The researchers glimpsed the blue-green colour of the "Millennium Planet" using the William Herschel Telescope on the Canary Islands and a custom- built computer program to disentangle the planet's extremely faint light from the background illumination of the night sky.
Although astronomers have detected over the past four years about 20 planets outside the solar system by the gravitational pull that makes their nearby stars wobble, this is the first time scientists have directly observed a planet's reflected light.
The observation, published in the Nature journal, was made by Andrew Collier Cameron, Keith Horne and David James of the University of St Andrews, and Alan Penny of the Rutherford Appleton Laboratory in Chilton, Oxfordshire. "This is the first time that a planet has actually been seen, so for the first time we've got some physical knowledge," Dr Penny said.
"It's an important step towards studying small planets like the Earth and studying for life on them. We've seen the light from this planet and we can make the first guess of what the atmosphere of this distant world might be like."
The planet does not yet have a name, although unofficially it is the "Millennium Planet". It orbits the star Tau Bootis, located some 50 light years away in the constellation of Bootis. Calculations made from the shifting colour of the reflected light suggests that the planet is nearly four times heavier than Jupiter, the solar system's biggest planet.
The "Millennium Planet" is 20 times nearer to its star than the Earth is from the Sun and is so close that even the Hubble Space Telescope cannot distinguish between the two objects. "The combination of close orbit, high mass and the fact that Tau Bootis is hotter and bigger than our Sun, means this planet is one of the hottest known," said Adam Burrow and Roger Angel, two Arizona University astronomers, in an accompanying commentary.
At a temperature of some 1,100C, the planet would be unsuitable for life, and like other gaseous planets would in any case have no solid surfaces for life to evolve in a watery medium. "If it was a solid planet it would be so small that we wouldn't be able to detect the light reflected from it. This technique can only work for large gas planets," Dr Collier Cameron said.
However, the success of the observation using a ground telescope has shown that it is technically feasible to decipher a planet's reflected light even though it is some 20,000 times fainter than its nearest star. Dr Penny said a mission involving sending telescopes into space would improve the chances of detecting smaller, solid planets.
"To be one of the first people on Earth to see a completely new planet is an awesome experience, especially on the eve of a new millennium," Dr Penny said. "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."
Within 20 years it will be possible to take photographs of planets outside the solar system, analyse the chemical composition of their atmosphere and even judge whether they harbour life. "The teenagers of today will have children who will put posters on their walls depicting another Earth- like planet, and they will wonder what it is like there," he said.