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2.10.2011 Vulcan's mates: The search for other Earths is hotting up

From The Economist

KEPLER, America’s planet-hunting space probe, is now really getting into its stride. The craft, which is armed with a telescope that can track more than 100,000 stars simultaneously, looks for slight diminutions of light caused by planetary transits. These transits are mini eclipses—the passage of the planet in question through the line of sight between its parent star and Kepler’s telescope. Transit detection can pick up much smaller planets than previous methods based on gravity-induced wobbles in the stellar parent. The hope is that, soon, it will find one as small as Earth.

On February 2nd America’s space agency, NASA, which controls Kepler, announced the latest results from the probe. So far, it has seen transit-like dips in the light from more than 1,000 stars. In the case of 170 of these the pattern of dips suggests at least two planets; for 45 stars it looks as if there may be at least three planets; in eight cases there may be four planets; in one case, five; and in one other instance, six.

Most of these dips represent only candidate planets at the moment, rather than confirmed ones. Though they have happened often enough to persuade Kepler’s researchers that the dips themselves are real, the way in which the team is conducting its initial search trades quantity of stars for precision of observation. This means that light from a star that Kepler is examining is sometimes “polluted” by light from other stars that appear near to it in the sky. If such a neighbour is a variable star (for example, a double star called an eclipsing binary in which two stars that orbit each other take it in turns to pass in front of one another), that can create the illusion of a transiting planet passing in front of the target star. Each of the candidate systems has therefore to be studied closely, to decide whether there really are planets involved.

In the case of some, that has already been done. As we reported last month, an object surprisingly similar to an astronomical wild-goose from the 19th century, the mythic planet Vulcan, was found orbiting a star dubbed Kepler 10. And in a paper published by Nature to coincide with NASA’s announcement of its candidates, that discovery was trumped six times over by an analysis of the most populous putative system, Kepler 11, 2,000 light-years from Earth. This shows that the planets in question are, indeed, real.

Jack Lissauer, of NASA’s Ames Research Centre, in California, and his team, have not only confirmed the existence of the six planets, they have worked out their orbital periods, diameters and, in all but one case, their masses. None is quite as small as Earth. They range in diameter from double the Earth’s to 4½ times, and in mass from 2 1/3 times the Earth’s to 13½ times. Nor are any of them orbiting in what astronomers fondly refer to as the “habitable zone” of Kepler 11—the distance from the star where water on the surface of an Earth-sized planet would be too cool to boil and too hot to freeze. Their orbital periods range from ten to 118 terrestrial days, which would put all but one of them inside the orbit of Mercury, were they going round the sun. The inner two appear, from their densities, to have a lot of water or methane or ammonia in them (or any mixture of the three), along with hydrogen and helium. The other three whose masses are known are less dense, so presumably have more hydrogen. The planet orbiting Kepler 10, by contrast, has an orbital period of a mere 20 hours and is so dense that it is probably made of iron.

None of which sounds all that encouraging for life-hunters. Such optimists, though, need not despair. According to Dr Lissauer, about 50 of the 1,200 or so candidate planets (if planets they be) are orbiting in the habitable zones of their parental stars. These candidates, you may be sure, will be subject to particularly intense scrutiny. The search for a new Earth has now begun in earnest.