Atmospheric characterization of HIP 67522 b with VLT/CRIRES+. VLT/CRIRES+ suggests a heavier planet and hints at deuterium fractionation
Abstract
Young transiting exoplanets provide unique opportunities to probe planetary atmospheres during the critical early phases of evolution. HIP 67522 b, a 17 Myr old hot Jupiter with an extraordinarily low bulk density, represents an ideal target for high-resolution transmission spectroscopy. We aim to characterize the atmospheric composition, thermal structure, and dynamics of HIP 67522 b using ground-based high-resolution near-infrared spectroscopy. We obtained high-resolution spectra 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. We detect H2O 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 is constrained to <1.8 km s-1 at 3σ, consistent with tidal locking. Retrieval analysis suggests a planetary mass of 29.8 3 Earth masses and a vertically isothermal atmosphere. This mass is two times larger than the mass estimated from JWST atmospheric observations and inconsistent at 3σ hence leaving a doubt on the actual planetary density of the planet. Using the mass derived from the CRIRES+ data, we derive a C/O ratio of 0.83 0.09, about 1.5 times solar, and a subsolar metallicity [C+O/H] = -0.8 0.4 which can be increased if the atmosphere is cloudy, a degeneracy our data alone cannot resolve. We report a tentative 2σ 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 intense escape rate to be explained.