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By Clive Cookson

Published: October 13 2006 03:00 | Last updated: October 13 2006 03:00

Astronomers have for the first time measured the day and night temperatures of a planet outside the solar system. Using Nasa's Spitzer Space Telescope the international team showed that the giant Jupiter-like planet, orbiting a star 40 light-years away, is always fiery hot on one side and cool on the other.

The observation, reported in Science, is the first time any kind of variation has been detected across the surface of an extrasolar planet. Upsilon Andromedae b, as the planet is called, is extremely close to its parent star, completing an orbit every 4.6 days. As a result it is "tidally locked" to the star, so that one side is constantly illuminated - and heated to about 1,500°C - while the other remains dark with a temperature close to freezing point.

"The atmosphere of the planet must be absorbing and re-radiating light fast enough that any heated gas circulating around the planet is cooled off before it reaches the dark side," said James Cho of Queen Mary, University of London. "This is the opposite of Jupiter, which is essentially the same temperature all around."