Rogue Planets Challenge Our Assumptions About Life in the Universe

Picture a planet, and a star is usually lurking nearby. In our own solar system, the sun is perhaps the most critical member—sending light and heat to make life possible on Earth. But what if we didn’t need the sun to survive?

Across the galaxy, astronomers have discovered planets roaming the cosmos without the guidance of a star. These free-floating bodies, called rogue planets, overturn the stereotype that big balls of hot gas are necessary for their existence. Now, scientists are investigating if stars are required for our livelihoods too.

Perhaps we need “step away from this Earth-centric view that life has to be the way that we know it,” says Barbara Ercolano, an astrophysicist at Ludwig Maximilian University of Munich. “Maybe [rogue planets] are the norm, and we are just thinking of them as exotic because we live in a different system.”

Astronomers have picked up hints of at least 70 wandering worlds in the Milky Way, but our galaxy may be home to 20 times more boundless planets than stars—amounting to trillions, models show. And this year, scientists precisely measured the mass and distance of a Saturn-sized rogue planet about 10,000 light-years from Earth, officially confirming the existence of one of these starless systems for the first time.

New projects, including NASA’s Nancy Grace Roman Space Telescope launched in August, could confirm hundreds more. And as scientists study these starless systems, they look a little less inhospitable, raising the possibility such planets—or more likely their orbiting moons—could sustain life.

(We’re still not sure what a planet really is.)

How Do Planets Go Rogue?

Planets can become loners in a few different ways. A small portion could be failed stars, although scientists are still working to understand this formation process. Ercolano says they could have formed in a fragmented molecular cloud in isolation but never accreted enough mass to burn fuel in their cores and become stars. Some may fall into a category of failed star called a brown dwarf, though the distinction is still debated.

Most of these outsiders, however, likely formed as typical planets do around a star, but then they got kicked out of the system. During their first few million years, planets are on chaotic orbits and not necessarily moving uniformly around their star. Young planets are more likely to intercept the orbit of another large object, getting gravitationally pushed out of the system. These planets are probably lower mass, perhaps the size of Mars, or on the outer skirts of the system.

“The catastrophic encounters probably have happened in every solar system that has formed,” Ercolano says.

But planets don’t get kicked out whenever. The orbital conditions need to be right. Currently, planets in our solar system, for example, are bound to our sun and don’t have enough energy to escape the star’s gravitational pull.

“We all only survive for so long because the orbits now are so stable,” says David Dahlbüdding, also an astrophysicist at Ludwig Maximilian University of Munich.

Can a Planet Host Life Without a Star?

In an early “Star Trek” episode, crew members land on a planet in a starless region—but with trees and an atmosphere like Earth. There are even hints of water.

“A basic ingredient for life, of course, is liquid water,” says Dahlbüdding. “For liquid water, you need enough warmth, enough heat, to keep it liquid.” He, Ercolano and their colleagues recently examined the habitability of rogue planet with moons in a paper in the Monthly Notices of the Royal Astronomical Society.

Unlike the Star Trek planet, a real rogue planet probably wouldn’t stay warm enough at its surface for us to live or harbor liquid water. Most of its heat would have been present during its formation, which would quickly dissipate.

Scientists have brainstormed other theories. A rocky planet could warm up as radioactive elements decay inside and release energy like a slow cooker—potentially allowing liquid water to exist beneath the surface. A planet with a liquid interior could also disperse heat to the surface through convection much like water in a boiling pot.

“These possibilities have all been speculated on, but I'm not aware of any study that has systematically gone through all those possibilities,” says Ercolano. “I think it's not to be excluded that you could have habitable conditions on the planet.”

A more likely home could be an orbiting moon around the planet, according to models. As the rogue planet gets ejected from a system, Dahlbüdding says its moons can also get ejected in a highly elliptical orbit. Some of the lunar bodies can be as big as Earth.

Moons can heat up internally through a process known as tidal heating—similar to rapidly squeezing a rubber exercise ball. As the moon takes an uneven orbit around the rogue planet, the planet exerts different gravitational pulls on it. Gravitational forces are stronger when the moon is closer to the planet and weaker farther away, flexing its interior and turning orbital energy into heat.

This happens in our own cosmic backyard, too. Take Jupiter’s moon, Io. Jupiter squeezes and heats Io so violently that its insides are popping out. As a result, Io is the most volcanically active body in our solar system and not suitable for human life.

“You want a lot of tidal heating to keep the water liquid, but at the same time, you don't want too much…because then the energy is dissipated a lot faster,” says Dahlbüdding. “You can't actually maintain these conditions for such long timescales on which maybe then life can evolve.”

But in that sweet spot, Dahlbüdding and his colleagues found tidal heating could provide enough warmth to keep surface water from freezing in space void of sunlight.

Can a Rogue Planet Keep Life Warm?

Generating heat is only part of the puzzle; a habitable world needs to retain that heat.

On Earth, carbon dioxide in the atmosphere traps heat through the greenhouse house effect. But a carbon dioxide-rich atmosphere isn’t so effective in an environment that’s much colder, say without a nearby star. The compound condenses into a liquid and loses a lot of its ability to retain heat in freezing environments.

Instead, Dahlbüdding and his colleagues modeled an atmosphere full of hydrogen molecules, which could have been present when the moon formed or produced through chemical processes afterwards. They absorb thermal radiation when they collide with one another. The team found the atmosphere along with tidal heating can effectively trap heat for 4.3 billion years, depending on the surface pressure, and even support liquid water on the surface.

In fact, life on Earth may have started with a hydrogen-rich atmosphere. Meteorite impacts could have produced a lot of hydrogen, which could have formed water and ammonia— helpful ingredients for life. Some research suggests that the last common universal ancestor—basically the organism from which all life on Earth evolved—may have used hydrogen as an energy source.

“We're not just learning about what these other planets are like, but we're also learning about ourselves, our own history,” says Andrew Gould, an astronomer at Ohio State University.

(NASA is preparing to explore alien worlds—by investigating Earth’s dark corners.)

How to Find a Rogue Planet

Finding more rogue planets could help answer some of these questions, butthey’re harder to detect than regular exoplanets spinning around stars (which number over 6,000). Imagine trying to find an object in the dark millions of miles of light without a flashlight. That seemingly impossible challenge has made the discovery of rogue planets a chance sighting.

In the early 2000s, scientists were scanning the skies for faint objects when they first began to come across planets with no stars. To date, no moon has been detected around a free-floating planet.

Scientists mainly find evidence of rogue planets by watching them briefly magnify the light of a distant, unrelated star as they pass in front of it—known as gravitational microlensing. Those alignments are rare and may only last for a few hours or days, though.

Ground-based telescope networks detect these bright blips, but they must compete with weather, daylight, atmospheric distortion and gaps in data collection. But our luck may soon change. NASA’s new Nancy Grace Roman telescope will detect gravitational microlensing with a field of view that is 100 times that of the Hubble Space Telescope and scan locations every 12 minutes. It could find as many as 400 Earth-sized rogue planets.

China’s proposed Earth 2.0 mission, expected to launch in 2028, is also planning to look at tens of millions of stars in the Milky Way’s center and potentially detect hundreds more rogue planets. Then the Extremely Large Telescope, an observatory in Chile slated to begin work in 2029, can follow up on any discovered candidates to examine its temperature, atmosphere and chemistry.

As for visiting a rogue planet, “eventually people will launch rockets and go there,” says Gould. But for now, that remains the realm of science fiction.  “Even to get to these planets…going at one-tenth the speed of light would take 100,000 years, so it's a long-term project to find any more information,” he says.