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It may not be long before we see other worlds

March 06, 2002

WE MAY actually see a planet around a nearby star within the next six months. A team of British astronomers hope to achieve this feat by focusing their search on white dwarfs-dimly glowing stars at the end of their lives.
        Although more than 80 planets outside our Solar System have been discovered, all were detected indirectly, by observing the minuscule effect their gravity has on the motion of their parent star, for instance. Seeing a planet directly is extremely hard because planets are so much fainter than stars. "It`s like trying to see a candle right next to a dazzling headlight," says Fraser Clarke of the University of Cambridge. It would be much easier if there were systems where the headlight was a lot fainter and the candle much farther from it.
        Clarke believes white dwarf planetary systems fit the bill. White dwarfs are burnt-out old stars, typically 10,000 times less luminous than the Sun.
        But should we expect to find planets around such elderly stars? In a paper to appear in Monthly Notices of the Royal Astronomical Society, a team led by Matt Burleigh of the University of Leicester, and including Clarke and Simon Hodgkin at Cambridge, admits that there won`t be any planets orbiting close to a white dwarf. They would have been cannibalised during its "red giant" phase, when it swells to monstrous proportions. But if a star puffs more than half its mass into space when it becomes a red giant, its gravitational grip would weaken, so any outer planets would spiral outwards to safety. "We have reason to believe that planets more than five times the Earth`s current distance from the Sun will survive into the white dwarf phase," says Clarke.
        The British team has been awarded time on the twin 8-metre Gemini telescopes in Hawaii and Chile (see picture). They are already looking at 50 nearby white dwarfs using near infrared wavelengths. "At such wavelengths, a planet three to five times the mass of Jupiter may be only 1000 times fainter than its stellar parent," says Clarke.
        Since all low-mass stars evolve into white dwarfs, Clarke says planets should be just as common around them as around other stars. That could mean about 10 per cent of white dwarfs would have planets. "Finding that many would be brilliant," says Clarke. "But we don`t know what we`re going to see."
        "I would expect most white dwarfs to have planets, so if anything I am more optimistic about detection then Burleigh`s team," says Steinn Sigurdsson of Pennsylvania State University. "I figure planets will be found."

Author: Marcus Chown
More at: www.arxiv.org/abs/astro-ph?0202194

New Scientist