The search for potentially habitable planets is focused on exoplanets—planets orbiting other stars—for good reason. The only planet we know of with life is Earth and sunlight fuels life here. But some estimates say there are many more rogue planets roaming through space, not bound to or warmed by any star.

Could some of them support life?

The term ‘Rogue Planet’ is a colourful term used to describe what are actually interstellar objects (ISOs). But in the case of rogue planets, the ISOs are planetary-mass objects, rather than less massive objects like ‘Oumuamua or 2I/Borisov, the only two confirmed ISOs to enter our Solar System.

Rogue Planets have been somehow ejected from their solar systems. Young solar systems are chaotic places, where bodies collide with each other and where migrating gas giants can perturb smaller terrestrial planets from their orbits, sending them on an interstellar journey. It’s also possible that rogue planets form in interstellar space similar to how stars form. A planet could coalesce out of a cloud of gas and dust, along with a system of moons orbiting it. Sub-brown dwarfs are also considered rogue planets, but since they’re just gas, life is unlikely. In any case, rogue planets aren’t gravitationally bound to any star or stars. They’re free-floating.

We don’t know how many of them there are. If you ask Neil deGrasse Tyson there are billions of them in the Milky Way, maybe even trillions. Could any of them host life? Possibly.

One scientist at Florida Tech University has been studying the issue. Manasvi Lingam is an assistant professor of Aerospace, Physics, and Space Sciences at Florida Tech and has researched multiple topics in astrobiology, including the habitability of planets and moons outside of solar systems. Lingam published, together with the prolific Avi Loeb, a book titled “Life in the Cosmos: From Biosignatures to Technosignatures.” In 2019 the pair published a paper in the International Journal of Astrobiology called “Subsurface exolife” which examined planets with subsurface oceans and their potential for life. But instead of focusing only on exoplanets orbiting other stars, they looked at rogue planets that may do the same.

The Milky Way over the Very Large Array. How many rogue planets are there in the Milky Way? Billions? Trillions? Credit: NRAO/AUI/NSF; J. Hellerman
The Milky Way over the Very Large Array. How many rogue planets are there in the Milky Way? Billions? Trillions? Credit: NRAO/AUI/NSF; J. Hellerman

If there are, as deGrasse Tyson says, billions or trillions of rogue planets in the Milky Way, then it’s possible that the nearest exoplanet to us isn’t actually an exoplanet, but a rogue planet. And some of those planets could also be prime targets in the search for life, according to Lingam. “We normally think of planets bound to stars, such as Mars, that could support life, but in reality, these types of life-supporting planets could just be floating out there in the vast void of space with rich biospheres,” he said.

In an interview with Discover magazine, Lingam said, “You can certainly think of having something that’s bigger than microbes,” Lingam says. “Even if it’s not as complex as the most complex things we see here [on Earth].”

Rogue planets floating through the frigid conditions in interstellar space seem unlikely to support life, on the surface anyway. But here in our own Solar System, there are planets and moons so far from the Sun that they may as well be in interstellar space. Take Jupiter’s moon Europa for instance. Its surface is frozen, but underneath that surface is an ocean of liquid water, making it a prime target in our search for life. Could some rogue planets be like Europa?

What would it take for a rogue planet to support life? A combination of things, probably.

Working with the assumption that life needs liquid water, then a rogue planet needs a source of energy to prevent the water from freezing. The most likely scenario is a planet similar to the moons Europa, Ganymede, and Enceladus. Strong evidence shows that these bodies have thick layers of ice on their surfaces, with
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