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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 10 ^14 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 10 ^12 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 $a^{\prime} =a/{(1-{e}^{2})}^{1/4}$ , 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.

We leverage Gaia DR2 parallactic distances to deliver new or revised estimates of planetary parameters and X-ray irradiation for a distance-limited (≲100 pc) sample of 27 gaseous planets (from super-Earths to hot Jupiters) with publicly available Chandra and/or XMM observations, for which we carry out a homogeneous data reduction. For 20 planets with X-ray-detected host stars we make use of the photoionization hydrodynamics code ATES to derive updated atmospheric mass outflow rates. The newly derived masses/radii are not consistent with the exoplanet.eu values for five systems: HD 149026b and WASP-38, for mass, and Au Mic b, HAT-P-20, and HAT-P-2 for radii. Notably, the lower mass implies a (Saturn-like) density of 0.86 ± 0.09 g cm ^−3 for HD 149026b. This independent estimate is consistent with the lowest values reported in the literature. Separately, we report on the X-ray detection of GJ 9827, HD 219134, and LHS 1140 for the first time. The inferred stellar X-ray luminosity of LHS 1140 ( ${1.34}_{-0.21}^{+0.19}\times {10}^{26}$ erg s ^−1 ) implies that LHS 1140 b is the least irradiated transiting super-Earth known to orbit within the habitable zone of a nearby M dwarf.