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02/21/2007 08:00:00

Astronomers Puzzled by Spectra of Transiting Planet Orbiting Nearby Star

PASADENA, Calif.—A team of astronomers led by Carl Grillmair of the California Institute of Technology has discovered some puzzling things about a Jupiter-sized planet that passes in front of a nearby star in the constellation Vulpecula.

Both the Grillmair team and groups from NASA's Goddard Space Flight Center and NASA's Jet Propulsion Laboratory are reporting today on their independent findings about two transiting exoplanets. These are the first spectra from planets outside our own solar system, and have been made possible by the NASA Spitzer Space Telescope's unexpectedly keen ability to study nearby stars.

According to Grillmair, an astronomer at Caltech's Spitzer Science Center, the planet studied by his group is named HD 189733b. The planet is about 62 light-years, or 360 trillion miles away from Earth, is about 10 percent larger than Jupiter, and has a "year" that lasts only two days. It orbits the star HD 189733, which is somewhat smaller and slightly redder than our own sun. And unexpectedly, the data doesn't show the presence of water.

"It's surprising," says Grillmair. "According to what the theoreticians tell us, we had expected to see a very structured spectrum that would have a particular shape because of the presence of water in the planet's atmosphere. But what we actually see is a very flat spectrum."

Spectral data is good for determining what's in a star—or planet, for that matter—because different substances can look very different when the light from them is split into separate colors by a prism. Scientists in the 19th century discovered that burning a substance and then looking at its light through a prism was an excellent way of figuring out what was being burned, and roughly the same procedure has been used ever since for finding out about the light-emitting things in the universe.

The problem with exoplanets, however, has been that the light of the star can be billions of times brighter than the planet itself. As a result, astronomers have previously been unable to study the spectra of planets outside our solar system due to the sheer distance and their inability to distinguish planet light from starlight.

"Normally, trying to see a planet next to a star is like trying to see a firefly next to an airport searchlight several miles away," Grillmair explains. "But in the case of our planet and the one being reported by the other teams, you can take the combined spectrum of the star and planet, and then when the planet passes behind the star, take another spectrum. By subtracting the second spectrum of just the star from the first, you can divine the spectrum of the planet itself."

Another key element to this discovery is that the observations are done in the infrared. The contrast between the star and the planet isn't as large in the infrared, so researchers can tease out the infrared spectrum of the planet. It remains impossible, with current technology, to do this in the visible light, even for transiting planets.

As for the apparent lack of water, Grillmair says there are at least four possibilities. First of all, there could really be no water, which he feels is not very likely. Second, there could be some other chemicals in the planet's atmosphere that emit radiation just where water absorbs it, thereby effectively camouflaging the signature of the water. This too seems unlikely. Third, the water could be hidden underneath an opaque cloud layer the Spitzer telescope can't see through. Fourth, a theoretical model suggests that, if the planet is in tidal lock (in other words, is so close to its sun that the same side always faces the same way), the atmospheric temperature profile on the day-side of the planet could be such that spectral features are suppressed.

But whatever the case, Grillmair thinks that a healthy collection of additional data during the Spitzer's final year or two of life could settle the matter—and teach us much about the worlds beyond our solar system.

"We really need more data to hammer this thing and knock down the noise," he says. "There will be 17 eclipses during the next year that will be visible to Spitzer, and I'd really like to look at every one of them."

So far, Grillmair and his team have been able to observe the planet for a total of 12 hours during two eclipses. A nearly tenfold increase in data would allow positive identifications of individual chemical elements, which has not been possible with the data returned so far.

"This type of data will undoubtedly be one of Spitzer's greatest legacies," Grillmair says. "Transiting extrasolar planets hadn't even been discovered when the Spitzer Space Telescope was designed, so this was all unanticipated."

NASA's Jet Propulsion Laboratory, located in Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

The other members of Grillmair's team are David Charbonneau of the Harvard-Smithsonian Center for Astrophysics; Adam Burrows of the Steward Observatory; Lee Armus, John Stauffer, Victoria Meadows, and Deborah Levine, all of the Spitzer Science Center; and Jeffrey Van Cleve of Ball Aerospace and Technologies Corp.

The Grillmair team's results will be published in an upcoming issue of Astrophysical Journal Letters. A report on the Goddard Space Flight Center team's study of the transiting exoplanet HD 209458b is being published this week in the journal Nature.

A separate paper by the JPL-led team on HD 209458b has been submitted to the Astrophysical Journal Letters. The JPL team, led by Mark Swain, also includes Caltech's Rachel Akeson and Chas Beichman

Written by Robert Tindol