Alain Lecavelier Des Etangs

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 ± 3 min) and a characteristic modulation timescale (1168 ± 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 ± 26 min) and the characteristic modulation timescale (2150 ± 110 d) remain tentative. These results highlight the importance of continued long-term monitoring of the AU Mic system.

Context . Following the 2022 alert of a TESS object of interest transiting TOI-5624 (a G7 V star ∼ 100 pc away), a CHEOPS campaign in 2023 detected four planetary signals at P _b ≈ 3.4, P _c ≈ 7.9, P _d ≈ 13.7, and P _e ≈ 21.5 days. These signals were later confirmed by additional TESS and CHEOPS photometry in 2024-2025. Aims . By using TESS and CHEOPS photometry, along with HARPS-N and SOPHIE high-resolution spectra, we determined the planet properties and performed a dynamical analysis of the system. Methods . After analysing the photometric data, we extracted and modelled the radial velocity (RV) time series using two independent methodologies, both within a Markov chain Monte Carlo 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. Results . We present the discovery of four transiting sub-Neptunes with radii of R _b = 2.314 ± 0.035 R _⊕ , R _c = 2.474 ± 0.042 R _⊕ , R _d = 3.584 _−0.050 ^+0.051 R _⊕ , and R _e = 3.247 _−0.043 ^+0.042 R _⊕ , along with masses of M _b = 9.4 ± 1.4 M _⊕ , M _c = 4.8 ± 1.9 M _⊕ , Md = 4.9 ± 2.2 M _⊕ , and M _e = 8.9 _−3.0 ^+2.9 M _⊕ . Our photometric analysis revealed that the outermost transiting planet TOI-5624e shows significant transit-timing variations (TTVs). Indeed, we found 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 labelled this non-transiting planet as TOI-5624 f and determined its minimum mass to be M _f sin i _f = 13.0 ± 3.7 M _⊕ . Conclusions . Among the known systems hosting more than four planets, the remarkable precision with which the radii have been measured (3σ) of the mass for at least three planets has previously been 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.

Exocomets have been detected orbiting several stars, using either absorption spectroscopy or photometry, yet their bulk composition remains essentially unconstrained. In particular, no direct measurement of the volatile-to-refractory ratio in individual exocomets has been reported to date. Here we present the first determination of the elemental C/Fe ratio in five exocomets transiting the young A-type star β Pictoris. Using new Hubble Space Telescope ultraviolet spectra, we model the absorption profiles of hundreds of lines from refractory ions and neutral carbon observed simultaneously, and constrain both the excitation state and the column densities of each species. After correcting for differential photoionisation, we find C/Fe≃20 in three well-characterised objects. These values are comparable to those measured in Solar System comets formed at large distances from the Sun and markedly higher than those found in carbonaceous chondrites, meteorites and terrestrial planets. This result provides the first direct measurement of the volatile-to-refractory ratio in exocomets and demonstrates that carbon-rich planetesimals formed in the outer regions of the β Pictoris system are currently being transported and destroyed within ∼1 au, revealing active large-scale redistribution of solids and volatiles in a young planetary system.

The radii of small exoplanets form two populations, super-Earths and sub-Neptunes, separated by a gap known as the radius valley. This feature could be produced by the removal of atmospheres by stellar or internal heating or by the lack of an initial envelope. We used transit photometry and radial velocity measurements to detect and characterize four exoplanets orbiting LHS 1903, a red dwarf star in the Milky Way’s thick disk. These four planets have orbital periods ranging from 2.2 to 29.3 days and span the radius valley within a single planetary system. The derived densities indicate that LHS 1903 b is rocky, whereas LHS 1903 c and LHS 1903 d have extended atmospheres. The most distant planet from the host star, LHS 1903 e, has no gaseous envelope, indicating that it formed from gas-depleted material.

We present the characterisation of the TOI-6041 system, a bright ( V = 9.84 ± 0.03) G7-type star hosting at least two planets. The inner planet, TOI-6041 b, is a warm Neptune with a radius of 4.55 _−0.17 ^+0.18 R _⊕ , initially identified as a single-transit event in TESS photometry. Subsequent observations with TESS and CHEOPS revealed additional transits, enabling the determination of its 26.04945 _−0.00034 ^+0.00033 orbital period and the detection of significant transit-timing variations (TTVs), exhibiting a peak-to-peak amplitude of about 1 hour. Radial-velocity (RV) measurements obtained with the APF spectrographs allowed us to place a 3 σ upper mass limit of 28.9 M _⊕ on TOI-6041 b. In addition, the RV data reveal a second companion, TOI-6041 c, on an 88 d orbit, with a minimum mass of 0.25 M _Jup . A preliminary TTV analysis suggested that the observed variations could be caused by gravitational perturbations from planet c; however, reproducing the observed amplitudes requires a relatively high eccentricity of about 0.3 for that planet. Our dynamical stability analysis indicates that such a configuration is dynamically viable and places a 1 σ upper limit on the mass of TOI-6041 c at 0.8 M _Jup . An alternative is the presence of a third, low-mass planet located between planets b and c, or on an inner orbit relative to planet b – particularly near a mean-motion resonance with planet b – which could account for the observed variations. These findings remain tentative, and further RV and photometric observations are essential in to better constrain the mass of planet b and to refine the TTV modeling, thereby improving our understanding of the system’s dynamical architecture.

Beta Pictoris is a young nearby A5V star, about 20 Myr old, that is embedded in a prominent debris disc. For the past 40 years, variable absorption features, produced by the gaseous tails of exocomets transiting the star, have been observed in the stellar spectrum. Yet, despite the large number of observations available, the origin and dynamical evolution of the exocomets remain poorly understood. Here we present new spectroscopic observations of β Pic obtained on April 29, 2025, with the Hubble Space Telescope and the High Accuracy Radial Velocity Planet Searcher. We report the detection of three strong exocomet signatures at low radial velocities (−7.5, +2.5, and +10 km/s) from a large set of lines from various species and excitation levels. We show that the three exocometary tails have different excitation states, indicating that they are located at different distances from the star. Using a detailed modelling of the excitation state of the transiting gas, which includes both radiative and collisional excitation, we derived the transit distance of the three exocometary gaseous tails to be 0.88 ± 0.08, 4.7 ± 0.3, and 1.52 ± 0.15 au. These values are much larger than previous estimates, which generally placed the transient features within 0.2 au. This reveals that gaseous tails produced by exocomets sublimating close to the star can expand and migrate over large distances while still remaining detectable in absorption spectroscopy. Our study provides a new method for measuring the transit distance of exocomets based on excitation modelling; this complements the acceleration method, which is only applicable to high-velocity objects.

With recent observational advancements, exocomet studies have entered a new era: we have moved from an epoch where exocomets' signatures were analysed to identify their true nature to a new epoch where individual exocomets' detections are studied in detail to characterise the observed bodies. In this context, as with other astronomical objects such as exoplanets and minor bodies in the solar system, a nomenclature system is needed to uniquely identify any observed exocometary body. Here, after outlining the purpose of a nomenclature and its required characteristics, we review the information on exocomets that can be included in the nomenclature. Finally, we propose an exocomet nomenclature scheme to identify the individual exocomets that have been discovered or yet to be discovered. Examples of nomenclature names of a few archetypal exocomets are provided.

With recent observational advancements, exocomet studies have entered a new era: we have moved from an epoch where exocomets’ signatures were analysed to identify their true nature to a new epoch where individual exocomets’ detections are studied in detail to characterise the observed bodies. In this context, as with other astronomical objects such as exoplanets and minor bodies in the solar system, a nomenclature system is needed to uniquely identify any observed exocometary body. Here, after outlining the purpose of a nomenclature and its required characteristics, we review the information on exocomets that can be included in the nomenclature. Finally, we propose an exocomet nomenclature scheme to identify the individual exocomets that have been discovered or yet to be discovered. Examples of nomenclature names of a few archetypal exocomets are provided.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics