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Last Updated:  Wednesday, 12 March, 2003, 23:45 GMT
Hubble sees evaporating world
By Dr David Whitehouse
BBC News Online science editor

The Hubble Space Telescope (HST) has observed a planet being destroyed because it orbits too close to its parent star.

Planet, Nasa
An artist's impression of the planet in its precarious orbit
The intense radiation and gravity of the star combine to heat and disperse the outer atmosphere of the Jupiter-sized world.

Instruments on Hubble have detected a vast cloud of hydrogen billowing out into space behind the superheated planet.

Eventually this world could become much smaller and lose all its hydrogen.

'Hot Jupiter'

Since the discovery of the first extrasolar planet in 1992, astronomers have detected more than 100 such worlds circling other stars.

HD209458, located about 150 light-years away, is similar to our Sun. Its planet, HD209458b, discovered in 1999, is similar to Jupiter.

About 15% of extrasolar planets orbit very close to their parent stars, typically with periods of a few days. HD209458b, for example, has a "year" that lasts only 3.5 days.

Because HD209458b is only seven million kilometres (4.3 million miles) from its star, astronomers call it a "hot Jupiter".

By chance, the orbit of the planet takes it in front of its star as seen from Earth - which is lucky for scientists.

During the three-hour eclipse, HD209458b hides a small part of the stellar disc, allowing the atmosphere of the planet to imprint its signature on to the light from the star. This reveals a wealth of information.

It was to observe this signature and determine what it tells us about the state of the planet that an international team of astronomers observed the eclipses with the Hubble Space Telescope, using the spectrograph that astronauts installed in February 1997.

Starlight expulsion

The ultraviolet light detected by the Hubble instrument allowed the hydrogen in the planet's upper atmosphere to be observed.

Unexpectedly, the observations showed this layer spread out over a huge area.

"We were astonished to see that the hydrogen atmosphere of this planet extends over 200,000 km," said Alfred Vidal-Madjar, of the Geneva Observatory, Switzerland.

Indeed, because the gas is detected well beyond the gravitational influence of the planet, astronomers believe it is escaping from HD209458b, literally being blown off the world by starlight.

It is estimated that at least 10,000 tonnes of hydrogen per second is being blown away from HD209458b.

Not only is the planet's upper atmosphere being subjected to intense heating, the star's strong pull of gravity is likely creating tidal forces that also distort its outer gas layers.

"The atmosphere is thus stretched, then hydrogen is pushed away by the starlight and strewn out in a large tail similar to those of comets," said Alain Lecavelier des Etangs, of the Paris Institute of Astrophysics, France.

This evaporation of the planets which are too close to their parent stars could explain the very few detections that are made of planets orbiting at less than seven million km from their parent stars.

Those planets either quickly evaporate, or become hydrogen-poor Neptune-like worlds.

The Hubble research is published in the journal Nature.

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10 Jan 02 |  Science/Nature

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