Scientists have observed, for the first time ever, the atmosphere of a planet around another star. Astronomers used the Hubble Space Telescope to detect layers of air around a distant world labelled HD 209458b in the constellation of Pegasus.

The planet, which has unofficially been named Osiris, is the size of Jupiter. It lies so close to its own sun that it zips round it in a “year” that is just 3.5 days long.

Intense ultraviolet radiation from the star heats the upper atmosphere so much that it inflates like a balloon. Its upper layer of hydrogen is bleeding out into space like steam escaping from a kettle.

The planet is losing the gas at a rate of 10,000 tons a second, more than three times that of water flowing over Niagara Falls. The star itself, 150 light years from Earth, is bright enough to be seen in binoculars.

Hubble was able to observe the escaping atmosphere like a comet’s tail by analysing the star’s light as it passed through it. The planet itself, dubbed a “hot Jupiter”, lies too close to the star to be seen directly by the space telescope and is a gassy rather than a rocky world.

Osiris lies 20 times closer to its parent star than the Earth is from the Sun. Mercury, the closest planet to our Sun, is ten times its distance from the star.

The results, by an international team, were published today in the journal Nature. Leader Gilda Ballester, of the University of Arizona, said: “With this detection we see the details of how a planet loses its atmosphere. This planet’s extreme atmosphere could yield insights into the atmospheres of other hot Jupiters.”

The Nasa picture is an artist’s impression of Osiris losing its atmosphere.

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