Evaporation of Close-in Sub-Neptunes by Cooling White Dwarfs

Abstract

Motivated by the recent surge in interest concerning white dwarf (WD) planets, this work presents the first numerical exploration of WD-driven atmospheric escape, whereby the high-energy radiation from a hot/young WD can trigger the outflow of the hydrogen–helium envelope for close-in planets. As a pilot investigation, we focus on two specific cases: a gas giant and a sub-Neptune-sized planet, both orbiting a rapidly cooling WD with mass M * = 0.6 M ⊙ and separation a = 0.02 au. In both cases, the ensuing mass outflow rates exceed 1014 g s−1 for WD temperatures greater than T WD ≳ 50,000 K. At T WD ≃ 18,000 K (22,000 K), the sub-Neptune (gas giant) mass outflow rate approaches 1012 g s−1, i.e., comparable to the strongest outflows expected from close-in planets around late main-sequence stars. Whereas the gas giant remains virtually unaffected from an evolutionary standpoint, atmospheric escape may have sizable effects for the sub-Neptune, depending on its dynamical history, e.g., assuming that the hydrogen–helium envelope makes up 1% (4%) of the planet mass, the entire envelope would be evaporated away so long as the planet reaches 0.02 au within the first 230 Myr (130 Myr) of the WD formation. We discuss how these results can be generalized to eccentric orbits with effective semimajor axis a′=a/(1−e2)1/4 , which receive the same orbit-averaged irradiation. Extended to a much broader parameter space, this approach can be exploited to model the expected demographics of WD planets as a function of their initial mass, composition, and migration history, as well as their potential for habitability.