arXiv subjects

S. G. Sousa

Publications and source records attributed to S. G. Sousa.

At least 37 records · Page 2Linked to original sources

arXiv2025

The star HIP 41378 potentially misaligned with its cohort of long-period planets

The obliquity between the stellar spin axis and the planetary orbit, detected via the Rossiter-McLaughlin (RM) effect, is a tracer of the formation history of planetary systems. While obliquity measurements have been extensively applied to hot Jupiters and short-period planets, they remain rare for cold and long-period planets due to observational challenges, particularly their long transit durations. We report the detection of the RM effect for the 19-hour-long transit of HIP 41378 f, a temperate giant planet on a 542-day orbit, observed through a worldwide spectroscopic campaign. We measure a slight projected obliquity of 21 $\pm$ 8 degrees and a significant 3D spin-orbit angle of 52 $\pm$ 6 degrees, based on the measurement of the stellar rotation period. HIP 41378 f is part of a 5-transiting planetary system with planets close to mean motion resonances. The observed misalignment likely reflects a primordial tilt of the stellar spin axis relative to the protoplanetary disk, rather than dynamical interactions. HIP 41378 f is the first non-eccentric long-period (P>100 days) planet observed with the RM effect, opening new constraints on planetary formation theories. This observation should motivate the exploration of planetary obliquities across a longer range of orbital distances through international collaboration.

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arXiv2025

A four-planet system orbiting the old thick disk star TOI-1203

TOI-1203 is a bright (V=8.6) G3 V star known to host a transiting warm sub-Neptune on a 25.5 d orbit. Here we report on an intensive high-precision radial velocity and photometric follow-up campaign carried out with the HARPS spectrograph and the CHEOPS space telescope. We found that TOI-1203 has an enhancement of $α$ elements relative to iron of [$α$/Fe]=$0.21\pm0.04$. With an age of $\sim$12.5 Gyr, TOI-1203 belongs to the old, $α$-element enhanced stellar population of the galactic thick disk. We spectroscopically confirmed the planetary nature of the 25.5 d sub-Neptune TOI-1203 d, measured its mass ($M_{d}=7.39\pm0.62~M_{\oplus}$) and refined its radius ($R_{d}=2.918_{-0.045}^{+0.046}~R_{\oplus}$). We discovered the presence of an additional transiting super-Earth on a 4.2 d orbit (TOI-1203 b) with a mass of $M_{b}=3.51_{-0.32}^{+0.33}~M_{\oplus}$ and a radius of $R_{b}=1.520_{-0.046}^{+0.045}~R_{\oplus}$. We also revealed the presence of two additional low-mass planets at 13.1 d and 204.6 d (TOI-1203 c and e), with minimum masses of $5.46_{-0.50}^{+0.51}~M_{\oplus}$ and $42.10_{-1.78}^{+1.83}~M_{\oplus}$. We found that the outer planet TOI-1203 e lies on an eccentric orbit with $e_{e}=0.152\pm0.029$. We performed a stability analysis of the system confirming that there are configurations consistent with the observed parameters that are dynamically stable over billion-year timescales. While analyzing the HARPS time series, we discovered that the FWHM of the HARPS cross-correlation function shows a significant long-period signal ($\sim$615 d) that has no counterpart in the radial velocity data or in the remaining HARPS ancillary time series. We significantly detected the same signal in the FWHM of the Th-Ar calibration lines used to compute the nightly wavelength solution, and attributed this systematic effect to a long-term variation of the HARPS instrumental profile.

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arXiv2025

Stellar-activity analysis of the nearby M dwarf GJ 526. Multi-dimensional Gaussian-process modelling of RV, FWHM and S-index

M dwarfs are the most abundant stars in the Galaxy, and their low masses make them natural favourites for exoplanet radial-velocity (RV) searches. Nevertheless, these stars often demonstrate strong stellar activity that is yet to be fully understood. We use high-precision ESPRESSO, HIRES, and HARPS spectroscopy to perform a stellar-activity analysis on the nearby early M dwarf GJ 526 ($d=5.4$ pc). We carry out joint modelling of: (i) stellar rotation in RV, FWHM, and Mount Wilson S-index through Gaussian processes, (ii) long-term trends in these three physical quantities, (iii) RV planetary signals. We constrain the stellar-rotation period to $P_\text{rot}=48.7\pm 0.3$ d, and discover a weak sinusoidal signature in RV, FWHM and S-index of period $P_\text{cyc}=1680^{+50}_{-40}$ d. We propose phase-space trajectories between RV and activity indicators as a novel means to visualise and interpret stellar activity. Current evidence does not support planetary companions of GJ 526.

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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

TOI-2322: two transiting rocky planets close to the stellar rotation period and its first harmonic

Context. Active regions on the stellar surface can induce quasi-periodic radial velocity (RV) variations that can mimic planets and mask true planetary signals. These spurious signals can be problematic for RV surveys such as those carried out by the ESPRESSO consortium. Aims. Using ESPRESSO and HARPS RVs and activity indicators, we aim to confirm and characterize two candidate transiting planets from TESS orbiting a K4 star with strong activity signals. Methods. From the ESPRESSO FWHM, TESS photometry, and ASAS-SN photometry, we measure a stellar rotation period of 21.28 $\pm$ 0.08 d. We jointly model the TESS photometry, ESPRESSO and HARPS RVs, and activity indicators, applying a multivariate Gaussian Process (GP) framework to the spectroscopic data. Results. We are able to disentangle the planetary and activity components, finding that TOI-2322 b has a $11.307170^{+0.000085}_{-0.000079}$ d period, close to the first harmonic of the rotation period, a $\leq 2.03 M_\oplus$ mass upper limit and a $0.994^{+0.057}_{-0.059}$ $\mathrm{R_\oplus}$ radius. TOI-2322 c orbits close to the stellar rotation period, with a $20.225528^{+0.000039}_{-0.000044}$ d period; it has a $18.10^{+4.34}_{-5.36}$ $\mathrm{M_\oplus}$ mass and a $1.874^{+0.066}_{-0.057}$ $\mathrm{R_\oplus}$ radius. Conclusions. The multivariate GP framework is crucial to separating the stellar and planetary signals, significantly outperforming a one-dimensional GP. Likewise, the transit data is fundamental to constraining the periods and epochs, enabling the retrieval of the planetary signals in the RVs. The internal structure of TOI-2322 c is very similar to that of Earth, making it one of the most massive planets with an Earth-like composition known.

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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

A comprehensive study on radial velocity signals using ESPRESSO: Pushing precision to the 10 cm/s level

We analyse ESPRESSO data for the stars HD10700, HD20794, HD102365, and HD304636 acquired via its Guaranteed Time Observations (GTO) programme. We characterise the stars' radial velocity (RV) signals down to a precision of 10 cm/s on timescales ranging from minutes to planetary periods falling within the host's habitable zone (HZ). We study the RV signature of pulsation, granulation, and stellar activity, inferring the potential presence of planets around these stars. Thus, we outline the population of planets that while undetectable remain compatible with the available data. A simple model of stellar pulsations successfully reproduced the intra-night RV scatter of HD10700 down to a few cm/s. For HD102365 and HD20794, an additional source of scatter at the level of several 10 cm/s remains necessary to explain the data. A kima analysis was used to evaluate the number of planets supported by the nightly averaged time series of each of HD10700, HD102365, and HD304636, under the assumption that a quasi-periodic Gaussian process (GP) regression is able to model the activity signal. While a frequency analysis of HD10700 RVs is able to identify a periodic signal at 20d, when it is modelled along with the activity signal the signal is formally non-significant. ESPRESSO data on their own do not provide conclusive evidence for the existence of planets around these three stars. ESPRESSO is shown to reach an on-sky RV precision of better than 10 cm/s on short timescales (<1h) and of 40 cm/s over 3.5 yr. A subdivision of the datasets showcases a precision reaching 20-30 cm/s over one year. These results impose stringent constraints on the impact of granulation mechanisms on RV. In spite of no detections, our analysis of HD10700 RVs demonstrates a sensitivity to planets with a mass of 1.7M$_{\oplus}$ for periods of up to 100 d, and a mass of 2-5M$_{\oplus}$ for the star's HZ. (abridged)

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arXiv2025

A systematic bias in template-based RV extraction algorithms

In this paper we identify and explore a previously unidentified, multi meter-per-second, systematic correlation between time and RVs inferred through TM and LBL methods. We evaluate the influence of the data-driven stellar template in the RV bias and hypothesize on the possible sources of this effect. We first use the s-BART pipeline to extract RVs from three different datasets gathered over four nights of ESPRESSO and HARPS observations. Then, we demonstrate that the effect can be recovered on a larger sample of 19 targets, totaling 4124 ESPRESSO observations spread throughout 38 nights. We also showcase the presence of the bias in RVs extracted with the SERVAL and ARVE pipelines. Lastly, we explore the construction of the stellar template through the 5 years of ESPRESSO observations of HD10700, totalling more than 2000 observations. We find that a systematic quasi-linear bias affects the RV extraction with slopes that vary from -0.3 m/s-1/h-1 to -52 m/s-1/h-1 in our sample. This trend is not observed in CCF RVs and appears when all observations of a given star are collected within a short time-period (timescales of hours). We show that this systematic contamination exists in the RV time-series of two different template-matching pipelines, one line-by-line pipeline, and that it is agnostic to the spectrograph. We also find that this effect is connected with the construction of the stellar template, as we are able to mitigate it through a careful selection of the observations used to construct it. Our results suggest that a contamination of micro-telluric features, coupled other sources of correlated noise, could be the driving factor of this effect. We also show that this effect does not impact the usual usage of template-matching for the detection and characterization of exoplanets. Other short-timescale science cases can however be severely affected.

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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

Two neighbours of the ultra-short-period Earth-sized planet K2-157 b in the warm Neptunian savanna

The formation and evolution of ultra-short-period (USP) planets is poorly understood. However, it is widely thought that these planets migrated inwards through interactions with outer neighbours. We aim to confirm and characterise the USP Earth-sized planet K2-157 b ($P_{\rm orb}$ = 8.8 h). To do so, we measured 49 radial velocities (RVs) with the ESPRESSO spectrograph and derived the properties of the system through an RV and transit model. We detect two additional super-Neptune-mass planets within the warm Neptunian savanna, K2-157 c ($P_{\rm orb, c}$ = $25.942^{+0.045}_{-0.044}$ d, $M_{\rm p, c} \, \textrm{sin} \, i$ = $30.8 \pm 1.9$ $\rm M_{\oplus}$), and K2-157 d ($P_{\rm orb, d}$ = $66.50^{+0.71}_{-0.59}$ d, $M_{\rm p,d} \, \textrm{sin}\,i$ = $23.3 \pm 2.5$ $\rm M_{\oplus}$). The mass of K2-157 b, $M_{\rm p,b}$ = $1.14^{+0.41}_{-0.42}$ $\rm M_{\oplus}$ ($<$ 2.4 $\rm M_{\oplus}$ at 3$σ$), together with its radius, $R_{\rm p}$ = 0.935 $\pm$ 0.090 $\rm R_{\oplus}$, make the planet compatible with a rocky composition. K2 data discard non-grazing transit configurations for K2-157 c ($i_{\rm c}$ $<$ 88.4$^{\circ}$ at 3$σ$), and ESPRESSO data constrain the eccentricities of K2-157 c and K2-157 d to $e_{\rm c}$ $<$ 0.2 and $e_{\rm d}$ $<$ 0.5 at 3$σ$. At a population level, we find that the trend that the closest USP planets tend to orbit late-type stars does not hold when scaling the orbital separation to the Roche limit, which suggests that the orbital distribution of the closest planets across spectral types is determined by tidal disruption. The orbital architecture of K2-157 is unusual, with only one similar case reported to date: 55 Cnc. The USP planets of these systems, being accompanied by massive, long-period, relatively spaced, and possibly misaligned neighbours, could have migrated inwards through eccentricity-based mechanisms triggered by secular interactions.

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arXiv2025

Three Hot Jupiters transiting K-dwarfs with a significant heavy element mass

Albeit at a lower frequency than around hotter stars, short-period gas giants around low-mass stars ($T_\mathrm{eff} < 4965$ K) do exist, despite predictions from planetary population synthesis models that such systems should be exceedingly rare. By combining data from TESS and ground-based follow-up observations, we seek to confirm and characterize giant planets transiting K dwarfs, particularly mid/late K dwarfs. Photometric data were obtained from the TESS mission, supplemented by ground-based imaging- and photometric observations, as well as high-resolution spectroscopic data from the CORALIE spectrograph. Radial velocity (RV) measurements were analyzed to confirm the presence of companions. We report the confirmation and characterization of three giants transiting mid-K dwarfs. Within the TOI-2969 system, a giant planet of $1.16\pm 0.04\,M_\mathrm{Jup}$ and a radius of $1.10 \pm 0.08\,R_\mathrm{Jup}$ revolves around its K3V host in 1.82 days. The system of TOI-2989 contains a $3.0 \pm 0.2\,M_\mathrm{Jup}$ giant with a radius of $1.12 \pm 0.05\,R_\mathrm{Jup}$, which orbits its K4V host in 3.12 days. The K4V TOI-5300 hosts a giant of $0.6 \pm 0.1\,M_\mathrm{Jup}$ with a radius of $0.88 \pm 0.08\,R_\mathrm{Jup}$ and an orbital period of 2.3 days. The equilibrium temperatures of the companions range from 1001 to 1186 K, classifying them as Hot Jupiters. However, they do not present radius inflation. The estimated heavy element masses in their interior, inferred from the mass, radius, and evolutionary models, are $90 \pm 30\,M_\oplus$, $114 \pm 30\,M_\oplus$, and $84 \pm 21\,M_\oplus$, respectively. The heavy element masses are significantly higher than most reported heavy elements for K-dwarf Hot Jupiters. These mass characterizations contribute to the poorly explored population of massive companions around low-mass stars.

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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

TOI-5005 b: A super-Neptune in the savanna near the ridge

The Neptunian desert and savanna have recently been found to be separated by a ridge, an overdensity of planets in the period range of $\simeq$3-5 days. These features are thought to be shaped by dynamical and atmospheric processes, but their roles are not yet well understood. Our aim was to confirm and characterize the super-Neptune TESS candidate TOI-5005.01, which orbits a moderately bright (V = 11.8) solar-type star (G2 V) with an orbital period of 6.3 days. We confirm TOI-5005 b to be a transiting super-Neptune with a radius of $R_{\rm p}$ = $6.25\pm 0.24$ $\rm R_{\rm \oplus}$ ($R_{\rm p}$ = $0.558\pm 0.021$ $\rm R_{\rm J}$) and a mass of $M_{\rm p}$ = $32.7\pm 5.9$ $\rm M_{\oplus}$ ($M_{\rm p}$ = $0.103\pm 0.018$ $\rm M_{\rm J}$), which corresponds to a mean density of $ρ_{\rm p}$ = $0.74 \pm 0.16$ $\rm g \, cm^{-3}$. Our internal structure modelling indicates that the overall metal mass fraction is well constrained to a value slightly lower than that of Neptune and Uranus ($Z_{\rm planet}$ = $0.76^{+0.04}_{-0.11}$). We also estimated the present-day atmospheric mass-loss rate of TOI-5005 b, but found contrasting predictions depending on the choice of photoevaporation model. At a population level, we find statistical evidence ($p$-value = $0.0092^{+0.0184}_{-0.0066}$) that planets in the savanna such as TOI-5005 b tend to show lower densities than planets in the ridge, with a dividing line around 1 $\rm g \, cm^{-3}$, which supports the hypothesis of different evolutionary pathways populating the two regimes. TOI-5005 b is located in a key region of the period-radius space to study the transition between the Neptunian ridge and the savanna. It orbits the brightest star of all such planets, which makes it a target of interest for atmospheric and orbital architecture observations that will bring a clearer picture of its overall evolution.

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arXiv2025

Ariel stellar characterisation III. Fast rotators and new FGK stars in the Ariel Mission Candidate Sample

The next mission dedicated to the study of planetary atmospheres is the Ariel space mission, planned for launch in 2029, which will observe a variety of planetary systems belonging to different classes around stars with spectral types from M to A. To optimise the scientific outcome of the mission, such stars need to be homogeneously characterised beforehand. In this work, we focus on a methodology based on spectral synthesis for the characterisation of FGK-type stars from the Ariel Tier 1 Mission Candidate Sample (MCS) which exhibit fast rotation. In addition, we analyse slow-rotating FGK-type stars, with either new observations or archival spectra available, consistently as in our previous work using the equivalent width (EW) analysis. To ensure consistency between our methods, we re-analysed a sample of FGK-type stars with the spectral synthesis method and compared it to our previous work. The results of our analysis show excellent agreement with the previous set of derived parameters. We also computed their orbital parameters establishing whether they belong to the Galactic thin or thick discs. With the current set of stellar parameters, we almost double the analysed hosts in the Ariel MCS to 353 stars in total. Using our homogeneous set of stellar parameters, we studied the correlations between stellar and planetary properties for the Ariel MCS analysed so far. We confirmed a close relationship between stellar mass (up to 1.8 solar masses) and giant planet radius, with more inflated planets at lower metallicity. We confirm that giant planets are more frequent around more metal-rich stars that belong to the thin disc, while lower-mass planets are also found in more metal-poor environments, and are more frequent than giant planets in the thick disc as also seen in other works in the literature.

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arXiv2025

Discovery of a cold giant planet and mass measurement of a hot super-Earth in the multi-planetary system WASP-132

Hot Jupiters generally do not have nearby planet companions, as they may have cleared out other planets during their inward migration from more distant orbits. This gives evidence that hot Jupiters more often migrate inward via high-eccentricity migration due to dynamical interactions between planets rather than more dynamically cool migration mechanisms through the protoplanetary disk. Here we further refine the unique system of WASP-132 by characterizing the mass of the recently validated 1.0-day period super-Earth WASP-132c (TOI-822.02), interior to the 7.1-day period hot Jupiter WASP-132b. Additionally, we announce the discovery of a giant planet at a 5-year period (2.7 AU). We also detected a long-term trend in the radial velocity data indicative of another outer companion. Using over nine years of CORALIE radial velocities (RVs) and over two months of highly sampled HARPS RVs, we determined the masses of the planets from smallest to largest orbital period to be M$_{\rm{c}}$ = $6.26^{+1.84}_{-1.83}$ $M_{\oplus}$, M$_{\rm{b}}$ = $0.428^{+0.015}_{-0.015}$ $M_{\rm{Jup}}$, and M$_{\rm{d}}\sin{i}$ = $5.16^{+0.52}_{-0.52}$ $M_{\rm{Jup}}$, respectively. Using TESS and CHEOPS photometry data, we measured the radii of the two inner transiting planets to be R$_{\rm{c}}$ = $1.841^{+0.094}_{-0.093}$ $R_{\oplus}$ and R$_{\rm{b}}$ = $0.901^{+0.038}_{-0.038}$ $R_{\rm{Jup}}$. We find a bulk density of $ρ_{\rm{c}}$ = $5.47^{+1.96}_{-1.71}$ g cm$^{-3}$ for WASP-132 c, which is slightly above the Earth-like composition line on the mass-radius diagram. WASP-132 is a unique multi-planetary system in that both an inner rocky planet and an outer giant planet are in a system with a hot Jupiter. This suggests it migrated via a rarer dynamically cool mechanism and helps to further our understanding of how hot Jupiter systems form and evolve.

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arXiv2025

TOI-512: Super-Earth transiting a K-type star discovered by TESS and ESPRESSO

One of the goals of the ESPRESSO guaranteed time observations (GTOs) at the ESO 8.2m telescope is to follow up on candidate planets from transit surveys such as the TESS mission. High-precision radial velocities are required to characterize small exoplanets. Aims. We intend to confirm the existence of a transiting super-Earth around the bright (V=9.74) K0-type star TOI-512 (TIC 119292328) and provide a characterization. Combining photometric data from TESS and 37 high-resolution spectroscopic observations from ESPRESSO in a joint Markov chain Monte Carlo analysis, we determined the planetary parameters of TOI-512b and characterized its internal structure. We find that TOI-512b is a super-Earth, with a radius of $1.54 \pm 0.10$ R$_\oplus$ and mass of $3.57_{-0.55}^{+0.53}$~M$_\oplus$, on a $7.19_{-6.1\cdot 10^{-5}}^{+7\cdot 10^{-5}}$ day orbit. This corresponds to a bulk density of $5.62_{-1.28}^{+1.59}$ g cm$^{-3}$. Our interior structure analysis presents a small inner core representing $0.13^{+0.13}_{-0.11}$ of the solid mass fraction for the planet, surrounded by a mantle with a mass fraction of $0.69^{+0.20}_{-0.22}$, and an upper limit of the water layer of $0.16$. The gas mass below $10^{-8.93}$ indicates a very small amount of gas on the planet. We find no evidence of the second candidate found by the TESS pipeline, TOI-512.02, neither in TESS photometry, nor in the ESPRESSO radial velocities. The low stellar activity makes it an interesting transmission spectroscopy candidate for future-generation instruments.

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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

KOBE-1: The first planetary system from the KOBE survey. Two planets likely residing in the sub-Neptune mass regime around a late K-dwarf

K-dwarf stars are promising targets in the exploration of potentially habitable planets. Their properties, falling between G and M dwarfs, provide an optimal trade-off between the prospect of habitability and ease of detection. The KOBE experiment is a blind-search survey exploiting this niche, monitoring the radial velocity of 50 late-type K-dwarf stars. It employs the CARMENES spectrograph, with an observational strategy designed to detect planets in the habitable zone of their system. In this work, we exploit the KOBE data set to characterize planetary signals in the K7V star HIP 5957 (KOBE-1) and to constrain the planetary population within its habitable zone. We used 82 CARMENES spectra over a time span of three years. We employed a GLS periodogram to search for significant periodic signals that would be compatible with Keplerian motion on KOBE-1. We carried out a model comparison within a Bayesian framework to ensure the significance of the planetary model over alternative configurations of lower complexity. We also inspected two available TESS sectors in search of planetary signals. We identified two signals: at 8.5d and 29.7d. We confirmed their planetary nature through ruling out other non-planetary configurations. Their minimum masses are 8.80+/-0.76ME and 12.4+/-1.1ME, corresponding to absolute masses within the planetary regime at a high certainty (>99.7%). By analyzing the sensitivity of the CARMENES time series to additional signals, we discarded planets above 8.5ME within the habitable zone. We identified a single transit-like feature in TESS, whose origin is still uncertain, but still compatible within 1sigma with a transit from planet c. We have explored future prospects for characterizing this system, concluding that nulling interferometry with the LIFE mission could be capable of directly imaging both planets and characterizing their atmospheres in future studies.

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