Maybe some Earth-size planets were just born to be wild.
Across the galaxy, countless "rogue" Earths may have been ejected from their planetary neighborhoods and are now zipping alone through space, previous studies have suggested. Now, new research says some of these worlds may have enough heat to support liquid water—and maybe life—without energy from a star.
Dubbed a Steppenwolf planet—since it "would exist like a lone wolf wandering the galactic steppe"—such a world could host a liquid ocean under an insulating layer of water ice and carbon dioxide snow, the study authors say.
Hydrothermal vents on the planet's seafloor may infuse the alien ocean with chemical energy sources. On Earth, nutrients from such deep-sea vents support bacteria and other life-forms that can exist without sunlight.
For now there's no proof Steppenwolf planets actually exist, said study co-author Dorian Abbot, a geophysicist at the University of Chicago. The idea was born of conversations between Abbot and his next-door neighbor, Eric Switzer, a cosmologist also at the University of Chicago.
"The idea was to consider extreme types of environments," Abbot said. "What if we take something Earthlike and turn off the sun? Can you image some environments that might be considered habitable?
"If we start to expand the sorts of things we think could harbor life, and if life can evolve and expand in these types of habitats, it could be everywhere, just floating around between the stars."
Internal Warmth A Big Driver on Earth
Previous studies have suggested that planets can get booted from their star systems due to gravitational tussles with more massive worlds.
For example, astronomers have found several huge, gassy planets that orbit very close to their stars, called hot Jupiters. It's thought these worlds actually formed farther out from their stars and migrated inward. (Related: "First Proof of Wet 'Hot Jupiter' Outside Solar System.")
"If a planetary system is configured like ours and the gas giants migrate in, that could toss out the small inner planets," said Robert Pappalardo, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, who wasn't involved in the new study.
We also know from studies of our own solar system that some of the moons around Jupiter and Saturn most likely harbor under-ice oceans. Jupiter's moon Europa, for example, appears to host an ice-covered ocean up to a hundred miles (160 kilometers) deep. (Related: "Could Jupiter Moon Harbor Fish-Size Life?.")
"The main difference is that an ice moon's energy is coming from tides, while on [Steppenwolf planets] it's coming from primordial heat left over from formation and [heat currently generated by] radioactive decay," study co-author Abbot said.
According to JPL's Pappalardo, "Europa isn't big enough to have significant radioactive decay anymore." Instead the moon is warmed by friction, as Jupiter's gravity pushes and pulls on the ice like a giant hand squeezing a tennis ball.
Earth is big enough that its insides have carryover heat from birth, and it has a generous amount of elements in the mantle that are still radioactive. It might not sound like much, but these internal heat sources are what drive plate tectonics on Earth, Abbot noted—energy from the sun simply controls the surface climate.
And where there's plate tectonics, there could be volcanic vents and other seismic activity.
"On a Steppenwolf, the planet is continuously exposing new rocks if plate tectonics are working," Abbot said. "That's a source of chemical energy that could support life."
For Steppenwolf Planets, Size Matters
Abbot and Switzer imagine a Steppenwolf as a planet that's roughly the same composition and age as Earth. The pair calculates that, without any energy input from a star, a planet would need to be slightly larger than Earth to host liquid water.
"We also assume that solid carbon dioxide gets outgassed by volcanoes that poke up through the ice, and the CO2 snows on top of the ice," Abbot said. "That would further insulate the system and make it easier to allow liquid water."
A rogue planet could still support a subglacial ocean without CO2 snow, the study says, but it would need to be 3.5 times the mass of Earth. By contrast, a Steppenwolf with a thick CO2 blanket could be just 0.3 times Earth's mass, the authors say.
In either case, "you could imagine a liquid layer hundreds of meters deep—you know, an ocean, not just a couple puddles," Abbot said.
Rogue Earths "Plausible"
The Steppenwolf concept is plausible, according to JPL's Pappalardo: "The universe is so grand, and our galaxy is so enormous, that small chance events can still happen lots of times," he said.
"It looks to me like the details of the model maybe could be improved in terms of going more thoroughly into the range of parameters," he said. "But the idea is an exciting one, and it's fascinating that they give the odds of actually detecting such a thing."
Based on their calculations, Abbot and Switzer say astronomers could spot a Steppenwolf planet via its reflected light if it comes within about 93 billion miles (150 billion kilometers) of Earth.
"It's possible that, at some point in the future if we find one of these, it might be easier to visit than the nearest star," Abbot said. Aside from the sun, the closest star to Earth is Proxima Centauri, at 4.2 light-years away.
And if we find something lurking in a Steppenwolf's ocean, he said, "it could be the first example of life from a different star."