The idea that life on Earth might have originated elsewhere, on Mars, for example, has gained currency in recent times as we’ve learned more about the transfer of materials between planets. Mars cooled before the Earth and may well have become habitable at a time when our planet was not. There seems nothing particularly outrageous in the idea that dormant bacteria inside chunks of the Martian surface, blasted into space by comet or asteroid impacts, might have crossed the interplanetary gulf and given rise to life here.
But what of an interstellar origin for life on Earth? The odds on meteoroids from a system around the average galactic field star not only striking the early Earth but delivering viable microbes are long indeed. But if we consider the Sun’s probable origin in a cluster of young stars, all emerging from the same collapsing cloud, the picture changes significantly.
Now we’re dealing with much smaller distances between stars and slow relative motion as well, conditions that could make such a transfer possible. A new paper makes the case that this process might well have been two-way:
…there is a definite possibility that bacteria carrying meteoroids of extrasolar origin have landed on the Earth. In reverse, it is possible that at least one other planetary system in our birth star cluster received a life-carrying asteroid from the Earth; and it is not excluded that the whole birth star cluster was ‘fertilized’ in this way by live bacteria from the Earth.
So write Mauri Valtonen (Turku University, Finland) and team in an upcoming paper. The authors believe bacterial exchanges between planets of different solar systems could only have occurred during the birth cluster stage of these systems, but given this constraint, it is possible that the Earth received large numbers of life-bearing bodies early on. The broad process of panspermia in which life spreads through an entire galaxy from a single source seems unlikely, but “…life-carrying bodies originating from our solar system may have found their way to our original neighbours, and …all conditions being optimal, life seeded by our system could have spread to many other solar systems.”
And now it gets interesting for future space missions. Because we may be getting into position to find our long lost relatives, the stars from the original cluster that gave birth to our Sun. They’ve long since moved away from us, but missions like Gaia may be able to track them down. Gaia will study a billion stars in the Milky Way, monitoring each some seventy times over the course of a five year period. The mission is expected to discover extrasolar planets, brown dwarfs and numerous other interesting objects, and will help us extend our picture of galactic structure three-dimensionally, perhaps pin-pointing our siblings.
Gaia is expected to be launched in 2011, a much improved version of the Hipparcos mission that did so much to catalog the more than 100,000 stars it looked at. It’s a significant upgrade, and other missions to be launched within the next two decades could go on to provide us with a look at planetary systems around the stars Gaia identifies as members of our original stellar family. If we ever confirm the existence of life on these planets, we may be looking at worlds that either gave birth to life here or received life’s impetus from Earth.
The paper is Valtonen et al., “Natural Transfer of Viable Microbes in Space from Planets in the Extra-Solar Systems to a Planet in our Solar System and Vice-Versa,” accepted by the Astrophysical Journal and available online.