DON鈥橳 worry about contact with aliens from other solar systems鈥攖hey may
be our distant cousins. According to an American astronomer, there is a slim
chance that microbes could be carried from one solar system to another on rocks
blasted from terrestrial planets by asteroid impacts, spreading life across the
Galaxy.
鈥淎bout one meteorite ejected from a planet belonging to our Solar System is
captured by another stellar system every 100 million years,鈥� Jay Melosh of the
University of Arizona told the Lunar and Planetary Science Conference in Houston
this week.
Although radiation would threaten stowaway microbes, Russell Vreeland of West
Chester University of Pennsylvania says it would be quite possible for
meteorites to carry well-protected organisms over interstellar distances.
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In the 1970s, astronomers Fred Hoyle and Chandra Wickramasinghe put forward
the still controversial theory, dubbed 鈥減anspermia鈥�. This says that comets
bombarding the Earth brought the bacteria and viruses from interstellar space
that started life here 4 billion years ago, and continue to bring in new
biological material today. Melosh argues that alien organisms might also come
from a distant planet similar to our own.
He is part of a group that earlier showed microbes could hitch a ride on
meteorites travelling between planets in our Solar System
(快猫短视频, 15 January 2000, p 19).
At the time, he didn鈥檛 think any
microbes could survive the millions of years a meteorite would take to travel
between stars. That view changed, however, after Vreeland successfully cultured
bacterial spores from a 250-million-year-old salt deposit in New Mexico
(快猫短视频, 21 October 2000, p 12).
The longer survival time makes the transfer of life conceivable, Melosh says.
Transfers between solar systems depend on gravitational interactions between
meteorites and other planets. As a starting point, Melosh considered rocks
blasted off the surface of Mars by impacts. His simulations show that Jupiter
can act as a slingshot, flinging roughly 500 kilograms of Martian rocks each
year right out of our Solar System in all directions. Their velocity averages 5
kilometres per second, so in a million years they would travel about 17 light
years鈥攆ar enough to reach nearby stars.
Most ejected meteorites would continue to drift in the interstellar void, but
a few would eventually pass near other planetary systems. 鈥淭he probability of
direct capture by an Earth-sized planet is very, very tiny,鈥� says Melosh.
However, the gravity of a Jupiter-sized giant planet can capture meteorites
passing within a hundred million kilometres of it, if the two are moving at
similar velocities in the same direction. The meteorite would then fall into an
eccentric orbit about the star.
It is still far from certain whether the meteorite would go on to collide
with a terrestrial planet, and Melosh鈥檚 calculations suggest that the likelihood
of such an event is low for a solar system like our own. The chances would be
higher, he says, if terrestrial planets orbited near to a Jupiter-sized
planet.
鈥淭he probabilities are pretty low,鈥� acknowledges Melosh. But they aren鈥檛
impossibilities, he adds. Wickramasinghe believes that his panspermia theory, in
which bacteria can drift on their own between solar systems, propelled by
radiation pressure, is a more likely scenario. 鈥淭he only advantage that you
might have [with] huge chunks of rock [is that] the interior is shielded totally
from any damaging radiation,鈥� he says.