What a star!
By Marcus Chown DON’T believe everything you read in books—our 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: “Unless astronomers narrow down their search to stars as exceptional as the Sun, they are wasting much of their time.” 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’s more, the Sun is orbiting very close to the “corotation radius” for the Galaxy—the place at which the angular speed of the spiral pattern matches that of the stars. 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’s 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’s Oort Cloud and bombard the Earth with comets. By being near the Galaxy’s 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. “There are fewer stars suitable for intelligent life than people realise,” says Gonzalez, who has submitted his findings to Astronomy & Geophysics. “I’m amazed at how little thought the SETI people put into selecting their stars.” Seth Shostak of the SETI Institute in Mountain View, California, disagrees. “Our 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.” “Most astronomers disagree with Gonzalez,” adds SETI researcher Dan Werthimer of the University of California at Berkeley. “Our Sun is pretty average. In any case,