WASHINGTON Rock-riding microbes tossed out of our solar system may have become star-trekking voyagers, by crossing deep space and planting themselves on planets circling other suns. This solar system-to-solar system seeding of life is called interstellar panspermia.

It is now commonly thought that meteorite-sized rock fragments can be ejected from one planet to land on another. Meteorite collections include rocks believed heaved our way from the planet Mars and the Moon. These celestial hunks of material reached Earth after those worlds were subjected to large impacts of asteroids or comets eons ago.

But Earth isn't only on the receiving end of chunky-style helpings of its planetary neighbors -- it works both ways.

Early bombardment of Earth by large objects likely blasted free fragments of our planet that were then sent careening through space. Some of that high-velocity flotsam could have made it to Mars and other places within our solar system.

In the mid 1980s, Jay Melosh, professor of planetary science at the University of Arizona in Tucson, did seminal work on how rocks can be blasted off a planets surface at high speed, but without melting in the process.

His research points out that rocks very close to a planets surface would be blown skyward, withstanding the blast without suffering too much damage. Moreover, Melosh also realized that organisms might make the trip off the surface of a planet without being killed.

Over the course of solar system history, perhaps a dozen or so rocks ejected from the surface of one of the terrestrial planets may fall onto the surface of a terrestrial planet in another solar system. Similarly, the Earth should have received a few such interstellar wanderers over the course of solar system history.

"Biologically, the most active place on Earth is the soil. Thats where most of the microbes are," Melosh told SPACE.com. In 1988, he proposed that there could be interplanetary transfer of life from Earth to Mars or from Mars to Earth.

"Theres no proof that this happened, but my research suggests that its plausible. Life could have been exchanged from Earth to Mars and Mars to Earth if conditions on Mars were ever hospitable. If Mars was once warmer and wetter, then theres a chance there has been that kind of exchange," Melosh said

Therefore, in this give-and-take uprooting of life process, we Earthlings could be transplanted Martians, or vice versa.

"The terrestrial planets are not biologically isolated," Melosh said.

Like buckshot, material that is ejected from a parent planet will make many close passes to other planets or interact with gravitational tugs, primarily from the giant planets.

As a result of these encounters, the orbits of ejecta alter with time, Melosh reported during the recent 32nd Lunar and Planetary Science conference, held March 12-16, in Houston, Texas. While ejected rocks may eventually end up falling onto a terrestrial planet, some of those objects leave the solar system each year, he said.

Perhaps 30 percent of all terrestrial planet ejecta is eventually catapulted from the solar system, Melosh said.

Could such ejecta be likened to tiny arks of microbes sent sailing out across interstellar space?

"Over the course of solar system history, perhaps a dozen or so rocks ejected from the surface of one of the terrestrial planets may fall onto the surface of a terrestrial planet in another solar system," Melosh said. "Similarly, the Earth should have received a few such interstellar wanderers over the course of solar system history," he said.

"So the solar system, right now, sailing through the galaxy, is constantly spitting out life-containing rocks," Melosh said. "They come from either Mars or the Earth or one of the other terrestrial planets," he said.