or the first time, astronomers have detected and made a preliminary chemical study of the atmosphere of a planet beyond the solar system.

The observations, announced yesterday, demonstrate that current and planned telescopes are on the verge of yielding striking discoveries about the atmospheres of worlds around other Sun-like stars — extrasolar planets — and could have the potential to identify chemicals there revealing possible extraterrestrial life.

The chemical analysis of the planet's atmosphere is considered preliminary, and not surprising: the Jupiter-like planet, around a star 150 light-years from Earth, is cloaked in a thick atmosphere hot enough to melt copper and bearing traces of sodium. The investigation, using the Earth-orbiting Hubble Space Telescope, was keyed to look for sodium, a readily detectable element that astronomers expected to be present.

Astronomers said they expected further observations to give them a more complete picture of the planet's atmosphere, and to widen the study to other extrasolar worlds.

"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," said Dr. David Charbonneau, an astronomer at the California Institute of Technology in Pasadena.

Dr. Charbonneau, a principal investigator on the project, and Dr. Timothy Brown of the National Center for Atmospheric Research in Boulder, Colo., announced the findings at a news conference at the National Aeronautics and Space Administration in Washington. Details of the research are to be published soon in The Astrophysical Journal.

"This is just a remarkable result," said one scientist not associated with the effort, Dr. Alan P. Boss, a theoretical astrophysicist at the Carnegie Institution of Washington. "The first detection of the atmosphere of a planet means that for the first time we have entered into a new phase in the era of extrasolar planet discovery and characterization."

As Dr. Boss and other scientists pointed out, the Hubble telescope was not designed specifically to study extrasolar planets. In fact, the first objects orbiting other stars were not even observed, and thus known to exist, until less than a decade ago, after the Hubble was launched. By now approximately 80 such objects, usually with the mass of Jupiter or greater, have been detected around relatively near Sun- like stars. The planets cannot be seen; their existence and mass are inferred from the wobbly motions of their parent stars, disturbances caused by the companion's gravity.

The planet now being studied was discovered in 1999. It orbits a yellow star designated HD 209458, in the constellation Pegasus and visible with small amateur telescopes.

Unlike any of the other newly discovered planets, the one around HD 209458 had an orbit that passed in front of its star edge-on to the line of sight from Earth. These passes are called transits. It was a transit of Venus across the face of the Sun that enabled astronomers to learn 300 years ago that Venus had a dense atmosphere, the first such discovery concerning another planet.

The presence of a planet around HD 209458 was confirmed when astronomers measured a dip in starlight each time the object passed in front of the star. Because of the planet's proximity to the star, it makes a transit once every three and a half days. So astronomers decided to use an instrument on the Hubble telescope, a spectrograph, to examine the starlight as it dimmed perceptibly when filtering through the planet's atmosphere.

The spectrograph breaks light into its different colors, or wavelengths. Different chemical elements and compounds in the atmosphere absorb the starlight differently, and thus have distinctive spectral signatures. The signature of sodium shows up in the yellow-green part of the spectrum.

Although sodium is only a trace element in the planet's atmosphere, and hydrogen and helium are presumably the principal constituents, sodium was chosen as the first target because it's "like a skunk scent — it's easy to detect, even if there's only a little bit of it there," Dr. Brown said.

Dr. Boss, of the Carnegie Institution, said the research was "a preview of coming attractions," especially given new telescopes on the ground and in space over the next decade that will be designed expressly to investigate extrasolar planets. Astronomers are expected to look closely for evidence of carbon dioxide, water vapor, ozone and methane.

"If you can find all four of those signatures in the same planet," Dr. Boss said, "you can make a very strong case that that planet is habitable, if not actually inhabited."