Rogue Planets Could be Habitable

Exoplanetsplanets that orbit other stars are the focus of our search for potential habitable planets. Earth is the only known planet with life. Sunlight fuels life on Earth. Some estimates suggest that there may be many other rogue planets in space that are not bound to any star or warmed by it.
Could they support our lives?

Rogue Planet is a term that describes interstellar objects (ISOs). However, in the case Oumuamua and 2I/Borisov are the only confirmed ISOs that have entered our Solar System, the ISOs are of planetary-mass.

Rogue Planets were somehow ejected out of their solar systems. Young solar systems can be chaotic, with bodies colliding with one another. Migrating gas giants can also perturb smaller terrestrial planets' orbits, sending them on interstellar travels. Rogue planets can also form in interstellar space, similar to the formation of stars. A cloud of dust and gas could form a planet, which would then be surrounded by a number of moons. Although sub-brown dwarfs can be considered rogue, they are just gas and unlikely to support life. Rogue planets don't have to be gravitationally bound with any star. They are free-floating.

We don't know how many there are. Neil deGrasse Tyson estimates that there are billions, if not trillions, of them in our Milky Way. Could they all be capable of hosting life? It could be.

The issue has been studied by one scientist from Florida Tech University. Manasvi Lingam, an assistant professor in Aerospace, Physics and Space Sciences at Florida Tech, has been researching multiple topics in Astrobiology, including the habitability and potential for life on moons and planets that are not part of our solar system. Lingam co-authored a book called Life in the Cosmos: from Biosignatures and Technosignatures with Avi Loeb. It was titled Subsurface exolife. This paper examined the potential for life on planets with subsurface waters. Instead of focusing on exoplanets that orbit other stars, they examined rogue planets.

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

DeGrasse Tyson claims that there are billions to trillions of rogue stars in the Milky Way. This means it is possible that the closest exoplanet is not an exoplanet but a rogue world. Lingam says that some of these planets could be prime targets for the search for life. He said that while we think of planets bound by stars like Mars as being capable of supporting life, in reality these kinds of life-supporting worlds could be just floating around in space without rich biospheres.

Lingam stated in an interview with Discover magazine that you can definitely think of something bigger than microbes. Even though it is not as complicated as some of the most complex things that we see on Earth,

It seems unlikely that life on the surface of rogue planets floating in frigid interstellar spaces would be supported by them. However, in our Solar System there are moons and planets that may be far away from the Sun. For example, Europa, Jupiter's moon. Although its surface is frozen, it is home to an ocean of liquid water beneath. This makes it a prime target for our search for life. Are there other rogue planets that could be similar to Europa?

What would it take to make a rogue planet support life? It would probably require a combination of all three.

Assuming that liquid water is essential for life, then a rogue world will need energy to keep the water from freezing. A planet that is similar to Enceladus, Ganymede and Europa is the most likely scenario. These bodies are strong evidence that they have thick layers and oceans of ice covering their surfaces. Europa may even contain twice the amount of water as Earth.

Based on data from the Galileo space probes, artists have created an impression of Europa's interior. Europa could contain twice the amount of water as Earth. Credit: NASA

The planet's interior would provide the heat necessary to prevent a rogue world from completely freezing. Earth's core is rich in geothermal energy. It is reasonable to suppose that other rogue planets also have this energy. Only a small percentage of Earth's energy comes directly from its core. This scenario is realistic for life, but it's not possible because the Sun provides more than 99.9% Earth's energy. A rogue planet would not have much energy to work with.

In interstellar darkness, rogue planets also face another problem. It would be possible for the same atmosphere to freeze in interstellar space if it had an atmosphere in its solar system. The Earth's atmosphere is crucial in maintaining heat and regulating our climate. Without one, how could rogue planets survive?

Perhaps they don't need one. Europa has a very low oxygen level. So does Ganymede. Enceladus' atmosphere is thicker than Earth's, but not as dense as Earth's. It is highly unlikely that any rogue planet would have a gaseous atmosphere capable to trap heat.

There is at least one exception. A dense hydrogen atmosphere could trap heat and resist freezing. It could store enough heat to prevent surface water freezing. It is not known if any rocky planets have hydrogen atmospheres. If so, it's extremely rare. However, experiments have shown that some organisms can survive in a hydrogen atmosphere.

A planet that has a large moon could have higher chances of supporting life. Tidal heating could be possible if the moon is large enough. Although tidal heat is not uncommon in our Solar System, the gas giant Jupiter can cause tidal heating on the moon Europa. Perhaps a rogue system of moons can produce similar results: the moon that is warm has a subsurface sea instead of the planet.

Images taken by NASA's Galileo spacecraft reveal the intricate details of Europa's icy surface. Image: NASA/JPL-Caltech

Lingam believes there is another possibility. It is theoretically possible for photosynthesis to occur if a rogue world is located near the galactic center. Lingam states that photosynthesis can be supported at a distance of less than 1,000 light-years from an active galactic nucleus (AGN).

Life can exist below the ocean floor without sunlight. Near hydrothermal vents at the ocean floor, Earth is home to entire biological communities. These vents are known as black smokers and produce a stream minerals that can be used to feed chemosynthetic bacteria. These bacteria attract other organisms to eat them. These organisms attract predators, which in turn attracts other species, creating a whole food chain. Similar communities could be found on rogue planets that have geothermal heating.

Hydrothermal vents could provide the energy necessary to sustain life without starlight. Credit: NOAA

Panspermia may be facilitated by the existence of rogue planets that carry life through interstellar spaces. Panspermia refers to the belief that the ingredients for life, or even life itself, can be spread across a galaxy via interstellar objects. Panspermia vehicles are ideal candidates for rogue planets. Our Solar System will have sent its own ISO and rogue planets out into interstellar spaces. They might be spreading life across the galaxy.

One advantage that rogue planets may have over Earth is their protection by an icy shield. Europa's ice layer is between 1030 km and 619 mi. It is thick. It can be thought of as an asteroid shield. Asteroid strikes can have devastating effects on a planet. They can lead to mass extinctions or change the course of evolution. A Chicxulub-sized impactor would be capable of disrupting life on a rogue world in the same way that it could on Earth. Perhaps not.

This is a lot of conjecture. What can we do to learn more about rogue worlds?

First, let's look at some. The Vera C. Rubin Observatory, which will be opening soon, will focus on finding transient phenomena and objects. The 10-year-long mission of the Rubin Observatory is to find 50 ISOs and rogue planets.

Once we have found one, we must find a way for it to be visited. In a paper called Interstellar Now!, Manasvi Lingam (with colleagues) tackled this problem. In-situ Study of Nearby Interstellar Objects and Missions to them. This is the only way to understand the composition of rogue planets and their chemical and isotopic structures. They discuss possible flybys of rogue worlds, and even how to get a lander on the surface.

Rubin Observatory lit up by the full moon at sunset. Credit: Rubin Observatory/NSF/AURA

We really need a sample. A high-speed impactor can be used for lower-mass ISOs like Oumuamua. The impactor could blast material off the surface, which could be captured by a spacecraft during flybys and then returned to Earth. It is unlikely that this would be possible for a large object of planet size. It is not clear how we would collect samples from a rogue world. This may not be possible technologically at this time.

ESA plans to launch a spacecraft that will visit an ISO when it enters the inner Solar System. It is called the Comet Interceptor, and it would launch before it knew its target. It would then be left at the Sun-Earth L2 location, where it would wait. The spacecraft would rendezvous with the ISO once it was located. Although the idea was based on long-period comests, it could be modified to ISOs or interstellar comets. It is easy to see how this could be developed further to visit a rogue planet.

NASA is currently working on a similar mission, the Extrasolar Object Interceptor (EOI) and Sample Return. NASA envisages sending a spacecraft towards Jupiter and waiting for an ISO. It would then be directed towards the ISO to collect a sample, and return it back to Earth.

It is impossible to travel to other star systems. Perhaps one day, but it is not likely anytime soon. Other star systems, thanks to rogue stars and other ISOs are providing us with the evidence we require. It is up to us to discover a way to examine it.

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