Atmosphere detected on distant planet

Astronomers discover sodium around world outside our solar system

November 28, 2001|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Astronomers using the Hubble Space Telescope say they have for the first time detected the atmosphere of a planet outside our solar system.

What they found wasn't very appealing - sodium, in an atmosphere hot enough to melt pocket change. But scientists were delighted they could learn anything at all about the environment on a planet 150 light-years away.

And they're hoping their discovery will be the first in a series that will compare the atmospheres of a host of "extra-solar" planets, perhaps eventually leading them to one that is hospitable to life.

"Ten years ago it was considered crazy to look for planets around other stars, and the thought of detecting an atmosphere was laughable," said astronomer David Charbonneau, a postdoctoral scholar at the California Institute of Technology and lead investigator on the project.

"Detection of an Earth-like planet will be much more difficult," he said, "but it's really worth thinking about now."

The discovery by Charbonneau and Timothy Brown, senior scientist at the National Center for Atmospheric Research, was announced yesterday at NASA headquarters in Washington. It is scheduled for publication in the Astrophysical Journal.

The first planet orbiting a star other than our sun was found six years ago. Astronomers had searched among thousands of nearby, sun-like stars, looking for a telltale "wobble" - movement in the star's light caused by the gravitational tug of large, but unseen, planets orbiting the stars.

They have since detected 75 such planets, and the number grows by about a half-dozen a year.

One of those planets was discovered two years ago circling a star designated HD 209458. The planet was nearly the size of Jupiter, a gaseous giant that circles the star every 3 1/2 days. Each time it passes in front of the star as seen from Earth, it dims the star's light slightly for three hours.

Charbonneau and Brown decided to analyze the starlight with Hubble's Imaging Spectrograph to see whether any of it was absorbed by atmospheric gases as the planet crossed in front of the star.

They looked specifically for signs of absorption by sodium, which Brown called "the spectral equivalent of skunk; you don't need very much of it to detect it."

And they found it - only a few parts per million and barely detectable above the levels of sodium in the star itself. But "it tells us there really is an atmosphere there," Charbonneau said.

It also proves that Hubble is capable of analyzing the atmospheric chemistry of extra-solar planets, something never contemplated when it was launched in 1990.

"The surprise is it turned out to be so easy for Hubble," said Bruce Margon, associate director for science at the Space Telescope Science Institute in Baltimore, who was not involved in the project.

"Now that the first measurement has been made, within just a few years we will be doing comparative planetary atmospherics."

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