H 2 -dominated Atmosphere as an Indicator of Second-generation Rocky White Dwarf Exoplanets

Abstract

<jats:title>Abstract</jats:title> <jats:p>Following the discovery of the first exoplanet candidate transiting a white dwarf (WD), a “white dwarf opportunity” for characterizing the atmospheres of terrestrial exoplanets around WDs is emerging. Large planet-to-star size ratios and hence large transit depths make transiting WD exoplanets favorable targets for transmission spectroscopy; conclusive detection of spectral features on an Earth-like planet transiting a close-by WD can be achieved within a medium James Webb Space Telescope program. Despite the apparently promising opportunity, however, the post-main sequence evolutionary history of a first-generation WD exoplanet has never been incorporated in atmospheric modeling. Furthermore, second-generation planets formed in WD debris disks have never been studied from a photochemical perspective. We demonstrate that transmission spectroscopy can identify a second-generation rocky WD exoplanet with a thick (∼1 bar) H<jats:sub>2</jats:sub>-dominated atmosphere. In addition, we can infer outgassing activities of a WD exoplanet based on its transmission spectra and test photochemical runaway by studying CH<jats:sub>4</jats:sub> buildup.</jats:p>