JWST transmission spectrum of HIP 67522b

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_ (>=7{sigma}), a modest detection of CO (3.5{sigma}), and weak detections of H_2_S and SO_2_ (~2{sigma}). 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 3x10 solar due to the amplitude of the SO_2_ feature.

These results point to the formation of HIP 67522b beyond the water snowline, where its envelope was polluted by icy pebbles and planetesimals. The planet is likely experiencing substantial mass loss (0.01-0.03M_{Earth}_/Myr), sufficient for envelope destruction within a gigayear. This highlights the dramatic evolution occurring within the first 100Myr of its existence.