Don’t worry about contact with aliens from other solar systems-they may be our distant cousins. According to an American astronomer, there is a slim chance that microbes could be carried from one solar system to another on rocks blasted from terrestrial planets by asteroid impacts, spreading life across the Galaxy.

“About one meteorite ejected from a planet belonging to our Solar System is captured by another stellar system every 100 million years,” Jay Melosh of the University of Arizona told the Lunar and Planetary Science Conference in Houston this week.

Although radiation would threaten stowaway microbes, Russell Vreeland of West Chester University of Pennsylvania says it would be quite possible for meteorites to carry well-protected organisms over interstellar distances.

In the 1970s, astronomers Fred Hoyle and Chandra Wickramasinghe put forward the still controversial theory, dubbed “panspermia”. This says that comets bombarding the Earth brought the bacteria and viruses from interstellar space that started life here 4 billion years ago, and continue to bring in new biological material today. Melosh argues that alien organisms might also come from a distant planet similar to our own.

He is part of a group that earlier showed microbes could hitch a ride on meteorites travelling between planets in our Solar System (New Scientist, 15 January 2000, p 19). At the time, he didn’t think any microbes could survive the millions of years a meteorite would take to travel between stars. That view changed, however, after Vreeland successfully cultured bacterial spores from a 250-million-year-old salt deposit in New Mexico (New Scientist, 21 October 2000, p 12). The longer survival time makes the transfer of life conceivable, Melosh says.

Transfers between solar systems depend on gravitational interactions between meteorites and other planets. As a starting point, Melosh considered rocks blasted off the surface of Mars by impacts. His simulations show that Jupiter can act as a slingshot, flinging roughly 500 kilograms of Martian rocks each year right out of our Solar System in all directions. Their velocity averages 5 kilometres per second, so in a million years they would travel about 17 light years-far enough to reach nearby stars.

Most ejected meteorites would continue to drift in the interstellar void, but a few would eventually pass near other planetary systems. “The probability of direct capture by an Earth-sized planet is very, very tiny,” says Melosh. However, the gravity of a Jupiter-sized giant planet can capture meteorites passing within a hundred million kilometres of it, if the two are moving at similar velocities in the same direction. The meteorite would then fall into an eccentric orbit about the star.

It is still far from certain whether the meteorite would go on to collide with a terrestrial planet, and Melosh’s calculations suggest that the likelihood of such an event is low for a solar system like our own. The chances would be higher, he says, if terrestrial planets orbited near to a Jupiter-sized planet.

“The probabilities are pretty low,” acknowledges Melosh. But they aren’t impossibilities, he adds. Wickramasinghe believes that his panspermia theory, in which bacteria can drift on their own between solar systems, propelled by radiation pressure, is a more likely scenario. “The only advantage that you might have [with] huge chunks of rock [is that] the interior is shielded totally from any damaging radiation,” he says.


Author: Jeff Hecht
New Scientist issue: 17th March 2001