Jay Melosh, a geophysicist at the University of Arizona's Lunar and Planetary Laboratory, has proved that rocks can be ejected from planets and remain cool enough to harbor life. New evidence from other researchers, reported in the October 27 issue of the journal Science, shows that any life aboard those rocks could survive a trip through Earth's atmosphere. Another group reported in the October 19 issue of Nature that microbes on Earth can survive 250 million years.

In light of the two new studies, we asked Melosh for his thoughts on panspermia, the theory that the seeds of life are everywhere in the universe.

SPACE.com: What's your view on the chances that a rock from Mars (or another planet or moon) is ejected and arrives at Earth with any potential microorganisms still alive?

SPACE.com: To what extent would you say either or both of the new studies lend support to panspermia?

Melosh: Both studies lend a healthy boost to the plausibility of panspermia. The first demonstrates that dormant organisms can survive at least 250 times longer than previously thought, making it much more likely that such organisms could endure the 10 million to 100 million year average trip times between Mars and Earth.

The second demonstrates clearly what we had previously only speculated about: that the conditions of launch, space transit and reentry are not too harsh for dormant spores and other microorganisms to survive.

SPACE.com: And, to stretch things a bit: Could a rock from another star system arrive at Earth with microorganisms intact?

Melosh: This is a stretch indeed. I just submitted a paper to Icarus that says that an interstellar journey is overwhelmingly improbable. However, a number of factors -- including the recent Nature article --are making me rethink this. It still seems very unlikely, but stand by for more work on this, suddenly very interesting, topic.

SPACE.com: What would be the nature of a journey for an interstellar rock?

Melosh: Any rock that might carry viable [dormant] organisms on an interstellar journey would have to be more than about 1 meter (3 feet) in diameter to provide shielding against galactic cosmic rays in the central region. This implies a moderately large [hence rare] impact ejection event, something producing a crater more than about 30 kilometers (18.6 miles) in diameter.

Our calculations show that such a rock ejected from, say, the Earth would have about a 30-percent chance of eventually encountering Jupiter and being ejected from the solar system (direct ejection into interstellar space is negligibly probable).

This entails a random walk among the planets of a few tens of millions of years, so our organisms must survive this period of dormancy, at least (but the Nature paper suggests that this is not impossible). Jupiter ejects most rocks from the solar system with a velocity relative to the Sun of about 5 kilometers (3.1 miles) per second.

Our rock now heads out into interstellar space where stars are sparse. If an organism can survive 100 million years in this mode (we estimate that radiation-hardy species such as Dienococcus Radiodurans can do this in a 3-meter, or 9-foot, diameter rock), it could travel a distance of about 2,000 light-years.

The optimum capture situation is if it encounters another Jupiter-size planet in another solar system. It is then captured by that planet (most probable for slow-approach velocities -- less than a few kilometers per second) into a bound orbit. Eventually the orbit is perturbed (altered) until it crosses the orbit of a terrestrial-type planet and collides with it.

This whole process (the subject of the upcoming Icarus paper) is very unlikely. However, if the increased possibility for reviving ancient dormant organisms is correct, then it is more likely than before (but still not probable!).

SPACE.com: Would you say that panspermia is about to experience a renaissance among researchers, and that the interstellar angle could become one of the hottest new areas of research?

Melosh: If new work were to make the interstellar angle more probable, then it might be a hot topic, although many origin-of-life researchers would be frustrated because the conditions under which life originated would be even less accessible!