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- For the first time, astronomers have detected light from
a planet in an alien solar system. This blue-green glint marks the start
of attempts to analyse the composition of extrasolar planets--and astronomers
are relishing the prospect of opening up a new field of research.
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- "We're going to see what giant planets in completely
different environments really look like," enthuses Roger Angel of
the University of Arizona in Tucson. He forecasts a "revolution in
planetary science". Since the first extrasolar planet was detected
in 1995, 28 have been found-- including 6 in the past month alone. Most
were revealed by detecting minute wobbles in the motion of their parent
stars. But attempts to observe the planets directly have been unsuccessful.
Earlier this year, for instance, astronomers conceded that what had appeared
to be a planet being ejected from its solar system was in fact a background
star (New Scientist, 3 July, p 13).
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- Now a team led by Andrew Cameron at the University of
St Andrews in Scotland has employed spectral analysis to separate the light
coming from a planet from the overwhelming glare of its parent star. Their
quarry was a giant planet orbiting Tau Boötis, a Sun-sized star 51
light years from Earth.
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- They knew the planet was much too dim and close to Tau
Boötis to show up on images of the star. But by observing Tau Boötis
using the 4.2-metre William Herschel Telescope at La Palma in the Canary
Islands, they were able to identify what appeared to be a faint replica
of the star's overall spectrum, Doppler shifted by the planet's orbital
speed of 75 000 metres per second. This, they argue, is caused by light
reflecting from the planet's surface.
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- The team pooled 48 hours of observations, fine-tuning
their understanding of the star's spectrum until they could subtract out
almost all its light. When they sifted through what remained, they found
a faint signature. "We found something snaking round the star, getting
brighter and dimmer," says Cameron.
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- The astronomers calculate that the planet is twice the
diameter and eight times the mass of Jupiter. It orbits Tau Boötis
so closely (see Diagram) that its atmosphere sizzles at 1500 °C. According
to theory, its clouds should include droplets of molten iron along with
sodium and potassium. But this means that the planet should absorb most
of the light hitting its atmosphere, reflecting just a small amount of
white light. The fact that it has a blue-green hue and is brighter than
expected puzzles theorists.
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- Another puzzle is that Steven Vogt of the Lick Observatory
near San Jose and Dave Charbonneau of the Harvard-Smithsonian Center for
Astrophysics in Cambridge, Massachusetts, have tried and failed to detect
light from the same planet, using the 10-metre Keck Telescope in Hawaii.
Cameron admits there is a 1 in 20 chance that the pattern he observed is
an artefact. "But it would have to be a pattern of noise spikes that
just happens to mimic a planet." He hopes to nail the case by getting
more telescope time to confirm the Tau Boötis sighting, and to take
a look at giant planets of other stars, such as 51 Pegasi.
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- And now Cameron's team has shown the way, other astronomers
are certain to jump on the bandwagon--which means the revolution predicted
by Angel shouldn't be long coming.
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