H$_2$-dominated Atmosphere as an Indicator of Second-generation Rocky White Dwarf Exoplanets

TL;DR

This paper explores the potential of using transmission spectroscopy to identify second-generation rocky exoplanets around white dwarfs with thick hydrogen atmospheres, considering their unique formation and photochemical characteristics.

Contribution

It introduces a novel approach to detect and analyze second-generation WD exoplanets' atmospheres, incorporating their evolutionary history and photochemical processes.

Findings

Transmission spectroscopy can identify H2-dominated atmospheres on WD exoplanets.

Outgassing activities can be inferred from spectral features.

Photochemical runaway can be tested through methane buildup analysis.

Abstract

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 (JWST) program. Despite the apparently promising opportunity, however, the post-main sequence (MS) 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 ($\sim1$ bar) H$_2$-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$_4$ buildup.