From Michael Paine
Your report on Jay Melosh’s work
(17 March, p 4) mentions how a
microbe-infested rock from Earth or Mars might reach another solar system, tens
of light years away. The odds are pretty small.
There is, however, a mechanism that might increase the chances of rocks being
exchanged between planetary systems. The stars might come to us.
In 1981 Jack Hills published a paper on comet showers and the steady-state
infall of comets from the Oort Cloud. While looking at a possible cause of comet
bombardments, he estimated the mean time between stars passing close to our
Sun.
Over the lifetime of the Earth, about four star systems may have come within
1000 astronomical units of the Sun. Every 100 million years or so a star system
passes within 3000 AU. It would be interesting to work out the odds of a
microbe-bearing Earth rock finding its way into this region of the Solar System
and then being picked up by a passing star and landing on a suitable planet. No
doubt they are extremely poor odds—but better than those on travelling 10
light years or more through empty space.
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While a star was passing near the Solar System, you would expect the Earth to
be bombarded by comets dislodged from the Oort Cloud over a period of several
years. The number of rocks launched bearing microbes could therefore increase
during this period, improving the odds of transpermia (the exchange of life via
meteoroids) succeeding.
Sydney
