If microorganisms could survive a journey through space inside meteoroids, could life from Earth be transferred to planets in other solar systems—or even vice versa? A new study suggests the possibility is much higher than scientists once thought.

Using computer simulations involving slow-moving rocks, scientists from Princeton University, the University of Arizona, and the Centro de Astrobiología (CAB) in Spain concluded that Earth could have exchanged rocks trillions of times with planets from other planetary systems during the solar system's infancy.

At the time—several billion years ago—the sun would have been in its native star cluster, with Earth and nearby planetary systems under heavy meteorite bombardment, said study co-author Amaya Moro-Martin, an astrophysicist at CAB.

The research, published in the journal Astrobiology, was presented this week at the European Planetary Science Congress in Madrid.

Scientists had previously considered the possibility that meteorites could escape from our solar system and land on a terrestrial planet in another system. But they had concluded that the chances were extremely slim because of the speeds of the objects involved.

"Everyone assumed the rocks would be ejected very fast—so fast they couldn't be captured by the next star. They were flying right by," said study leader Edward Belbruno, a Princeton mathematician.

Space Rocks Sneaking Up on Stars

Belbruno, Moro-Martin, and colleagues considered a new scenario: a low-velocity process called weak transfer.

When they factored in much slower speeds of around 100 meters (330 feet) per second, along with other considerations, the researchers found a strong case for lithopanspermia—the idea that biologic material can be spread through pieces of planetary rock hurled into space by collisions and other events. (Also see: "Life Ingredients Found in Superhot Meteorites—A First.")

"Our idea is that, instead of leaving Earth fast, you leave slowly and sort of sneak up on the next star," said Belbruno, who demonstrated the principles of weak transfer in 1991 with a Japanese probe trying to enter the moon's orbit.

In its youth, the solar system would have still been embedded in the sun's native stellar cluster, when the stars were close together and moving very slowly relative to each other. Before the cluster slowly dispersed, the research suggests, a window of opportunity had opened up for lithopanspermia to occur.

Did Life on Earth Come From Other Planets?

Rocks have already intermingled within our solar system: A number of meteorites found on Earth originate from Mars, others from the moon. This new model opens up the possibility of large rock quantities being exchanged between different planetary systems within a star cluster.

Under the weak-transfer scenario, as many as 12 out of 10,000 rocks cast off by our solar system and its closest neighbor in the sun's birth cluster could have been captured by the other. Earlier simulations put the odds at around one in a million.

The model also boosts the odds that life-bearing rocks could seed other worlds under certain conditions.

For one, microorganisms like bacterial spores would have to survive a journey fraught with hazards. "Things like UV radiation and cosmic rays would basically fry the poor guys," Moro-Martin said.

The bigger the rock, however, the better the chances that the life-forms could hide long enough to survive an interstellar journey, she noted. (Related: " Space Poison Helped Start Life on Earth?")

Although life on Earth is largely thought to have originated here, the notion that it could have spread to other worlds via weak transfer leaves open the intriguing, opposite scenario.