Craft probes alien planet's atmosphere.

Astronomers have long wondered what the atmospheres of planets beyond the solar system might be like. Researchers this week reported that they have now gotten their first whiff.

The new observations, the researchers say, demonstrate that telescopes will ultimately have the capability to measure the composition of a variety of extrasolar planets' atmospheres and search for chemical markers of life beyond Earth.

"This is a huge step forward in extrasolar-planet research and one of the biggest discoveries ever in planetary science," comments theorist Sara Seager of the Institute for Advanced Study in Princeton, N.J. "For the first time, we can start to understand what the atmospheres [of extrasolar planets] are made of."

In the new study, astronomers homed in on a hot, gaseous planet. It's about two-thirds as heavy as Jupiter and orbits the sunlike star HD 209458, located 150 light-years from Earth. Researchers were able to detect the atmosphere of the distant planet because it periodically passes directly between its parent star and Earth. During those times, light from the star travels through layers of the planet's outer atmosphere, so less light reaches Earth.

Using the Hubble Space Telescope, astronomers found that whenever the unseen planet crossed in front of the star, they detected a slight decrease in starlight of a particular wavelength. That wavelength corresponds to the radiation absorbed by sodium atoms, which the researchers therefore say must be present in the planet's atmosphere.

The technique "is like looking at a very bright search light through a dense fog," explains Seager. "If some [wavelengths of the light] get more dimmed than others ... we can tell what the fog is made of."

David Charbonneau of the California Institute of Technology in Pasadena and Timothy M. Brown of the National Center for Atmospheric Research in Boulder, Colo., announced the findings on Nov. 27 in Washington, D.C. Their team will report the study in the ASTROPHYSICAL JOURNAL.

Charbonneau and his colleagues got their first inkling that they could study the planet's atmosphere in 1999, shortly after two other teams independently discovered the orbiting body. The two teams discerned the unseen planet by detecting the slight wobble it induces in the motion of its parent star. Residing much closer to the star than the distance at which Mercury orbits the sun, the planet whips around HD 209458 in just 3.52 days.

Soon after that discovery, Charbonneau, Brown, and another group of astronomers examined the star to see if its planet would periodically block some of the starlight. An observer can see that dip in brightness only if the plane in which the planet orbits is aligned edge on with respect to Earth. The body circling HD 209458 is the only known extrasolar planet to have that alignment. The dimming that researchers detected revealed the planet's mass and radius (SN: 11/20/99, p. 324).

The alignment, combined with the planet's proximity to its parent star, offered "a fantastic opportunity to learn about the planet's atmosphere," Charbonneau says.

The intense heat from HD 209458 keeps the planet's atmosphere inflated like a hot-air balloon. Seager and other researchers calculated that the atmosphere is tenuous enough that each time the planet passes in front of the star, some starlight filters through the atmosphere's outer layers rather than being blocked altogether.

Models suggest that planetary atmospheres contain only trace amounts of sodium. Nonetheless, Charbonneau's team searched for this element because even small amounts would absorb enough light to be detected by Hubble's spectrometer.

Hubble did detect a few parts per million of sodium, but that's considerably less than any of the models predicted. The atmosphere may simply contain less sodium than expected, notes Seager. Another explanation is that the alien planet's atmosphere contains dense, high-altitude clouds. These clouds would prevent starlight from penetrating deep into the atmosphere, where much of the sodium may lie.

Charbonneau, Brown, and their colleagues now plan to search for methane, water vapor, potassium, and other chemicals in the atmosphere.

The scorchingly hot planet circling HD 209458 isn't likely to support life. But astronomers can apply the same search technique to probe the atmospheres of cooler, more hospitable extrasolar planets. The atmospheres of these planets may contain oxygen and other chemicals produced by organisms, Charbonneau notes.

He emphasizes, however, that this method of studying planetary atmospheres works only if a planet passes directly in front of its star as seen from Earth. So-called hot Jupiters, such as the planet closely orbiting HD 209458, have about a 1 in 10 chance of having that alignment. The odds are much less for a planet that lies farther from its star because its orbit must be much more precisely aligned if starlight passing through the atmosphere is to reach Earth.

A network of telescopes that Charbonneau and his colleagues have just begun to use has the potential to find one correctly positioned planet each month, he estimates.

The group recently employed another method to study the atmosphere of the planet circling HD 209458. This time, the researchers used Hubble to measure the intensity of starlight reflected by the planet, rather than the amount transmitted through its atmosphere. The team is now analyzing the data to find variations in the intensity of the reflected light at several wavelengths. The observations may reveal the size of clouds and particles in the atmosphere, Charbonneau says.