Probing the Atmosphere of the 17 Myr, Gas Giant HIP 67522b Using JWST

The sample of young, transiting exoplanets have larger radii compared to their older counterparts. However, such comparisons are complicated by the challenges of measuring their masses in the presence of strong stellar activity. In this study, we report a 3-5μm atmospheric transmission spectrum of the 17 million-year-old, gas giant, HIP 67522b using the NIRSpec/G395H grating of JWST. We robustly detect significant absorption from H₂O and CO₂. Best-fit atmospheric models range between 30 and 100 times solar metallicity, with a subsolar C/O. The strength of the molecular features demonstrates that the planet has to be < 20 M_⊕ – dispelling the prior classification of HIP 67522b as a hot Jupiter based solely on its radius (10 R_⊕) and positioning it as a precursor to the more common sub-Neptunes. With a density of < 0.10g/cm³, HIP 67522b stands as the youngest super-puff known, boasting one of the lowest densities among all known planets.