet-host stars. We used updated data from the X-exoplanets database, archival X-ray spectra of M-type stars (including AU Mic and Proxima Cen), and new XMM-Newton X-ray data obtained for the CARMENES project. These data were complemented at longer wavelengths with publicly available HST, FUSE, and EUVE spectra. A total of 75 stars are carefully analyzed to obtain a new calibration between X-ray and EUV emission. Results. Two distinct relationships between stellar X-ray emission (5-100 A) and EUV_H (100-920 A) or EUV_He (100-504 A) radiation are obtained to scale the emission from late-type stellar coronae. A total of 48 systems with reported planetary He I 10830 A studies, exhibit a robust relationship between the planetary He I feature and the ionizing XUV_He received by the planet, corrected by stellar and planetary radii, and the planet's gravitational potential. Some outliers could be explained by a different atmospheric composition or the lack of planetary gaseous atmospheres. This relation may be used to predict the He I 10830 A absorption in exoplanet atmospheres.

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arXiv2025

Transit-timing variations in the AU Mic system observed with CHEOPS

AU Mic is a very active M dwarf with an edge-on debris disk and two transiting sub-Neptunes with a possible third planetary companion. The two transiting planets exhibit significant transit-timing variations (TTVs) that are caused by the gravitational interaction between the bodies in the system. Using photometrical observations taken with the CHaracterizing ExOPlanet Satellite (CHEOPS), our goal is to constrain the planetary radii, the orbital distances and periods of AU Mic b and c. We aim to determine the superperiod of the TTVs for AU Mic b and to update the transit ephemeris for both planets. Based on the observed TTVs, we study the possible presence of a third planet in the system. We conducted high precision photometric observations with CHEOPS in 2022 and 2023. We used Allesfitter to fit the planetary transits and to constrain the planetary and orbital parameters. We combined our new measurements with results from previous years to determine the periods and amplitudes of the TTVs. We applied dynamical modelling based on TTV measurements from the 2018-2023 period to reconstruct the perceived variations. The orbital distances and periods for AU Mic b and c agree with the results from previous works. However, the values for the planetary radii deviate slightly from previous values, which we attribute to the effect of stellar spots. AU Mic c showed very strong TTVs, with transits that occurred ~80 minutes later in 2023 than in 2021. Through dynamical analysis of the system, we found that the observed TTVs can be explained by a third planet with an orbital period of ~12.6 days and a mass of 0.203+0.022-0.024 M_E. We explored the orbital geometry of the system and found that AU Mic c has a misaligned retrograde orbit. Due limited number of observations the exact configuration and planetary parameters could not be determined. Further monitoring with CHEOPS may improve these results.

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