Cold giant planets evaporated by hot white dwarfs

TL;DR

This paper explores how intense EUV radiation from hot white dwarfs can cause atmospheric evaporation of orbiting giant planets, leading to observable signatures and influencing white dwarf composition.

Contribution

It is the first to assess the impact of EUV irradiation from white dwarfs on orbiting giant planets and links planetary evaporation to observed white dwarf atmospheric pollution.

Findings

Giant planets around white dwarfs undergo significant hydrodynamic escape.

Evaporated volatiles can be detected as absorption lines in white dwarf spectra.

A large fraction of hot white dwarfs show signs of ongoing planetary atmospheric accretion.

Abstract

Atmospheric escape from close-in Neptunes and hot Jupiters around sun-like stars driven by extreme ultraviolet (EUV) irradiation plays an important role in the evolution of exo-planets and in shaping their ensemble properties. Intermediate and low mass stars are brightest at EUV wavelengths at the very end of their lives, after they have expelled their envelopes and evolved into hot white dwarfs. Yet the effect of the intense EUV irradiation of giant planets orbiting young white dwarfs has not been assessed. We show that the giant planets in the solar system will experience significant hydrodynamic escape caused by the EUV irradiation from the white dwarf left behind by the Sun. A fraction of the evaporated volatiles will be accreted by the solar white dwarf, resulting in detectable photospheric absorption lines. As a large number of the currently known extra-solar giant planets will survive the metamorphosis of their host stars into white dwarfs, observational signatures of accretion from evaporating planetary atmospheres are expected to be common. In fact, one third of the known hot single white dwarfs show photospheric absorption lines of volatile elements, which we argue are indicative of ongoing accretion from evaporating planets. The fraction of volatile contaminated hot white dwarfs strongly decreases as they cool. We show that accretion from evaporating planetary atmospheres naturally explains this temperature dependence if more than 50 per cent of hot white dwarfs still host giant planets.