Upsilon Andromeda b, a gas giant 40 light years from Earth that orbits very close to its parent star, is a two-faced planet of extremes, observations with Nasa’s Spitzer Space Telescope show. One hemisphere is turned permanently towards its sun, with temperatures of up to 1,650C (3,000F). The other endures perpetual night, and cold ranging from minus 20C (-4F) to minus 230C (-382F).

“If you were moving across the planet from the night side to the day side, the temperature jump would be equivalent to leaping into a volcano,” said Brad Hansen, of the University of California, Los Angeles, who led the research.

His colleague, James Cho, of Queen Mary, University of London, said that the temperature extremes “make for a very unusual climate and weather”.

This strange world, which orbits Upsilon Andromeda, a star visible with the naked eye in the constellation Andromeda, was discovered in 1996. Details of its bizarre climate, however, could be seen only after the launch in 2003 of the Spitzer telescope, which observes infra-red light. Planets orbiting other stars are difficult to detect and study because the light they emit is swamped by that of their parent suns, but they stand out better when viewed in infra-red light.

Dr Hansen’s team, who publish their results today in the journal Science, found that more infra-red light was visible when the planet’s day side was in Earth’s view than when the night side was turned towards us. This allowed them to calculate temperatures for both.

Upsilon Andromeda b is a kind of gas giant known as a “hot Jupiter”, which orbits its parent star at very close quarters: its “year” lasts 4.6 days and it is one sixth as far from its star as Mercury is from the Sun.

Its odd climate is thought to occur because its rotation has become tidally locked to the star, so that it rotates on its axis in unison with its orbit. This means that one side is permanently turned towards the star while the other never sees it, in much the same way as the Moon orbits the Earth so that its far side is never visible. Consequently, its weather must be extremely different to what is known of the gas giants of our own solar system: Jupiter, Saturn, Uranus and Neptune.

Dr Cho said: “The atmosphere of the planet must be absorbing and reradiating light fast enough that any heated gas circulating around the planet is cooled off before it reaches the dark side. This is the opposite of Jupiter, which is essentially the same temperature all around.”

Click here for a skymap of Upsilon Andromeda

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