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