(Correction appended Sept. 26, 2012.)
Time was, the solar system was raining rocks. You only need to look at the cratered face of airless bodies like Mercury and the moon to get a sense of the cosmic crossfire that took place back when the local worlds were just forming and much of the debris that helped make them up was still flying free. Even now, planets and moons occasionally swap rubble, with odd bits of, say Mars, blasted into space by a long-ago meteor spiraling slowly in to get snagged by Earth.
This kind of planetary tissue exchange long ago gave rise to the concept of panspermia — the idea that life on Earth may not have originated here at all, but rather was imported in the form of organic building blocks or even microorganisms from far away. Earth, in turn, may have similarly seeded other worlds. The catch is that the solar system is a limited place, with Earth the only place we know of that’s currently capable of supporting wandering biology.
Things get a lot more interesting if you expand the pool of candidate worlds to include those in other solar systems. This idea, called lithopanspermia, has always seemed like a nifty possibility, but not one worth much thought. The physics of interstellar transfer are so complex that it would, for practical purposes, be impossible for any debris to make such a journey. Or that was the belief. But a new paper published in the journal Astrobiology gives new energy to the lithospermia idea — concluding that interstellar transfer of life might be a whole lot more possible than anyone expected.
For astrophysicists, the easiest part of both panspermia and lithopanspermia has paradoxically been the biology itself. The universe is fairly awash in water, hydrocarbons and even amino acids — and all of them can be carried aboard free-flying space rubble. In 2011, geologists announced that a meteorite that landed on Earth in 2000 not only contained amino acids and other prebiotic materials, but that all of them existed in different stages of complexity — meaning that the meteor had actually been cooking them up en route, probably with the help of traces of on-board water and heat released by radioactive material.
But if organic cargo can survive — and even thrive — on such a long journey, there’s still the matter of how you ship it from sender to receiver, and here’s where lithopanspermia ran into trouble. Old models of interstellar transfer relied on the idea of rubble being flung out of a solar system by gravitational encounters with large bodies like Jupiter, meaning that they’d be traveling at speeds of about 8 km per second — or nearly 18,000 mph. That’s way too fast for the rocks ever to be captured by the gravity of another star system, even if they did reach one. “It is very unlikely that even a single meteorite originating on a terrestrial planet in our solar system has fallen onto a terrestrial planet in another solar system, over the entire period of our solar system’s existence,” wrote astrophysicist H. Jay Melosh of the University of Arizona in a 2003 paper that attempted to put the lithopanspermia idea to rest once and for all. If our rocks can’t get out, other rocks have no greater chance of getting in.
That, however, is only if you stick with the old model for how the debris was set free in the first place. A team of researchers from Princeton University, the University of Arizona and Centro de Astrobiologia in Spain took a different approach, developing computer models of a slow-boat transit method known as weak transfer. Under this process, rubble spirals slowly outward through a solar system until it reaches a spot so far from its parent sun that it requires only a slight perturbation to nudge it into interstellar space. “At this point,” says Princeton astrophysicist Edward Belbruno, one of the authors of the paper, “you’re escaping so slowly that randomness and chaos theory is involved in getting you out.”
The problem is that low speed can also mean slow transit time to the next solar system, with a trip lasting 1.5 billion years or more, longer than even the toughest organic material could survive. About 4.5 billion years ago, however, when the sun was just being born, it was part of a tight grouping of nascent stars known as the local cluster that was comparatively densely packed — and that could have cut transit times dramatically.
“After about 100 to 200 million years, the stars scattered, and the transfer likelihood went dramatically down,” says Belbruno. “But you do have a window.” Encouragingly, analyses of terrestrial rocks reveal that Earthly organics may indeed have formed in the solar system’s comparative babyhood, directly within the departure window.
On its face, the number of rocks that would reach another solar system seems small — 5 to 12 out of every 10,000. But since trillions of rocks per year make the low-speed escape, that means a whopping one billion in that same year might be captured by neighboring worlds — and we could be on the receiving end of similar numbers from elsewhere. It may still be unlikely that anyone alive today will ever meet an alien— but the odds just went up a little that we all could be the aliens.
(An earlier version of this story mentioned the 5 to 12 per 10,000 number, but did not mention that such a low figure could still add up to one billion per year.)