The Featherweight Giant: Unraveling the Atmosphere of a 17 Myr Planet with JWST

Abstract

The characterization of young planets (< 300 Myr) is pivotal for understanding planet formation and evolution. We present the 3-5$\hat{I}¼$m transmission spectrum of the 17 Myr, Jupiter-size ($R$ $\sim$10$R_{\oplus }$) planet, HIP 67522 b, observed with JWST/NIRSpec/G395H. To check for spot contamination, we obtain a simultaneous $g$-band transit with SOAR. The spectrum exhibits absorption features 30-50% deeper than the overall depth, far larger than expected from an equivalent mature planet, and suggests that HIP 67522 b's mass is $<$20 $M_{\oplus }$ irrespective of cloud cover and stellar contamination. A Bayesian retrieval analysis returns a mass constraint of $13.8\pm 1.0M_{\oplus }$. This challenges the previous classification of HIP 67522 b as a hot Jupiter and instead, positions it as a precursor to the more common sub-Neptunes. With a density of $<$0.10g/cm${}^3$, HIP 67522 b is one of the lowest density planets known. We find strong absorption from H${}_2$O and CO${}_2$ ($\ge 7\ddot{I}ƒ$), a modest detection of CO (3.5$\ddot{I}ƒ$), and weak detections of H${}_2$S and SO${}_2$ ($\simeq 2\ddot{I}ƒ$). Comparisons with radiative-convective equilibrium models suggest supersolar atmospheric metallicities and solar-to-subsolar C/O ratios, with photochemistry further constraining the inferred atmospheric metallicity to 3$\times$10 Solar due to the amplitude of the SO${}_2$ feature. These results point to the formation of HIP 67522 b beyond the water snowline, where its envelope was polluted by icy pebbles and planetesimals. The planet is likely experiencing substantial mass loss (0.01-0.03 M${}_{\oplus }$ Myr${}^{-1}$), sufficient for envelope destruction within a Gyr. This highlights the dramatic evolution occurring within the first 100 Myr of its existence.

Accepted for publication in The Astronomical Journal; 32 pages, 18 figures, 7 tables