spacetoday.net: Astronomers observe "evaporating" exoplanet

Posted: Thu, Mar 13, 2003, 11:04 AM ET (1604 GMT)
Evaporating exoplanet illustration (ESA)

European and American astronomers reported Wednesday that an extrasolar planet is losing its atmosphere to the intense heat of its nearby star. In a paper published in the current issue of the journal Nature, astronomers said they detected an envelope of hydrogen surrounding and trailing the exoplanet HD209458b, a gas giant orbiting just 7 million kilometers from its star, 150 light-years from the Earth. The planet was not observed directly by astronomers; instead, they used a spectrograph on the Hubble Space Telescope to detect a decrease in emissions from the star caused by the hydrogen cloud when the planet passed between it and the Earth. The cloud is likely caused by the extreme heating of the planet's atmosphere, allowing the hydrogen molecules in its atmosphere to escape. This process could continue until nothing is left of the planet, currently two-thirds as massive as Jupiter, but a rocky core.