Wandering Planets Could Support Life

July 1, 2001 -- Take Earth, gently fling it out of the solar system into the voids of interstellar space…

… and we’d die.

Without warming sunlight, temperatures would cool to minus 400 degrees Fahrenheit.

“Not a good place for life,” observes David Stevenson, a planetary scientist at the California Institute of Technology. (A few microbes that live a few miles underground and get their energy from Earth’s still-hot core might survive, but pretty much every other critter would freeze to death.)

Not to worry. Earth won’t be going anywhere anytime soon.

A Galaxy of Rogue Planets

But elsewhere in our Milky Way galaxy, there could be hundreds of billions of Earth-size planets adrift in the dark reaches of space. Ruminating about the fates of these lonely, lost worlds, Stevenson has concluded some of them could actually be cozy, if pitch-black, homes of alien life.

“This is not a discovery,” Stevenson, says. “It’s just an idea. And it’s fun.”

Stevenson reports on his fun idea in today’s issue of the journal Nature.

Planets coalesce from the disk of dust and gas left over after the birth of a star. Small clumps clump into larger clumps. Larger clumps clump into asteroid-size mini-planets, which then clump into planets.

But the gravitational tugging between planets and mini-planets likely swung many of them straight out of the neighborhood, like pebbles flying out of a slingshot.

Warm Blanket of Hydrogen

The difference between Stevenson’s hypothetical exiled worlds and the frozen wasteland that Earth would be in interstellar space lies in the atmospheres. Moving through clouds of hydrogen gas, Stevenson surmises the wandering planets could accumulate a dense blanket of hydrogen on the way out.

Under high pressures — as much pressure as at the bottom of Earth’s oceans — hydrogen acts like a thermal blanket, trapping heat from volcanic activity, and the trapped heat would keep the surface above freezing temperatures. (Earth’s atmosphere of nitrogen and oxygen doesn’t share that insulating quality.)

“There would be running water,” Stevenson says. In the oceans, life could arise near geothermal vents, similar to creatures found on Earth’s ocean floors. Still, the available energy would be about 5,000 times lessthan that on Earth.

“Life would have to be simple,” he says. “Certainly you wouldn’t have much of it.”

Lightning and Clouds

What might the surface of such a planet look like?

“If you happened to be standing near a volcanic eruption,instead of pitch darkness, you would see a landscape lit up witha dull-red glow and the sky would most likely be cloudy, withwater, ammonia and methane clouds in layers,” Stevenson says. “Youmight not actually see a beautiful starry sky.”

The rogue planets could even have lightning, but not Earth’s constantly shifting weather, which is driven by sunlight and the temperature differences between day and night. “The temperature would be very uniform from place to place,” Stevenson says. “It would be a dull place in that sense, and of course it would be totally dark.”

Reasonable FantasyOther scientists agree there is little doubt that planets are ejected from primordial planetary systems.

“It’s really a very logical conclusion from what we know about planet formation,” says Alan Boss of the Carnegie Institution in Washington, one of the leaders in the search for planets around other stars.

Less certain is whether they would have the atmosphere and other means to sustain even simple life.

“Just because you can make an atmosphere doesn’t mean you can keep it,” says Jonathan Lunine at the University of Arizona. “You have to ask whether that atmosphere, when perturbed very slightly, would return to its regular state or just collapse onto the surface.”

No one has ever seen a rogue planet, and chances of finding one are slim to none. About the only way would be if one happened to cross in front of a star.

“To be frank, I debated whether to submit this (forpublication),” Stevenson says. “In the end I decided thatideas play an important role in science, even when they don’thave an immediately testable aspect.”

If Stevenson’s idea is true, then life could be spread all through the galaxy — and we’d never see it.

Reuters and the Associated Press contributed to this report.