The Fate of Oceans on First-Generation Planets Orbiting White Dwarfs

TL;DR

This paper explores the potential for water retention on planets orbiting white dwarfs, considering stellar evolution, planetary migration, and water loss mechanisms, and finds that some planets could retain surface water despite stellar death.

Contribution

It provides a detailed analysis of water retention on post-main-sequence planets, highlighting conditions under which planets can preserve surface oceans around white dwarfs.

Findings

Planets with substantial water reservoirs may retain surface water after stellar evolution.

Migration timing influences water retention, with later white dwarf ages favoring water preservation.

Stellar evolution can dehydrate planetary systems, but some planets may still hold onto surface oceans.

Abstract

Several groups have recently suggested that small planets orbiting very closely around white dwarf stars could be promising locations for life to arise, even after stellar death. There are still many uncertainties, however, regarding the existence and habitability of these worlds. Here, we consider the retention of water during post-main-sequence evolution of a Sun-like star, and during the subsequent migration of planets to the white dwarf's habitable zone. This inward migration is driven by dynamical mechanisms such as planet-planet interactions in packed systems, which can excite planets to high eccentricities, setting the initial conditions for tidal migration into short-period orbits. In order for water to persist on the surfaces of planets orbiting white dwarfs, the water must first survive the AGB phase of stellar evolution, then avoid being lost due to photoevaporation due to X-ray and extreme ultraviolet (XUV) radiation from the newly-formed white dwarf, and then finally survive the tidal migration of the planet inwards to the habitable zone. We find that while this journey will likely desiccate large swaths of post-main-sequence planetary systems, planets with substantial reservoirs of water may retain some surface water, especially if their migration occurs at later white dwarf cooling ages. Therefore, although stellar evolution may pose a challenge for the retention of water on exoplanet surfaces, it is possible for planets to retain surface oceans even as their host stars die and their orbits evolve.