A new paper asks us to think about the possibilities of planets that have been kicked out of a predictable orbit:

In a new paper, Alberto Fairén from the Center of Astrobiology in Madrid, Spain, and I look into the possibility that planets wandering through interstellar space could also host life. These “rogue” planets may have been ejected out of their original solar system during the early, chaotic phase of planetary formation.

There are two general types of rogue planets: gas giants like Saturn and Neptune, and rocky Earthlike planets. While the chances for life on gas giants is extremely remote, rocky migrating planets could in principle host microbial life. To do that, they would need internal heat from the decay of radioactive elements that keep water—or some other suitable solvent—liquid beneath a frozen surface.

This is similar to what we believe happens on Jupiter’s moon Europa, where chemical, thermal, or even osmotic gradients could act as a life-sustaining energy source. If the planet is really large and there is more radiogenic heat available than on Earth, the planet could also retain a thick hydrogen or nitrogen atmosphere, which may make life feasible.

There is even more to the astrobiological potential of rogue planets, however. Not only could they hold microbial life bottled up in their subsurface, they may be able to distribute life throughout the galaxy. In our paper we suggest two ways that this kind of panspermia might occur.

Dirk Schulze-Makuch, “The Astrobiological Potential of Rogue Planets” at AirSpaceMag (August 24, 2021) The paper is open access.

Wandering planets is a romantic idea but there may be something in it.

Actually, Earth — admittedly pinned in place — is a “rogue planet” in one sense. Years ago, biologist Peter Ward and astronomer Donald Brownlee wrote a book called Rare Earth: Why Complex Life is Uncommon in the Universe. The title speaks for itself. Astronomer Guillermo Gonzalez and business prof Jay Richards followed up with The Privileged Planet Both books made the point, coming from different perspectives, that Earth is unusually, well, even ridiculously well-suited for life.

It may be that generally, the planets that feature life forms are rogue planets. Maybe we should look for wanding planets that pack moons:

You may also wish to read: New class of Hycean exoplanets may feature life. The new James Webb Telescope will enable much clearer resolution for the composition believed necessary for hosting life. Exoplanets that have been overlooked because they are un-Earthlike may feature oceans that extremophiles could live in, Cambridge astronomers say.