DON鈥橳 believe everything you read in books鈥攐ur Sun is no ordinary star.
And its very uniqueness has implications for SETI, the search for
extraterrestrial life, claims Guillermo Gonzalez of the University of Washington
in Seattle: 鈥淯nless astronomers narrow down their search to stars as exceptional
as the Sun, they are wasting much of their time.鈥�
The Sun is a single star whereas most stars are in multiple systems. But that
apart, textbooks say the Sun is pretty average. However, after trawling through
the data on the Sun, Gonzalez has found many idiosyncrasies. It is among the
most massive 10 per cent of stars in its neighbourhood. It also has 50 per cent
more heavy elements than other stars of its age and type, and about a third of
the variation in brightness.
The most unusual aspects of the Sun concern its orbit around the centre of
the Galaxy, says Gonzalez. Its orbit is significantly less elliptical than that
of other stars of its age and type, and hardly inclined at all to the Galactic
plane. What鈥檚 more, the Sun is orbiting very close to the 鈥渃orotation radius鈥�
for the Galaxy鈥攖he place at which the angular speed of the spiral pattern
matches that of the stars.
Advertisement
Gonzalez argues that these exceptional characteristics made it possible for
intelligent life to emerge on Earth. He points out that stable planetary orbits
such as the Earth鈥檚 are much more likely around single stars like the Sun. For a
massive star with inhabitable planets that are relatively far away, stellar
flare-ups would be little threat to the planets. Heavy elements are essential to
make planets like Earth, and a star with a stable light output is essential for
life.
As for the orbit of the Sun, its circularity prevents it plunging into the
inner Galaxy where life-threatening supernovae are more common. And its small
inclination to the Galactic plane prevents abrupt crossings of the plane that
would stir up the Sun鈥檚 Oort Cloud and bombard the Earth with comets. By being
near the Galaxy鈥檚 corotation radius, the Sun avoids crossing the spiral arms too
often, an event that would expose it to supernovae, which are more common
there.
Because life-bearing stars have to be close to the corotation radius, that
rules out more than 95 per cent of stars in the Galaxy in one fell swoop. 鈥淭here
are fewer stars suitable for intelligent life than people realise,鈥� says
Gonzalez, who has submitted his findings to Astronomy & Geophysics.
鈥淚鈥檓 amazed at how little thought the SETI people put into selecting their
蝉迟补谤蝉.鈥�
Seth Shostak of the SETI Institute in Mountain View, California, disagrees.
鈥淥ur targets are all very close to the Sun. They share our Galactic
neighbourhood and motions. If the Sun is the most suitable type of star to be
scrutinised, then we are, indeed, looking in all the best places.鈥�
鈥淢ost astronomers disagree with Gonzalez,鈥� adds SETI researcher Dan Werthimer
of the University of California at Berkeley. 鈥淥ur Sun is pretty average. In any
case, you don鈥檛 need a star exactly like our Sun for life.鈥�