The challenges of interstellar travel are immense. However, one scientist has proposed an easier (albeit very slow) way to move between stars: let free-floating planets carry us. If she's right, it could change how we search for life elsewhere in the universe.

Science fiction makes hopping between stars look easy, deepening the puzzle as to why aliens have not visited us. It's likely, however, that the truth is a great deal harder. Warp drives and other faster-than-light proposals may well be impossible. Even accelerating spacecraft capable of carrying humans to substantial fractions of the speed of light would take barely conceivable amounts of energy.

If it takes hundreds of years to reach the nearest star system, any interstellar vessel will need to be large enough to carry thousands of crew to create a self-sustaining civilization in transit. Building something suitably large would impose heavy costs on even a very advanced civilization, but Irina Romanovskaya of Houston Community College has an alternative suggestion. In the International Journal of Astrobiology, she explores the possibility of hitching a ride on a passing planet.

We know some planets travel through space without an accompanying star, known as free-floating or rogue planets. Some were probably thrown out by the internal dynamics of star systems, while others may have formed independently. Occasionally, such planets will happen to pass near – or even inside – a star system, just as comets Oumuamua and Borisov did to ours in recent years, and the star W0720 did 70,000 years ago. 

Romanovskaya argues such close passages would provide opportunities for those within that star system. These civilizations; “Would most likely encounter serious or insurmountable technical problems when using spacecraft to transport large populations over interstellar distances,” she writes.

They could, however, hop on board and wait for the planet to pass by some other star. To reach there, they might take up temporary residence on a minor planet like Sedna, with an orbit that goes from relatively close to the star to tolerably close to the passing planet.

Moreover, the most likely times for star systems to eject planets are at their beginnings and ends. For a civilization trapped in a dying star system, boarding a planet ejected by a star transitioning to a red supergiant may be the one way to survive.

Naturally, there are challenges. Free-floating planets would be hard, if not impossible, to steer. Instead of taking travelers straight to the nearest promising star system, it would be a much longer journey to some more distant star, and the first few to be approached might not be very inviting.

The freezing cold and cosmic radiation between the stars would make the surface of any such planet quite unsuitable for life. On the other hand, Romanovskaya argues nuclear power could make interior caverns comfortably warm. Most, if not all, of the elements needed to sustain civilizations for tens of thousands of years would be present naturally, rather than needing to be taken from the world the colonists left. Even gravity would be provided.

If Romanovskaya is right, we need to rethink our assumptions about where to search for extraterrestrial intelligence. Currently, if we detect what looks like a radio transmission from space – such as the 1977 “Wow! Signal” or 2019s BCL1 that appeared to come from Proxima Centauri – we keep looking in the same place. However, since it often takes years for radio data to be analyzed, a free-floating planet could have moved from our perspective in that time. We might need to expand the scope of such follow-up searches.

The work also provides an interesting twist on the Fermi Paradox, often phrased as “why aren't they here.” Most scenarios for colonizing the galaxy assume even slow colonization will eventually grab every habitable world. However, transport by rogue planets could be so slow that much of the galaxy, particularly towards the outer edges, could remain unexplored for billions of years.