Chemical detected in atmosphere of distant star's planet

WASHINGTON -- Astronomers have made the first measurement of a chemical in the atmosphere of a planet orbiting a distant star, using a technique that could help them find Earth-like bodies around other suns.

Using the Hubble Space Telescope, astronomers analyzed light shining through a planet's atmosphere as it orbited a star 150 light-years away. The changes in the color of the light proved the planet's atmosphere contained sodium.

"This is the first measurement ever of any atom in the atmosphere of an extrasolar planet," said Timothy Brown, a scientist at the National Center for Atmospheric Research and a co-author of the study. "This proves it is possible to do a detailed analysis of an atmosphere so far away."

Brown's co-author, David Charbonneau of the California Institute of Technology, said the achievement illustrates how far astronomers have advanced in the search for other worlds.

"Ten years ago it was considered crazy to talk about planets about other stars," Charbonneau said. Since then, astronomers have found 76 planets orbiting stars outside of the solar system, he said.

"All of sudden," Charbonneau said, "looking for Earth-like planets seems very reasonable."

The Charbonneau-Brown study focused on a planet that orbits a sun-like star called HD209458 in the constellation Pegasus, some 150 light-years from Earth.

The planet is about 70 percent the size of Jupiter, the largest planet in our solar system, but it orbits just 4 million miles from its parent star. As a result, the planet whips around the star every 3.5 days.

Earlier studies by Charbonneau and Brown had shown that the distant planet caused the light from HD209458 to dim slightly. The planet was, in effect, casting a shadow, when viewed from Earth, as it orbited the star.

When the planet moves in front of its parent star, light from the star passes through the planet's atmosphere on its way to the Earth. As the light passes through, chemicals in the planet's atmosphere absorb some colors in the light.

Different compounds have different color signatures, said Brown. He said in the initial test, the researchers looked only for sodium.

"Sodium is the spectral equivalent of skunk," Brown said. "You don't need much to detect it."