Prof Suzanne Aigrain

Abstract:

Context . Young transiting exoplanets provide unique opportunities to probe planetary atmospheres during critical early phases of evolution when atmospheric escape and contraction are most active. HIP 67522 b, a 17 Myr old hot Jupiter with an extraordinarily low bulk density (<0.20 g cm −3 ), represents an ideal target for high-resolution transmission spectroscopy. Aims . We aim to constrain the mass and characterise the atmospheric composition, thermal structure, and dynamics of HIP 67522 b using ground-based high-resolution near-infrared spectroscopy with VLT/CRIRES+, complementing recent JWST observations. Methods . We obtained 92 high-resolution spectra ( R ≈ 10 5 ) with VLT/CRIRES+ in the K2166 band during a transit on 30 January 2025. We applied cross-correlation techniques and Bayesian nested sampling retrievals to constrain molecular abundances, temperature structure, and atmospheric dynamics. Results . We detected H 2 O at 20σ and CO at 5σ, confirming the extremely extended atmosphere of this low-mass giant. A velocity offset of −2.9 ± 0.2 km s −1 indicates day-to-night winds. The rotation velocity has been constrained to <1.8 km s −1 at 3σ, consistent with tidal locking. The retrieval analysis suggests a planetary mass of 27.7 −5.5 +5.9 Earth masses and statistically favours a two-temperature atmospheric structure with a discrete change at mbar pressures over an isothermal profile. This mass is twice as high as the mass estimated from JWST atmospheric observations and inconsistent at 3 σ , casting doubt on the actual planetary density of the planet. No matter the choice of atmospheric model, we derived a supersolar C/O ratio that is about 1.5 times solar, along with a supersolar metallicity that might further increase if the atmosphere is cloudy, which is a degeneracy that our data alone cannot resolve. We report a tentative 2 σ detection of HDO with an extreme enrichment factor of ∼1000 relative to the protosolar D/H ratio. If confirmed, this would be the first detection of deuterium in an exoplanet atmosphere and would require an intense escape rate to confirm its presence.

Abstract:

The radii of small exoplanets form two populations, super-Earths and sub-Neptunes, separated by a gap known as the radius valley. This could be produced by the removal of some atmospheres by stellar or internal heating, or the lack of an initial envelope. We use 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. The planets have orbital periods 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, while 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 it formed from gas-depleted material.