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Wednesday, August 18, 1999 Published at 17:26 GMT 18:26 UK


Planet found orbiting two stars

By BBC News Online Science Editor Dr David Whitehouse

Astronomers may have discovered a Jupiter-sized planet that orbits two stars, the first time this has been seen.

It was found by a team from the USA, Israel and Australia using a telescope at the Mount Stromlo observatory in Australia. The discovery was made thanks to a "micro-lensing" event.

These occur when an object like a dim star or a planet passes directly between Earth and another star. The gravity of the intervening star or planet acts like a magnifying glass, improving the view of the more distant star.

Double vision

On 19 June astronomers saw such a brightening which lasted 100 days. After only a few days, however, they realised the specific pattern of light they saw could not be due to just one star - they needed two stars and a planet to explain it.

They suggest that the two stars are both smaller and dimmer than our Sun and orbit each other at a distance of about 150 million miles.

Outside this a Jupiter-sized planet orbits the pair of them at a distance of about 650 million miles.

More to come?

Planets have been found in double star systems before. But they have only been orbiting one of the pair, not both of them. Most stars in the galaxy belong to double (or more) systems, so it is likely that there are many planets that orbit two (or more) stars, waiting to be seen.

Detecting planets circling other stars is just one use of micro-lensing. Several teams of astronomers are monitoring the light from millions of stars every night looking for this form of stellar brightening.

They hope they may be able to obtain evidence about the Universe's "dark matter" which is known to be out there, but which has so far defied analysis.

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