The search for potentially habitable planets focuses on exoplanets – planets orbiting other stars, for good reason. The only planet we know there is life is Earth, and sunlight powers life here. But some estimates suggest that there are many more rogue planets roaming space that are not attached to or heated by a star.
Could some of them support life?
The term ‘rogue planet’ is a colorful term used to describe what interstellar objects (ISOs) actually are. But in the case of rogue planets, the ISOs are objects of planetary mass and no less massive objects like ‘Oumuamua or 2I / Borisov, the only two confirmed ISOs that have entered our solar system.
Rogue planets were somehow expelled from their solar systems. Young solar systems are chaotic places where bodies collide with one another and where wandering gas giants can tear smaller terrestrial planets out of their orbits and send them on an interstellar journey. It is also possible for rogue planets to form in interstellar space, much like stars. A planet could grow together from a cloud of gas and dust together with an orbiting lunar system. Sub-brown dwarfs are also considered rogue planets, but since they are just gas, life is unlikely. In any case, rogue planets are not gravitationally tied to one or more stars. You are floating freely.
We don’t know how many there are. If you ask Neil deGrasse Tyson, there are billions of them in the Milky Way, maybe even trillions. Could one of them harbor life? Possibly.
A Florida Tech University scientist investigated the subject. Manasvi Lingam is an assistant professor of aerospace, physics, and space science at Florida Tech and has researched several topics in astrobiology, including the habitability of planets and moons outside of solar systems. Lingam published a book with prolific Avi Loeb called “Life in the Cosmos: From Biosignatures to Technosignatures”. In 2019, the two published an article in the International Journal of Astrobiology entitled “Subsurface Exolife,” which examined planets with subterranean oceans and their potential for life. But instead of just focusing on exoplanets orbiting other stars, they looked at rogue planets that could 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. Hellermann
If, as deGrasse Tyson says, there are billions or trillions of rogue planets in the Milky Way, then it is possible that the exoplanet closest to us is not an exoplanet but a rogue planet. And some of these planets could also be prime targets in the search for life, according to Lingam. “Usually we think of planets attached to stars like Mars that could support life, but in reality these types of life-sustaining planets could just float out there in the vast void of space with rich biosphere,” he said.
In an interview with Discover magazine, Lingam said, “It’s easy to imagine having something 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 due to the cold 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 might as well be in interstellar space. Take, for example, Jupiter’s moon Europa. Its surface is frozen, but beneath that surface is an ocean of liquid water, making it a prime target in our search for life. Could some rogue planets be like Europe?
What does a rogue planet need to sustain life? A combination of things, probably.
Assuming that life needs liquid water, a rogue planet needs a source of energy to keep 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 surface, with oceans of water underneath. Europe could even have twice as much water as the earth.
Artist’s impression of the interior of Europe, based on data from Galileo spacecraft. Europe could have twice as much water as the earth. Photo credit: NASA
The heat preventing a rogue planet from completely freezing would come from within the planet. The earth has a lot of geothermal energy emanating from its core. It is reasonable to assume that some rogue planets have the same thing. 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, while realistic, poses a challenge to 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 with an atmosphere in its own solar system, that same atmosphere in interstellar space would freeze and fall to the ground. The earth’s atmosphere plays a crucial role in maintaining warmth and mitigating our climate. How could rogue planets do without one?
Maybe they don’t need one. Europe has an extremely weak oxygen atmosphere. Ganymede too. Enceladus has a thicker atmosphere, but nothing like that of Earth. A rogue planet is very unlikely to maintain a gaseous atmosphere capable of trapping heat.
There is at least one exception. An extremely dense hydrogen atmosphere could resist freezing and possibly trap heat. It could trap enough heat to prevent surface water from freezing. We don’t know if there are rocky planets with hydrogen atmospheres, and if so, they are extremely rare. However, experiments show that at least some organisms can live in a hydrogen atmosphere.
A rogue planet with a massive moon might have better chances of sustaining life. A moon of sufficient mass could cause the planet to be heated by the tides. Tidal warming does not seem to be uncommon, although in our own solar system the gas giant Jupiter in the moon Europa provides tidal warming. So maybe something similar can happen in a rogue planetary system with its own moons: The moon stays warm and has an underground ocean instead of the planet.
Images from NASA’s Galileo spacecraft show the intricate details of Europe’s icy surface. Image: NASA / JPL-Caltech
Lingam says there is another option. If a rogue planet is near the galactic core and the galaxy has an active galactic core (AGN), it is theoretically possible that it will receive enough light for photosynthesis to take place. According to Lingam, there is enough energy to support photosynthesis less than 1,000 light years away from an AGN.
We know that life can exist without sunlight, at the bottom of an ocean. The earth is home to entire biological communities in the vicinity of hydrothermal springs on the ocean floor. These openings are called black smokers and produce a stream of minerals that serve as nourishment for chemosynthetic bacteria. These bacteria attract other organisms that feed on them. These organisms, in turn, attract predators and an entire food chain manifests itself. Geothermal rogue planets could have similar communities.
Life without energy from a star could rely on hydrothermal sources. Image credit: NOAA
If some rogue planets carry life through interstellar space, they may play a role in panspermia. Panspermia is the idea that either the ingredients for life or life itself can spread across a galaxy by hitchhiking on interstellar objects. Rogue planets appear to be ideal candidates for vehicles for panspermia. Our solar system will have sent its own rogue planets and ISO into interstellar space. Perhaps they are spreading life across the galaxy.
Rogue planets with frozen surfaces and subterranean oceans might have an advantage on planets like Earth: They are protected by an icy shield. Europe has a layer of ice that is between 10 and 30 km thick. Think of it as an asteroid shield. We know that asteroid strikes can have devastating effects on a planet, cause mass extinctions and change the entire course of evolution. Would an impactor the size of the Chicxulub impactor be able to disrupt life on a rogue planet as it did on Earth? Maybe not.
So far, much of it is guesswork. How can we find out more about rogue planets?
First we have to put our eyes on some. The upcoming Vera C. Rubin Observatory will specialize in the search for transient objects and phenomena. The Rubin Observatory has a 10 year mission and has been able to find up to 50 ISOs, including rogue planets, during that time.
Once we find some, we need to find a way to visit one. Manasvi Lingam and colleagues raised this problem in a paper entitled “Interstellar Now! Missions to and sample returns from nearby interstellar objects. ”The authors of this paper say that in situ exploration of these objects is the next step. It is the only way to study the composition of a rogue planet and its chemical and isotopic structure. They talk about possible options for rogue planets soaring past and even for the lander to come to the surface.
Ruby Observatory at sunset lit by a full moon. Photo credit: Rubin Observatory / NSF / AURA
But what we really need is a pattern. For lower mass ISOs, similar to ‘Oumuamua’, a high speed impactor could be used. It could blast material from the surface that is collected by a spacecraft during a flyby and returned to Earth. But for an object the size of a planet, that’s probably impossible. It is not clear how we could collect a sample from a rogue planet. That may be technically unattainable, at least for the time being.
ESA has a plan to send a space probe to an ISO if it enters our inner solar system. It’s called the Comet Interceptor and would launch before it knows what its target is. The spacecraft would be parked at point Sun-Earth L2 where it would wait. Once a suitable ISO was found, the spacecraft would be sent on a rendezvous with it. The idea focuses on long-period comets, but could be adapted to ISOs, at least interstellar comets. It’s not hard to see how it could evolve to visit a real rogue planet.
NASA is working on a similar mission called Extrasolar Object Interceptor and Sample Return. NASA envisions launching a spacecraft towards Jupiter and waiting for an ISO to approach. Then it would be directed to the ISO to collect a sample and return it to Earth.
We cannot travel to another star system. Maybe one day in our science fiction future, but not anytime 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.