Aerosols and hydrocarbons in the atmosphere of a white dwarf planet

Key Points:

• The JWST NIRSpec PRISM observations of WD 1856 b were reduced using two independent pipelines, FIREFLy and Juniper, both extracting transmission spectra and fitting transit light curves with custom noise and outlier treatments to handle the unique grazing transit geometry.
• Due to WD 1856 b’s large size relative to its white dwarf host and grazing transit, the transmission spectrum was modeled using the wavelength-dependent maximum transit depth at mid-transit, accounting for overlap area and nightside thermal emission contamination, rather than the usual planet-to-star radius ratio squared.
• Atmospheric retrievals with the POSEIDON code included 25 free parameters modeling molecular abundances, temperature profile, and aerosols; results indicate a warm (~400 K) atmosphere with methane and clouds/hazes, consistent across both data reductions.
• Mie scattering retrievals tested various aerosol compositions, finding several species (e.g., KCl, NH3 ice, organic hazes) can explain the observed scattering slope, but current data do not strongly favor a specific aerosol type.
• Thermal evolution modeling using substellar cooling grids and constraints from atmospheric retrievals suggests WD 1856 b was reheated about 4–4.5 Gyr after the white dwarf formed, favoring reheating during high-eccentricity migration over common-envelope evolution, with irradiation from the white dwarf playing a minor role.