The first-ever good look at the upper atmosphere of a planet outside our solar system has turned up what a University of Arizona scientist has dubbed a "cometary planet, " a hot Jupiter-like planet quickly evaporating as it revolves close to its sun-like, yellow star.

"This planet looks more like a comet than a planet in the ultraviolet," said Gilda Ballester, a research scientist at the UA Lunar and Planetary Laboratory.

The planet orbits only 4 million miles from its star. Because it is so close to its star, it is seared by the heat of the star, and the upper layers of its atmosphere are very hot and inflated. Atomic hydrogen, the lightest and most abundant element, extends the farthest, to about 3 times the size of the planet. Some of this hydrogen escapes the gravitational pull of the planet and forms an extensive, comet-like tail =AD a hydrogen tail long enough to circle Earth five times.

"The hydrogen will just keep evaporating away during the lifetime of the star," Ballester said.

Ballester is the U.S. member of a science team led by Alfred Vidal-Madjar of the Institut d'Astrophysique de Paris reporting on the discovery in the March 13 issue of Nature. The scientists used an ultraviolet instrument on the Hubble Space Telescope to observe the atmosphere of a planet called HD 209458b. The star HD 209458 is 150 light years away in constellation Pegasus.

Discovered in 1999, the planet is unique among exoplanets (planets outside our solar system) because it is the only one with an orbit that crosses the face of its star that is near and bright enough to be observed. The planet has only 70 percent of the mass of Jupiter. But its disk is inflated to 30 percent bigger than Jupiter in size.

"Two things have come together for this discovery," Ballester said. "One is that the Hubble Space Telescope now carries a sensitive instrument for observing in the ultraviolet. And two is the discovery of this planet that transits in front of a star."

That discovery "was like opening up a lab" for researchers who study the composition and structure of planetary atmospheres, she added. One of their most powerful techniques is to observe how atmospheres absorb starlight at ultraviolet wavelengths. Such observations can only be made from space because Earth's ozone layer filters out UV light.

Ballester worked with her colleagues in Paris on their study of HD 209458b in 2001. Throughout 2002, the French scientists analyzed the complex data one way, while Ballester did a completely different kind of analysis in Tucson.

"It took a long time, but we independently confirmed that what we were seeing was real," she said.

The planet revolves around its sun in 3 and 1/2 days. The team observed the planet during three transits. Each transit - the time during which they could observe it in front of the star - lasts 3 hours.

"We observed that the planet and its upper atmosphere covered 15 percent of the area of that star. The disk of the planet itself covers just one percent of the star. That means the planet has a huge hydrogen atmosphere controlled by stellar forces," Ballester said.

"The other big result is that as the hydrogen is extracted from the planet, it is then pushed by starlight, forming a tail that makes the planet look more like a comet."

Team member Alain Lecavelier des Etangs has made a simulation of the hydrogen escape and the formation of the tail structure. The modeled planet's comet-like tail, 124,000 miles long, would stretch almost 5 times around the Earth.

"We knew from the 'blue-shifted' absorption of the starlight that the tail points toward us, away from the star," Ballester said. (Astronomers observe blue-shifted light, or light at shorter wavelengths, from approaching objects, just as they observe "red-shifted" light, or light at longer wavelengths, from receding objects.)

"And I have to chuckle about this because a colleague of mine has been searching for comets around other stars. It's funny, because now we know a 'comet' has always been there!"

The next steps in this project will be to make new UV observations and to model the upper planetary atmosphere in more detail. In particular, "We need to better understand the effects of the strong UV input and heating from the star, as well as how the atmosphere interacts with the stellar wind," Ballester said.

"Solar wind, the phenomenon that causes our aurorae on Earth, is one of the major elements that define the properties of planets' upper atmospheres. When you think this planet, it seems to have elements of everything we study. It is really very complicated, but this is like another type of planet," she said.

Members of the discovery team are Alfred Vidal-Madjar (team leader), Alain Lecavelier des Etangs, and Jean-Michel Desert (Institut d' Astrophysique de Paris, CNRS, France); Gilda E. Ballester (University of Arizona), Roger Ferlet and Guillaume Hebrard (Institut d' Astrophysique de Paris) and Michel Mayor (Geneve Observatory, Switzerland).

Ballester, a native of Puerto Rico who has a physics doctorate from Johns Hopkins University, is expert in ultraviolet studies of planetary atmospheres. Her work has been based on observations with the Hubble Space Telescope (HST) and early on with the International Ultraviolet Explorer (IUE). She was a member of the HST WFPC 2 camera team, and also has expertise with Hubble's Space Telescope Imaging Spectrograph (STIS), the instrument used on this project.

Ballester joined the UA in 2000. Several of her colleagues at LPL, who include Voyager team and IMAGE mission scientists, have used UV spectroscopy in making major discoveries about planetary atmospheres in our own solar system. Ballester now carries that work to exoplanets. Other UA planetary scientists and physicists have been long involved in theoretical modeling that led to the recent Hubble Space Telescope discovery of sodium in this same planet's lower atmosphere using the same technique, she noted.

Related Links (include animation & art )

Institut d' Astrophysique de Paris http://www.iap.fr/exoplanetes/index_en.html

Ballester, LPL http://vega.lpl.arizona.edu/~gilda

Hubble Telescope, ESA http://hubble.esa.int

Space Telescope Science Institute http://hubblesite.org/news/2003/08