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Telescopes and space missions

Light glimpsed from extra-solar planet

16 Dec 1999

Since the detection of the first 'extrasolar' planet four years ago, almost 30 planets have been discovered outside the solar system. Nearly all of them were detected as a result of the planet causing the star around which it orbits to "wobble". Now, Andrew Cameron from the University of St Andrews and colleagues claim to have seen light reflected by the planet orbiting around the star tau Bootis (Nature 402 751). The planet - first discovered in 1997 - is eight times the mass and 1.8 times the size of Jupiter, and blue-green in colour. Tau Bootis is 50 light years from Earth.

Light from the planet is almost completely obliterated by the bright glare from the star. Although the reflected signal is 10-20,000 times weaker than the glare from tau Bootis, the planet studied by Cameron and co-workers is still the brightest and hottest planet among the extra-solar planets discovered so far. It also has one of the shortest orbital periods – just over three days – of any known planet.

The light from the planet was detected by exploiting the high orbital speed of the planet. By looking for Doppler shifts caused by this motion, Cameron and colleagues were able to find a faint trace of the light reflected by the planet. The chances of the signal being due to random noise is just 5 percent.

The team hope to confirm their findings during further observations in March and to apply the technique to other star systems later in the year. However, the method only works on planets with very short orbits. “If you want to go looking for Earth-like planets you’re going to have to use another technique,” says Cameron.

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