Life on Lonely Planets?

The universe may be awash with more planets suitable for life than previously thought, an astronomer argues in today's Nature. He suggests that lone planets, slung out of solar systems by the gravity of other planets, could retain dense atmospheres and volcanic heat suitable for sustaining primitive life.

In the last 5 years, astronomers have gathered more and more evidence of planets surrounding several stars other than the sun. If any of these extrasolar planets--or similar ones throughout the universe--have an atmosphere and temperatures like Earth's, they could harbor life. But astronomers have thought these conditions could arise only if a planet is a precise distance from a star: not too close to burn off the atmosphere, not too far to freeze all water. Such constraints frustrate David Stevenson of the California Institute of Technology. "I'm motivated in part by what I see as a tendency to restrict the thinking about possible locations for the origin of life," he says. "I wanted to open up the discussion."

Stevenson argues it is very likely that many solar systems have tossed out planets. This could happen, he says, when a lighter planet gets drawn into the gravitational embrace of a heavier one but, instead of orbiting like the moon around Earth, gets flung away--similar to the way the Pioneer spacecraft that explored Jupiter in the 1970s was ejected from the solar system by the gravitational field of Jupiter. A planet wandering through the cosmos, with no sun to warm it, could still have life nurtured by heat radiating from its core, Stevenson says. And a spacefaring planet could retain a dense atmosphere--which would trap heat to warm the surface--if its gravity were strong enough, he argues.

If lone planets exist they are beyond the reach of today's telescopes, Stevenson says, because they would be much smaller and dimmer than stars, and their outer atmospheres would be too cold to be spotted by infrared detectors. "There is not much prospect of detecting them directly," he notes. But the idea they may be out there somewhere is provocative, says Robert Reasenberg of the Harvard-Smithsonian Center of Astrophysics in Cambridge, Massachusetts. It's "important for people to think in broader terms, and for that alone, it is an interesting piece," he says.