Illustration: G. BACON (STScI/AVL)
Astronomers using the Hubble Space Telescope have directly detected the atmosphere of a planet outside our solar system that lies more than 150 light-years away, NASA announced yesterday. The discovery of the element sodium in the planet's atmosphere, the scientists say, is the first step in measuring more complex chemical compositions of planetary atmospheres far from Earth.

The planet investigated in the new study orbits the sun-like star HD 209458, located in the constellation Pegasus. When the planet passed between the star and the telescope, light from the sun was filtered through the planet's atmosphere. The planet, a gas giant 220 times more massive than Earth, contains sodium in its outer layers, but Hubble's spectrometer confirmed the presence of additional sodium in the planet's atmosphere. This finding, says lead investigator David Charbonneau of the California Institute of Technology, "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."

The current research focused only on sodium and did not look for gases expected in a life-sustaining atmosphere. Indeed, the planet's atmosphere, heated to more than 2,000 degrees Fahrenheit as a result of its close proximity to HD 209458, makes it a highly improbable home. Future observations of the planet's atmosphere, the researchers say, will look for additional chemicals, such as methane, water vapor and potassium. But even if life-sustaining elements are not found, co-author Timothy Brown won't mind. "It's hugely exciting," he says, "to nail down anything at all about something as mysterious as planets outside our solar system."