Astronomers have made the first direct detection of the atmosphere of a planet orbiting a star outside our solar system and have obtained the first information about its chemical composition. Their unique observations demonstrate that it is possible to measure the chemical makeup of extrasolar planet atmospheres and potentially to search for chemical markers of life beyond Earth.

The discovery was made by David Charbonneau of the California Institute of Technology in Pasadena and the Harvard-Smithsonian Center for Astrophysics (CfA), Timothy Brown of the National Center for Atmospheric Research (Boulder, Colo.), Robert Noyes of the CfA, and Ronald Gilliland of the Space Telescope Science Institute in Baltimore. The scientists used NASA’s Hubble Space Telescope to detect the atmosphere of a planet as it passed in front of the star HD 209458, a sunlike star about 150 light years away in the constellation Pegasus.

“This opens up an exciting new phase of extrasolar planet exploration, where we can begin to compare and contrast the atmospheres of planets around other stars,” says Charbonneau.

The planet orbiting HD 209458 was first detected in 1999 by David Latham of the CfA and colleagues, as well as independently by another group, because of the “wobble” it induced on its parent star. Charbonneau (then a Harvard astronomy graduate student working with Noyes) made follow-up observations together with Brown, which established that the planet actually passes in front of the star during its orbit. Charbonneau, Brown, Noyes, and Gilliland then turned to the Hubble Space Telescope because its unique location outside the Earth’s atmosphere permitted detection of the very subtle signature of gases in the planet’s atmosphere, which appear during the time the planet is in front of the star.

Based on the earlier observations, the planet was strongly believed to be a “gas giant,” very similar to Jupiter. That is, like Jupiter it must be composed largely of gases rather than solid rocks like the Earth. But in contrast to Jupiter, it orbits extremely close to its parent star – only about 4 million miles above the star’s surface. Because the planet is so close to the star, its atmosphere is calculated to be much hotter than the Earth’s – about 2,000 degrees Fahrenheit (1,100 degrees Centigrade).

Theoretical calculations predict that such a hot planetary atmosphere should show the signature of chemical constituents such as sodium, when starlight shines through the atmosphere during the planet’s passage in front of the star. The team used the Hubble Space Telescope Imaging Spectrograph to search for these signatures, and successfully detected the presence of sodium gases in the planet’s atmosphere.

The observations were not tuned to look for gases expected in a life-sustaining atmosphere (which is improbable for a planet as hot as the one observed). Nevertheless, this unique observing technique opens a new phase in the exploration of exoplanets, say astronomers. Such observations could potentially provide the first direct evidence for life beyond Earth by measuring unusual abundances of atmospheric gases caused by the presence of living organisms.