NASA/JPL/University of Arizona/University of Colorado
Underneath, they're not so cold. Rogue planets with frozen surfaces may have liquid oceans beneath that could support life. Scientists believe a subsurface ocean is most likely present on Europa (above), one of Jupiter's moons.
Underneath, they're not so cold. Rogue planets with frozen surfaces may have liquid oceans beneath that could support life. Scientists believe a subsurface ocean is most likely present on Europa (above), one of Jupiter's moons. NASA/JPL/University of Arizona/University of Colorado
Here is a link to an interesting paper from the astro pre-prints archive. It's likely that the process of planet formation is a messy affair with newborn worlds being kicked into deep space by gravitational interactions with their siblings. What happens to these planets after they are orphaned?
The authors Dorian S. Abbot, Eric R. Switzer argue that even though these worlds will wander forever in the frozen depths of space, they might still harbor life. Taking a cue from Europa, the frozen ocean moon of Jupiter, they claim
"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."
These dark space planets might even be found if they came close enough to the solar system. "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 1000 AU of Earth." That's about 1000 times the distance of the Earth from the Sun.