The Steppenwolf: A proposal for a habitable planet in interstellar space

TL;DR

This paper explores the potential for rogue planets to sustain subsurface liquid oceans due to geothermal heat, proposing that Earth-like rogue planets with increased mass or water content could remain habitable in interstellar space.

Contribution

It introduces a model showing how rogue planets could maintain liquid oceans beneath ice layers, expanding the possibilities for extraterrestrial habitability beyond traditional star-bound planets.

Findings

A ~3.5 Earth-mass rogue planet can sustain a subglacial ocean.

Planets with higher water fractions or cryo-atmospheres require less mass (~0.3 Earth masses).

Such planets could be detected via reflected light and thermal IR if passing within ~1000 AU.

Abstract

Rogue planets have been ejected from their planetary system. We investigate the possibility that a rogue planet could maintain a liquid ocean under layers of thermally-insulating water ice and frozen atmosphere as a result of geothermal heat flux. We find that a rogue planet of Earth-like composition and age could maintain a subglacial liquid ocean if it were ~3.5 times more massive than Earth. If a rogue planet had about ten times higher water mass fraction or a thick cryo-atmospheric layer, it would need to be only ~0.3 times the mass of Earth to maintain a liquid ocean. Such a planet could be detected from reflected solar radiation and its thermal emission could be characterized in the far-IR if it passed within O(1000) AU of Earth.