2024/09/24 by Pa Chia Thao, Andrew W. Mann, Thao, Pa Chia +68 · 1 voice · 13 citations
Physics and Astronomy · #Stellar, planetary, and galactic studies #Astronomy and Astrophysical Research #Gamma-ray bursts and supernovae
paper · pdf · doi:10.48550/arxiv.2409.16355
The characterization of young planets (< 300 Myr) is pivotal for understanding planet formation and evolution. We present the 3-5μm transmission spectrum of the 17 Myr, Jupiter-size (R ∼10R⊕) 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⊕ irrespective of cloud cover and stellar contamination. A Bayesian retrieval analysis returns a mass constraint of 13.8±1.0M⊕. 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/cm3, HIP 67522 b is one of the lowest density planets known. We find strong absorption from H2O and CO2 (≥7σ), a modest detection of CO (3.5σ), and weak detections of H2S and SO2 (≃2σ). 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 3×10 Solar due to the amplitude of the SO2 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⊕ Myr-1), sufficient for envelope destruction within a Gyr. This highlights the dramatic evolution occurring within the first 100 Myr of its existence.