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It's easy enough to imagine how a meteor that accreted in space and spent its life flying could eventually find its way into the gravity field of a planet if it came too close. Harder to figure is what it takes to get biologically contaminated material from the surface of one planet to another. Something, after all, has to launch the stuff in the first place. Typically that something is a meteor strike, which hurls debris into space, where it slowly drifts from one world to the next. Earth and Mars have exchanged material this way for billions of years, though more in the early days of the solar system, when the cosmic bombardment was greater.
The kind of life that can get started on the warm, wet surface of a planet, contaminate its rocks and hitch a ride to the world next door is a lot more complex than the mere prebiology that can get cooked up in space. Most of those organisms--probably the single-celled kind like those the ALH84001 scientists thought they found--couldn't live through the shock heating that occurs when debris is blasted into space, but the ones deep within the rock might. Surviving the hundreds of thousands or millions of years it would take to travel from world to world would not be impossible. Earthly bacteria that live in extreme environments may go dormant or even freeze-dry until conditions improve and they stir to life again.
In June, investigators from the University of Colorado at Boulder studied bacteria found in the Atacama region of South America, where rain almost never falls and temperatures go from 13F (-11C) at night to 133F (56C) the next day. Microbes nonetheless thrive there, sucking energy from traces of carbon monoxide in the air and extracting moisture from exceedingly rare snowfalls. The rest of the time they hibernate. There's no reason an adaptation that nifty should be confined to earthly life.
Whatever biology is flitting about out there would not even have to be limited to traveling from planet to planet; it could also hop from solar system to solar system. This idea, known as lithopanspermia, was long considered impossible. Not only would the transit times between solar systems be prohibitively long for even the hardiest bacteria--on the order of 1.5 billion years--but the speed a space rock needs to travel to escape the gravity of its home solar system is too great for it to be captured by another. In September, however, a team of researchers from Princeton University, the University of Arizona and the Centro de Astrobiologa in Spain figured out a neat solution that sidesteps these problems.
Most lithopanspermia models assumed that the only way a rock could escape a solar system was if it passed too close to a large body like Jupiter and was gravitationally ejected at a speed of about 18,000 m.p.h. (29,000 km/h). But the investigators in the recent study used a computer to model a slow-boat escape known as weak transfer, in which a rock gradually drifts out through a solar system until it's so far from its parent sun that the slightest flutter in its trajectory could tip it into interstellar space.