Our recent look at panspermia concepts was largely devoted to the transmission of life via microbes or spores here in our own Solar System. The even richer question of how life might pass from star to star is far more problematic, but as a follow-up to that earlier story, I want to look at work that graduate student Jess Johnson did with Jonathan Langton and advisor Greg Laughlin at the University of California, Santa Cruz. Their work suggests that while life might readily survive an interstellar journey, it is unlikely to wander close enough to seed another system.

Ponder the era here on Earth known as the Late Heavy Bombardment (LHB). After the period of planetary accretion ended some 4.4 billion years ago, life apparently began. But 3.8 to 4 billion years ago, the LHB saw the planet again pummeled, causing debris to be ejected into space. Looking specifically at the mass that is ejected at 16.7 kilometers per second in the direction of the Earth’s motion (this is Solar System escape velocity), Johnson, Langton and Laughlin found that a substantial amount of rock (about 5 X 1021 grams) would have been blasted free of the Sun.

Remember, this is a period after life has started, so biological material could presumably be involved in any materials lifted into space. But what could survive the 20,000 g’s the ejecta would have experienced, and then cope with vacuum, radiation, cosmic ray strikes and ultimate re-entry and collision upon arrival? Bacillus subtilis is a common bacteria that needs no oxygen to survive, uses carbon and nitrogen as nutrients and forms spores when it lacks the nutrients to thrive. The dormancy period we’re talking about runs into the tens of millions of years, obviously long enough for an interstellar journey — even our glacially slow (by interstellar standards) Voyager spacecraft could make it to the Centauri stars in 75,000 years or so if they were pointed in that direction.

Here’s a striking fact: A viable sample of Bacillis has been found in the stomach of a mosquito encased in amber that has been dated at 25 million years old. Moreover, Bacillus passes all the other tests, able to survive impact pressures upon arrival, capable of enduring 33,800 g’s and, shielded by a sufficient outer encasement of rock, more than able to withstand the radiation hazards of the journey. In deriving the amount of ejected materials (the 5 X 1021 grams mentioned above), the Santa Cruz team chose only those fragments of rock greater than one metre in diameter to ensure the necessary shielding.

So everything looks promising for interstellar panspermia except the possibility that such life-bearing rocks may make their way to another stellar system. Producing calculations on the odds of capture, the trio found a result discouraging for interstellar panspermia theorists:

The results of our work found that, although there are microrganisms that are easily capable of surviving all of the challenges of interstellar travel, the probability of capture by another planetary system is vanishingly small. It should be noted that this in no way negates the possibilty of transport between worlds in our own system, a situation that seems quite possible.

A poster on this work (though without the later results) can be found here.

Related: An upcoming paper by William Napier and Janaki Wickramasinghe (Cardiff Centre for Astrobiology) in Monthly Notices of the Royal Astronomical Society discusses the Solar System’s movement through the plane of the galaxy, suggesting that the chances of comet collision go up every 35 to 40 million years. The potential for disaster on Earth is obvious, but the paper argues that such impacts help life to spread. Says Chandra Wickramasinghe, the Centre’s director, “This is a seminal paper which places the comet-life interaction on a firm basis, and shows a mechanism by which life can be dispersed on a galactic scale.” Wickramasinghe collaborated with Fred Hoyle in the 1981 book Evolution From Space. More in this news release.