Clusters of living worlds would hint life came from outer space

By Joshua Sokol, New Scientist

Panspermia would give rise to some lively stellar neighbourhoods (Image: NASA/JPL-Caltech)

Does life spread like an interstellar infection? If we spot it on clusters of planets, that might suggest it doesn’t stay put wherever it evolves.

The theory that life crosses space to reach new worlds, called panspermia, is hard to test. Life on Earth could have been seeded by just one microbe-laden rock, but there are too many rocks to check, even if we had a foolproof test for extraterrestrial life.

“That’s not a very effective strategy of testing whether life came from outer space,” says Henry Lin of Harvard University. He says the answer lies in mapping life across the galaxy.

Future probes like NASA’S James Webb Space Telescope will scrutinise the atmospheres of planets in other solar systems for possible signs of biological activity. If life spreads between planets, inhabited worlds should clump in space like colonies of bacteria on a Petri dish. Otherwise, Lin says, its signature would be seen on just a few, randomly scattered planets.

Radiating life

Lin argues that if we find 25 worlds with life on one side of the sky and 25 lifeless ones on the other, it might mean the sun sits on the edge of a panspermia bubble – a strong sign that life radiated outward. “We would have smoking-gun evidence that panspermia actually happens,” he says.

But panspermia would be harder to confirm from the bubble’s centre. If there are biosignatures all around as far as we can see, for example, we can’t draw conclusions one way or the other. And if we see only scattered life, Lin says, that could suggest either that panspermia doesn’t happen or that it proceeds so slowly as to be rare.

Sara Seager of the Massachusetts Institute of Technology, an expert on the hypothetical biosignatures the technique relies on, doubts Lin’s scenarios will come in handy any time soon. “It would be great if there’s a time in which we have so many biosignatures that we see clumps throughout the galaxy. But I don’t know when that time will be,” she says. “Until we find biosignatures we can’t actually proceed with any of this work.”

Whether we manage to detect biosignatures or not, Lin thinks his work might have a second life in the distant future, if humans achieve interstellar travel. The spread of humans and other organisms riding our coat-tails would follow the same growth pattern, he says.

“Even if panspermia doesn’t happen, we might be the ones to bring it about. Maybe this paper will be useful a thousand years from now,” he says.

Journal reference: arxiv.org/abs/1507.05614