Did life crash to Earth from space on slow moving rocks?

Microorganisms that crashed to Earth embedded in the fragments of distant planets might have been the sprouts of life on Earth, according to new research from Princeton University, the University of Arizona, and the Centro de Astrobiología (CAB) in Spain.
The researchers report in the journal Astrobiology that under certain conditions there is a high probability that life came to Earth — or spread from Earth to other planets — during the solar system’s infancy when Earth and its planetary neighbors orbiting other stars would have been close enough to each other to exchange lots of solid material.
The work will be presented at the 2012 European Planetary Science Congress on Sept. 25.
The findings provide the strongest support yet for “lithopanspermia,” the idea that basic life forms are distributed throughout the universe via meteorite-like planetary fragments cast forth by disruptions such as volcanic eruptions and collisions with other matter.
Eventually, another planetary system’s gravity traps these roaming rocks, which can result in a mingling that transfers any living cargo.
Previous research on this possible phenomenon suggests that the speed with which solid matter hurtles through the cosmos makes the chances of being snagged by another object highly unlikely.
But the Princeton, Arizona and CAB researchers reconsidered lithopanspermia under a low-velocity process called “weak transfer,” wherein solid materials meander out of the orbit of one large object and happen into the orbit of another. In this case, the researchers factored in velocities 50 times slower than previous estimates, or about 100 meters per second.
Using the star cluster in which our sun was born as a model, the team conducted simulations showing that at these lower speeds the transfer of solid material from one star’s planetary system to another could have been far more likely than previously thought, explained first author Edward Belbruno, a mathematician and visiting research collaborator in Princeton’s Department of Astrophysical Sciences who developed the principles of weak transfer.
The researchers suggest that of all the boulders cast off from our solar system and its closest neighbor, five to 12 out of 10,000 could have been captured by the other. Earlier simulations had suggested chances as slim as one in a million.
“Our work says the opposite of most previous work,” Belbruno said. “It says that lithopanspermia might have been very likely, and it may be the first paper to demonstrate that. If this mechanism is true, it has implications for life in the universe as a whole. This could have happened anywhere.”