Hell planet gets solar hammering

Mark Peplow

Astronomers propose new planet category for dying gas giants.

Astronomers have detected oxygen and carbon in the atmosphere of a planet outside our Solar System. But these elements aren't sticking around, nor can they be taken as signs of life, say the researchers.

Planet HD 209458b, also known as Osiris — named after the Egyptian god of the afterlife — is being scorched by its sun so hard that these raw elements are being ripped from the atmosphere in a wind of up to 35,000 km per hour.

This unusual planetary 'weather' has prompted the scientists to suggest that other planets may be taking even more of a buffeting, losing all their atmosphere to become dead remnants of evaporated gas giants. Osiris isn’t on its way to becoming such a planet, but its existence leads researchers to speculate that the harsher varieties may also exist.

The researchers propose calling the new category 'chthonian' planets, from the Greek word for denizens of the underworld. Further observations from Hubble could find proper chthonian planets.

The team from the Institut d'Astrophysique de Paris, France, used the Hubble Space Telescope to watch the planet, which is 150 light years away, slightly bigger than Jupiter, and orbits a star very similar to our own Sun once every three-and-a-half days. They saw that a vast cloud of carbon and oxygen surrounds the planet, which is carried along by an updraft of evaporating hydrogen.

Some scientists have proposed that the Earth was also once a gas giant that was scoured down to its rocky core by the hot, young Sun. The observations of Osiris may lend some credence to this theory — if it is happening to Osiris, it could have once happened to the Earth too, they say.

Hotter than hell

The discovery of the two key elements of organic life — carbon and oxygen — on another planet shouldn’t excite people who are searching for extraterrestrial life, says Alfred Vidal-Madjar, one of the astronomers who made the discovery. The surface of Osiris has a temperature of more than 1,000 ºC, so these elements wouldn't stand a chance of forming the complex structures needed for life, he says.

Other planets in our Solar System, such as Jupiter and Saturn, also contain these elements. But they are present in molecules of methane and water deep down in the atmosphere, rather than floating free.

Osiris has a history of proving planetary firsts. It was the first extrasolar planet to be found with an atmosphere — and the first to be shown to have an atmosphere containing hydrogen.

References

Vidal-Madjar, A. et al. Astrophysics. Journal of Astrophysics, In press (2004).

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