The Astrobiological Potential of Rogue Planets

Not only could they hold subsurface life, they may spread it throughout the galaxy.

According to the so-called “giant impact” theory, Earth's moon was created when another large body smashed into our planet early in its history. Such collisions may be one way life spreads throughout the galaxy.

When we think about extraterrestrial life on another planet, one common assumption is that we’re talking about a planet orbiting a star, which provides it with the necessary energy. However, that may not always be so. 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.

If a wandering planet passes close to a habitable rocky planet within a solar system, the outer layer of the rogue planet might be torn apart by gravitational disturbances, and the resulting debris could end up on the habitable world. Dormant life that had been trapped in the icy shell of the rogue planet may become active again and establish a biosphere on the receiving planet.

Alternatively, the two planets could collide, or come close to colliding. This is generally believed to have a sterilizing effect, as when a Mars-sized object (probably a rogue planet!) collided with the early Earth, resulting in the creation of our Moon. But the distribution of energy and matter during such an event would be very uneven, and some localities might not experience temperatures high enough for sterilization. A common atmosphere may even form, which I think was the case for early Earth and Moon shortly after the cataclysmic impact. Some (dormant) microbial life may survive suspended in such an atmosphere for a long time, before eventually settling down when the surface has cooled off and become more habitable again.

Two other scenarios have been discussed in the literature—one by Chandra Wickramasinghe and the other by Manasvi Lingam and Avi Loeb, who suggested that a rogue planet may be caught by the gravitational force of a solar system to enter orbit around a star. Neptune’s moon Triton is thought to have been captured in this way by the planet’s gravitation. A subsequent transfer of life would then be more straightforward, especially if the planetary system is closely packed with planets and moons.

All these scenarios for panspermia are highly speculative, as they can only occur under very constrained environmental conditions, which are likely to be quite rare. However, given the estimates for how many rogue planets exist—as many as 60 for every star in our galaxy—we suggest looking further into the mechanisms by which these wanderers could spread life to other worlds.

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