Dominique J. M. Petit Dit de la Roche

HIP 67522 is a one of the youngest multi-planetary systems discovered to date. The 17 Myr-old, Sun-like star is part of the Scorpius-Centarus OB association and is known to host two Saturn-sized planets in a near 2:1 mean motion resonance. We analysed photometric transits observed with the CHaracterising ExOPlanet Satellite (CHEOPS), the Transiting Exoplanets Survey Satellite (TESS), and multiple ground-based facilities to search for transit timing variations induced by planet-planet gravitational interactions, as well as transit depth variations, linked with the changing coverage of active regions on the stellar surface. We do not detect any transit timing variations for HIP 67522 b exceeding two minutes, contrary to previous results from the James Webb Space Telescope. In addition, we found that the observed transit depth of planet b changes strongly over time, with a >30% variation in amplitude. The high sensitivity and blue bandpass of the CHEOPS satellite also enabled us to identify multiple starspot crossings and measure their properties, including spot temperatures and sizes. We also measured the shear of the host star’s differential rotation by modelling the out-oftransit photometric variability and concluded that HIP 67522 exhibits a supersolar differential rotation. Based on internal structure models and planet formation simulations, we find that more than 20% of the mass of HIP 67522 b, at the system’s current age, is likely made of gaseous envelopes. Evolutionary simulations further indicate that both planets in the system will experience significant photoevaporation, evolving into either Earth size planets with completely stripped atmospheres or sub-Neptune-sized planets.

Accurate modeling of stellar active regions (ARs) remains a major bottleneck for radial-velocity and transmission-spectroscopy studies aimed at finding Earth-like planets. While much effort has been devoted to AR modeling, their center-to-limb variations (CLVs) have been largely overlooked. We took a step toward remedying this by measuring the CLV of the 6173 Å continuum intensity for sunspots (the whole spot, separate umbrae, and penumbrae), faculae, network, and the quiet Sun using the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). This study is based on four simple round α -sunspots and their surroundings, as well as one strongly evolving region. After correcting for stray light, we find that relative to the quiet Sun, all components except for the umbra display reduced darkening toward the limb. Additionally, strongly evolving ARs do not appear to display significantly altered CLV profiles compared to stable ARs. Faculae and network show contrast enhancements that peak near µ ≈ 0.3 before declining toward the limb, reaching maxima of approximately 4% and 2%, respectively, in contrast excess relative to the quiet Sun, while the spot-to-quiet-Sun contrast rises to approximately 15% near the limb. For both types of AR, this change in CLV behavior near the limb is likely related to the 3D structure of the ARs and the rapidly changing viewing geometry. This behavior is not captured by synthetic CLVs based on PHOENIX and ATLAS model atmospheres with solar values and a different effective temperature, underscoring the need for more realistic treatments of stellar activity.

The center-to-limb variations (CLVs) of photospheric and chromospheric spectral lines were obtained in 2025 July and August using drift scans from the echelle spectrograph of the 0.7 m Vacuum Tower Telescope at the Observatorio del Teide (ODT) in Tenerife, Spain. This instrument can observe four spectral regions simultaneously, enabling multi-line spectroscopy with high spectral resolution of various activity features and the quiet Sun in the lower solar atmosphere. The initial results of Halpha observations demonstrate the diagnostic potential of drift scans obtained with a ground-based, high-resolution telescope. Data products include spectroheliograms and maps of physical parameters such as line-of-sight velocity, line width, and line-core intensity. The combination of the CLV from photospheric and chromospheric lines, as well as the wide range of formation heights of the selected lines, renders this dataset ideal for characterizing stellar and exoplanet atmospheres.

The center-to-limb variations (CLVs) of photospheric and chromospheric spectral lines were obtained in 2025 July and August using drift scans from the echelle spectrograph of the 0.7 m Vacuum Tower Telescope at the Observatorio del Teide (ODT) in Tenerife, Spain. This instrument can observe four spectral regions simultaneously, enabling multi-line spectroscopy with high spectral resolution of various activity features and the quiet Sun in the lower solar atmosphere. The initial results of H α observations demonstrate the diagnostic potential of drift scans obtained with a ground-based, high-resolution telescope. Data products include spectroheliograms and maps of physical parameters such as line-of-sight velocity, line width, and line-core intensity. The combination of the CLV from photospheric and chromospheric lines, as well as the wide range of formation heights of the selected lines, renders this dataset ideal for characterizing stellar and exoplanet atmospheres.

Context. Transmission spectroscopy is a powerful tool for understanding exoplanet atmospheres. At optical wavelengths, this technique 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 activity levels of the host star. Methods. We obtained three nights of spectrophotometric data with the FORS2 instrument on the VLT, covering a wavelength range of 340–1100 nm. These were divided into 10 nm binned spectroscopic light curves, which were fit with a combination of Gaussian processes and parametric models to obtain a transmission spectrum. 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 ΔT = + 644₋₂₆₃⁺⁴²⁷ K for the former and a stellar coverage fraction of around 30% with a temperature of ΔT = +231 ± 72 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(H₂O)=− 2.01_−0.86^+0.54 and log(NH₃)=−3.4_−5.20^+0.96, respectively, and place 3σ upper limits on TiO (−7.65) and K (−7). We see a lack of evidence of Na, which we attribute to the presence of clouds. 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.

Context. The nature of the close-in rocky planet 55 Cnce is puzzling, despite it having been observed extensively. Its optical and infrared occultation depths show temporal variability, in addition to a phase curve variability observed in the optical. Aims. We wish to explore the possibility that the variability originates from the planet being in a 3:2 spin–orbit resonance, and 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. Methods. JWST/NIRCam (Near Infrared Camera) observed five occultations (secondary eclipses) of the planet – of which four were observed within a week – simultaneously at 2.1 and 4.5 µm. While the former gives band-integrated photometry, the latter provides a spectrum between 3.9–5.0 µm. Results. 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 119₋₁₉⁺³⁴ ppm at 2.1 µm and 4.5 µm, 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 µm varies between 873–2256 K and is lower than the expected day-side temperature of bare rock with no heat redistribution (2500 K), which is indicative of an atmosphere. Our atmospheric retrieval analysis of occultation depth spectra at 4.5 µm 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. Conclusions. The observed variability could be explained by stochastic outgassing of CO/CO₂, which is also hinted at by retrievals. Alternatively, the variability observed at both 2.1 and 4.5 µm could be the result of a circumstellar patchy dust torus generated by volcanism on the planet.

Transmission spectroscopy is a proven technique for studying a transiting exoplanet’s atmosphere. However, stellar surface inhomogeneities – spots and faculae – alter the observed transmission spectra: the stellar contamination effect. The variable nature of the stellar activity also makes it difficult to stitch together multi-epoch observations and evaluate any potential variability in the exoplanet’s atmosphere. This paper introduces SAGE, a tool that corrects for the time-dependent impact of stellar activity on transmission spectra. It uses a pixelation approach to model the stellar surface with spots and faculae, while fully accounting for limb-darkening and rotational line-broadening. The current version is designed for low- to medium-resolution spectra. We used SAGE to evaluate stellar contamination for F- to M-type hosts, testing various spot sizes and locations, and quantify the impact of limb-darkening. We find that limb-darkening enhances the importance of the spot location on the stellar disc, with spots close to the disc centre impacting the transmission spectra more strongly than spots near the limb. Moreover, due to the chromaticity of limb-darkening, the shape of the contamination spectrum is also altered. Additionally, SAGE can be used to retrieve the properties and distribution of active regions on the stellar surface from photometric monitoring. We demonstrate this for WASP-69 using TESS data, finding that two spots at midlatitudes and a combined coverage fraction of ~1% are favoured. SAGE allows us to connect the photometric variability to the stellar contamination of transmission spectra, enhancing our ability to jointly interpret transmission spectra obtained at different epochs.

Wasp-15 b is an inflated hot Jupiter orbiting a bright host star. Its low density and consequent large atmospheric scale height make it an excellent candidate for atmospheric characterization using transmission spectroscopy. In fact, it has previously been observed with the FORS2 spectrograph on the VLT, but large systematics have so far prevented this data from being used. Here, we show that Gaussian Process modelling can remove systematic noise features with amplitudes up to that of the transit signal, allowing us to achieve a precision comparable to later data without the systematics. We present the first transmission spectrum of the atmosphere of Wasp-15 b and compare it to theoretical spectra to discuss the implications.

Commercial disco balls provide a safe, effective and instructive way of observing the Sun. We explore the optics of solar projections with disco balls, and find that while sunspot observations are challenging, the solar disk and its changes during eclipses are easy and fun to observe. We explore the disco ball’s potential for observing the moon and other bright astronomical phenomena.