Poof goes an atmosphere.

Blasted by the heat and radiation from its planet's parent star, the atmosphere of a distant planet is blowing off into space. Astronomers have now detected carbon and oxygen escaping from the upper reaches of the searingly hot planet HD209458b, which orbits its star at just one-eighth the distance that Mercury orbits the sun. This is the first time carbon and oxygen have been found in an extrasolar planet, researchers report in an upcoming Astrophysical Journal Letters.

Alfred Vidal-Madjar of the Astrophysics Institute of Paris and his colleagues suggest that the carbon and oxygen is dragged from HD209458b's atmosphere by the evaporation of hydrogen, which was previously detected in the planet's upper atmosphere (SN: 3/15/03, p. 164). If this process persists, the planet could be stripped to its dense core in a few billion years.

Like the other 125 or so extrasolar planets known, HD209458b is too faint to be imaged and was discovered indirectly, through the tug it exerts on the star it so closely orbits. The planet, which lies 150 light-years from Earth, also makes its presence known in another way. During each orbit, it passes directly between its star and Earth. Thus, once every orbit, the starlight reaching Earth must filter through the planet's extended atmosphere.

Analyzing the light with a spectrograph aboard the Hubble Space Telescope, Vidal-Madjar's team found oxygen and carbon atoms in a vast oblong envelope surrounding the planet. Some planets may lie even closer to their parent stars, but the heat and gravity there may have whittled the planets' masses down to undetectable levels, the team says.--R.C.