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Thursday, 16 December, 1999, 16:08 GMT
Astronomers 'see' planet orbiting distant star
"Millennium" planet impression by David A Hardy
By BBC News Online Science Editor Dr David Whitehouse

Light from a planet circling another star has been detected for the first time.

This significant finding, revealed exclusively last month by BBC News Online, is published in detail in the journal Nature on Wednesday.

In making the discovery, UK scientists from St Andrews University in Scotland and the Central Research Laboratories in England have devised a method to extract the faint light signal from a planet in orbit around its host star, even though the star itself is at least 10,000 times brighter.

The technique could now be used to find other planets elsewhere in the Universe.

"This is a tremendous coup for British astronomers", said Ian Halliday, head of the Particle Physics and Astronomy Research Council.

Commenting on this first, direct visible detection of what is generally referred to as an exoplanet, Lord Sainsbury, the UK Minister for Science said: "This discovery was made possible by the innovative work of British scientists.

"It is exciting to be able to look, for the first time, at the light from a planet outside our own Solar System. It is another step in the search for life elsewhere in the Universe."

Visual proof

The team of researchers included Dr Andrew Collier-Cameron, Professor Keith Horne, Dr David James and Dr Alan Penny.

They knew that a planet slightly bigger than our own Jupiter was circling very close to the star called Tau Boo, which is some 50-light-years away from Earth.

The planet, slightly larger than Jupiter, would be very hot
The planet, slightly larger than Jupiter, would be very hot
They knew this because there was a wobble in the star's motion produced by the gravitational influence of the nearby planet. But the team wanted visual proof.

The method they devised for detecting the light reflected off the planet relied on the fact that the world was orbiting the star every 3.3 days. The movement of the two bodies produced very different light signals that the scientists believed could be separated out from each other.

A computer program was written to scan the spectrum looking for the difference in wavelength changes in the light coming from both bodies. The trick was being able to pick out the tiny component in the spectrum that was doing the opposite to the star.

Telescope time

"We had the idea one morning in February 1997, when our collaborator Alan Penny was visiting St Andrews," Dr Collier-Cameron told BBC News Online.

"At coffee time, the three of us got to wondering just what the brightness ratio would be for a 'hot Jupiter', and after destroying a couple of envelopes we decided that one part in 10,000 was about right.

"That set me going, as that's the kind of signal-to-noise level that Jean-Francois Donati and I regularly achieve in our stellar magnetic-field mapping work. We did a couple of simulations, and on the strength of that managed to wangle a few hours of twilight time on the Herschel Telescope that spring."

What the team had to obtain was a high-quality spectrum of the star-planet system. They needed a series of spectra taken at various times during the 3.3-day planetary orbit. At one point in the orbit, the star would be moving away from us and the planet towards us. At another point in the orbit, the situation would be reversed.

Knowing this, the planet's light could then be extracted from the combined star-planet spectrum.

Blue-green colour

"The eureka moment came in May, once most of this year's observations were complete," said Dr Collier-Cameron. "At that stage, as now, we were 95% certain that we had something."

The detection is not completely certain, however, and requires more work and observations. The team plan to observe Tau Boo again in the spring.

The team also wants to try the same technique out on some other suspected exoplanets, which may well turn out to be easier targets than Tau Boo. The strong tilt in the orbit of the planet around Tau Boo made detection harder than it would have been had the system been orbiting edge-on.

The observations herald a new branch of planetary science. The spectra, albeit somewhat lacking in detail, of the planet orbiting Tau Boo are a landmark. They show the planet to have a blue-green colour. More precise observations of other planets may tell us what their atmospheres are made of and, possibly, tell us if the conditions are right for life.

The Tau Boo exoplanet, which is about eight times the mass and about 1.8 times the size of Jupiter, has unofficially been named the "Millennium Planet".

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See also:

22 Nov 99 | Sci/Tech
Light detected from distant planet
16 Nov 99 | Sci/Tech
Extrasolar planet detected
25 Nov 99 | Sci/Tech
We saw it too, say astronomers
30 Nov 99 | Sci/Tech
...and then six come along at once
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