Astronomers first discovered the planet, dubbed HD 209458b, in 1999, around a star in the Pegasus constellation 150 light-years from Earth. The Jupiter-like object is unique because its three-and-a-half-day orbit takes it in front of a star close enough and bright enough to be observed. Using an ultraviolet spectrograph on board the Hubble Space Telescope, Alfred Vidal-Madjar of the Astrophysical Institute of Paris and his colleagues studied the planet in October and November 2003. "What¿s key here is that we have detected oxygen and carbon in atomic form and in the outermost layers of the planet where we would not normally expect them," says study co-author Gilda Ballester of the University of Arizona. "These species are 10 times heavier than hydrogen atoms, so a force stronger than gravity is driving them up along with the hydrogen gas into the very extended envelope around the planet."
Astronomers first detected hydrogen evaporating from HD 209458b¿s atmosphere last year, and they now calculate that the gaseous envelope is being ripped away at speeds greater than 35,000 kilometers an hour. "We speculate that even heavier elements such as iron are blown off at this stage as well," comments team member Alain Lecavelier des Etangs of the Astrophysical Institute of Paris. The researchers propose that HD 209458b is a novel type of extrasolar planet because of its distinct evaporation pattern. They further surmise that detection of more such planets, thought to be remnants of gas giants, may soon be within reach of both Earth- and space-bound telescopes. "It has been speculated that Venus, Earth and Mars may have lost their entire original atmospheres during the early part of their lives, " Vidal-Madjar says. "Their present atmospheres have their origins in asteroid and cometary impacts and outgassing from the planet interiors."