- Steven J Desch: School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA.
- Alan P Jackson: School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA.
The interstellar objects 1I/'Oumuamua and 2I/Borisov confirm the long-held expectation that bodies from one stellar system will be carried to another, allowing, interstellar panspermia. Life might be transferred between stellar systems, depending on the nature of the bodies and how they escaped their systems. 2I/Borisov appears to be a comet, with no more likelihood of carrying life than Solar System comets. In contrast, the nature of 1I/'Oumuamua has been difficult to determine. We review various hypotheses for its origin, including ejection of N ice from the surface of an exo-Pluto, formation in a molecular cloud by freezing of H, and a derelict solar sail of alien construction. Of these, the N ice fragment hypothesis is uniquely falsifiable, plausible, and completely consistent with all observations. The possibility of interstellar panspermia would be made more probable if 'Oumuamua originated on a dwarf planet rather than a comet, although substantial challenges to transfer of life would remain. Of proposed mechanisms for interstellar panspermia, transfer of life via rocky meteoroids is perhaps less improbable.