Rogue Planets Could be Habitable

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

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 oceans of water underneath. Europa could even have twice as much water as Earth.

Artist’s impression of Europa’s interior, based on data obtained by Galileo space probes. Europa could have twice as much water as Earth. Credit: NASA

The heat that prevents a rogue planet from freezing completely would come from the planet’s interior. Earth has a lot of geothermal energy emanating from its core. It’s reasonable to assume that some rogue planets have the same. Of course, only a tiny percentage of Earth’s energy comes from its core. The Sun provides over 99.9% of Earth’s energy, so this scenario, though realistic, is challenging for life. A rogue planet would have very little energy to work with.

Rogue planets face another problem in the cold darkness of interstellar space. If it started out in its own solar system with an atmosphere, that same atmosphere would freeze and fall to the ground in interstellar space. Earth’s atmosphere plays a critical role in preserving heat and moderating our climate. How could rogue planets get by without one?

Maybe they don’t need one. Europa has an extremely tenuous oxygen atmosphere. So does Ganymede. Enceladus has a thicker atmosphere, but nothing like Earth’s. It’s highly unlikely that a rogue planet would retain a gaseous atmosphere capable of trapping heat.

There’s at least one exception. An extremely dense hydrogen atmosphere could resist freezing and potentially trap heat. It could trap enough heat to keep surface water from freezing. We don’t know if there are any rocky planets with hydrogen atmospheres, and if there are they’re exceedingly rare. But experiments show that at least some organisms can live in a hydrogen atmosphere.

A rogue planet with a massive moon might have better odds of supporting life. A massive enough moon could cause the planet to undergo tidal heating. Tidal heating doesn’t seem to be rare, though in our own Solar System the gas giant Jupiter causes tidal heating in the moon Europa. So maybe in a rogue planet system with its own moons, a similar thing can happen: it’s the moon that stays warm and has a subsurface ocean instead of the planet.

Images from NASA’s Galileo spacecraft show the intricate detail of Europa’s icy surface. Image: NASA/JPL-Caltech

Lingam says that there’s another possibility. If a rogue planet is near the galactic core, and the galaxy has an active galactic nucleus (AGN) then it’s theoretically possible that it receives enough light for photosynthesis to take place. According to Lingam, there’s enough energy to support photosynthesis less than about 1,000 light-years from an AGN.

We know life can exist without sunlight, down at the bottom of an ocean. Earth hosts entire biological communities near hydrothermal vents on the ocean floor. These vents are called black smokers, and they produce a stream of minerals that serve as food for chemosynthetic bacteria. These bacteria attract other organisms that feed on them. Those organisms, in turn, attract predators and an entire food chain manifests. Rogue planets with geothermal heating could have similar communities.

Life without energy from a star could rely on hydrothermal vents. Credit: NOAA

If some rogue planets do carry life through interstellar space then they may play a role in panspermia. Panspermia is the idea that either the ingredients for life or life itself can spread throughout a galaxy by hitching rides on interstellar objects. Rogue planets seem like ideal candidates for vehicles for panspermia. Our Solar System will have sent its own rogue planets and ISO out into interstellar space. Maybe they’re spreading life throughout the galaxy.

Rogue planets with frozen surfaces and subsurface oceans might have one advantage over planets like Earth: they’re protected by an icy shield. Europa has a layer of ice that’s between 10–30 km (6–19 mi.) thick. Think of it as an asteroid shield. We know that asteroid strikes can have dire effects on a planet, can cause mass extinctions and change the whole course of evolution. Would an impactor the size of the Chicxulub impactor be able to disrupt life on a rogue planet the way it did on Earth? Maybe not.

So far, a lot of this is conjecture. How can we find out more about rogue planets?

First, we have to lay our eyes on some. The upcoming Vera C. Rubin Observatory will specialize in finding transient objects and phenomena. The Rubin Observatory has a 10-year mission, and during that time it could find as many as 50 ISOs, including rogue planets.

Once we find some, we have to find a way to visit one. Manasvi Lingam and colleagues tackled that problem in a paper titled “Interstellar Now! Missions to and Sample Returns from Nearby Interstellar Objects.” The authors of that paper say that the in-situ study of these objects is the next step. It’s the only way to study a rogue planet’s composition and its chemical and isotopic structure. They talk about possible options for flybys of rogue planets and even getting a lander to the surface.

Rubin Observatory at sunset, lit by a full moon. Credit: Rubin Observatory/NSF/AURA

But what we really need is a sample. For lower-mass ISOs similar to ‘Oumuamua, a high-speed impactor could be used. It could blast material from the surface to be collected by a spacecraft during a flyby and returned to Earth. But for a planet-sized object, that’s likely impossible. It’s not clear how we could collect a sample from a rogue planet. That may be out of reach technologically, at least for now.

The ESA has a plan to send a spacecraft to visit an ISO as it enters our inner Solar System. It’s called the Comet Interceptor and it would launch before it knew what its target is. The spacecraft would be parked at the Sun-Earth L2 point where it would wait. Once a suitable ISO was found, the spacecraft would be sent to rendezvous with it. The idea is centred around long-period comets but could be adapted to ISOs, at least interstellar comets. It’s not too difficult to see how it could be further developed to visit an actual rogue planet.

NASA’s working on a similar mission called the Extrasolar Object Interceptor and Sample Return. NASA envisions launching a spacecraft toward Jupiter and waiting for an ISO to approach. Then it would be directed toward the ISO to gather a sample and return it to Earth.

We can’t travel to another star system. Maybe one day in our science fiction future, but not any time soon. But thanks to rogue planets and other ISOs, other star systems are sending us the evidence we need. We just have to find a way to study it.

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Evan Gough

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