ASTRONOMERS in Britain have earned a place in history by making the first direct sighting of a planet orbiting another sun. The new world, which has been called the "millennium planet'', is 55 light-years from earth, nearly twice the size of Jupiter and probably blue in colour with high clouds, they report.

Sophisticated computer software was used to untangle the planet's light from the glare of its parent star, Tau Bootis, which is 30,000 times brighter. As a result the astronomers were able to "see'' the planet, or at least detect its reflected light. No-one has ever achieved this before. Until now almost every other known extra-solar planet has been detected indirectly by measuring the "wobble'' its gravity imparts on the star it orbits.

In the past four years, 28 nearby sun-like stars, including Tau Bootis, have been assumed to possess planets by this method. In November, two teams in the United States detected the "shadow'' of one of these planets. They saw a brief dimming in the light of the star HD 209458 as the planet crossed its face. But although the observation was a major step forward it pales next to the UK scientists' achievement: the first extra-solar planet discovery made by UK astronomers.

Capturing light from the surface of the planet provides scientists with a potential goldmine of information. For the first time it is possible to say something meaningful about the composition of a planet in another solar system.

The astronomers know the millennium planet is about 1.8 times the diameter of Jupiter but eight times its mass, orbiting close to the star and very hot with a surface temperature of around 1,100 degrees Celsius. They can also assume that the planet is bluish in colour and with a cloudy atmosphere. Despite being a gas giant with no solid core it would probably contain solid particles. The planet is thought to be composed mainly of heated hydrogen but its blue colour suggests the presence of magnesium, possibly in the form of silicate droplets.

Further detailed analysis of the planet's faint light spectrum is expected to reveal more precisely of what it is made.

One of the astronomers, Dr Alan Penny, from the Rutherford Appleton Laboratory in Didcot, Oxfordshire, southern England, said: ``For the first time it has been possible to capture light from an extra-solar planet and use it to make some sense of the planet's physical constituents. Our discovery is a major step in finding out what these planets are really like, a step that could lead to finding planets like earth.''

Dr Penny is one of the chief scientists behind the Darwin mission, a European plan to launch a giant space telescope in 2012 specifically designed to search for earth-like life-sustaining planets in other solar systems. But he doubted there was life on the millennium planet.

''There probably won't be any life there,'' he said. ``This is a gas giant, like Jupiter. It's too hot and has no solid surface and no liquid water. You can speculate but if there were any life it wouldn't be life as we know it.''

The astronomers made the discovery with the 4.2-metre William Herschel telescope on La Palma in the Canary Islands. A first attempt in 1998 produced no clear sign of the planet. But computer analysis revealed a faint copy of the star's spectrum embedded in the starlight from Tau Bootis.

The faint light signal wobbled back and forth in the opposite direction to the star's own smaller wobble. This told the astronomers they were picking up light reflected from the surface of a closely orbiting planet tilted to the line of sight.

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