- Dirk Schulze-Makuch: Astrobiology Group, Center for Astronomy and Astrophysics, Technische Universität Berlin, 10623 Berlin, Germany.
- Alberto G Fairén: Centro de Astrobiología (CSIC-INTA), 28850 Madrid, Spain. ORCID
There are two types of rogue planets, sub-brown dwarfs and "rocky" rogue planets. Sub-brown dwarfs are unlikely to be habitable or even host life, but rocky rogue planets may have a liquid ocean under a thick atmosphere or an ice layer. If they are overlain by an insulating ice layer, they are also referred to as Steppenwolf planets. However, given the poor detectability of rocky rogue planets, there is still no direct evidence of the presence of water or ice on them. Here we discuss the possibility that these types of rogue planets could harbor unicellular organisms, conceivably based on a variety of different energy sources, including chemical, osmotic, thermal, and luminous energy. Further, given the theoretically predicted high number of rogue planets in the galaxy, we speculate that rogue planets could serve as a source for galactic Panspermia, transferring life to other planetary systems.