OXYGEN AND CARBON DISCOVERED IN EXOPLANET ATMOSPHERE 'BLOW OFF'
From Lori Stiles, UA News Services, 520-621-1877,
February 2, 2004

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Contact Information
Gilda Ballester
520-621-4073


Alfred Vidal-Madjar
+33-1-44-32-80-73


Lars Lindberg Christensen
Hubble European Space Agency Information Centre
+49-89-3200-6306
+49-173-3872-621 (cellular)


Related Web sites listed at end of story
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An international team of astronomers has for the first time detected oxygen
and carbon in the atmosphere of a planet beyond our solar system.

Hydrogen gas flowing out from the planet at near-sonic speed is dragging
heavier oxygen and carbon up from the lower atmosphere like dust in a
whirlwind.

Oxygen and carbon atoms surround the planet in an extended envelope. This
tells scientists they for the first time are seeing a planet "blow off" its
atmosphere. Some argue that early Venus and Earth may have lost their
original atmospheres by the same kind of stunning hydrodynamic outflow.

The planet is a hot Jupiter-like planet 150 light years away, orbiting star
HD 209458 in the Pegasus constellation. It is unique among extrasolar
planets because its orbit crosses in front of a star close and bright enough
to be observed. The planet revolves completely around its star every 3 and
1/2 days, partly eclipsing its star during each 3-hour transit, or pass
across the face of the star.

Astronomers led by Alfred Vidal-Madjar of the Institut d¹Astrophysique de
Paris, CNRS, France, used the Hubble Space Telescope for observations of the
planet, called HD 209458b, in October and November 2003. They used Hubble¹s
sensitive ultraviolet spectrograph to probe the structure and chemical
make-up of the planet¹s atmosphere during the transits. Such observations
can only be made from space because Earth¹s ozone layer filters out UV
light.

"In some ways our discovery of oxygen and carbon is not so surprising," said
Gilda Ballester of the University of Arizona Lunar and Planetary Laboratory,
a member of the team. "These are common species which have been identified
in the lower atmospheres of our jovian planets, bound up in molecules such
as methane and water.

"What's key here is that we have detected oxygen and carbon in atomic form
and in the outermost layers of the planet where we would not normally expect
them," Ballester said. "These species are 10 times heavier than hydrogen
atoms, so a force stronger than gravity is driving them up along with the
hydrogen gas into the very extended envelope around the planet."

Vidal-Madjar, along with Ballester and other team members, discovered in
earlier Hubble Space Telescope observations that the planet has a huge
hydrogen atmosphere. They reported it in Nature in March 2003.

The planet¹s visible disk eclipses 1.5 percent of its star during a transit,
Ballester said. But its expansive upper atmosphere covers between 8 and 15
percent of the star. When the astronomers saw UV absorption signatures of
oxygen and ionized carbon in the planet¹s inflated upper atmosphere, they
knew it was produced in an atmospheric blow off.

Planet HD 209458b is only 4 million miles away from its star, so its lower
atmosphere is already extremely hot, around 1,800 degrees Fahrenheit (1,000
degrees Celsius). Ultraviolet light adds a lot of energy to the planet¹s
upper atmosphere, heating it up to around 18,000 degrees Fahrenheit (10,000
degrees Celsius). Ultraviolet light also splits gas molecules into atoms and
ions. Absorbed UV light is also what produces the kinetic energy that fuels
the planet¹s explosive gas outflow.

Team member Jack McConnell of York University, Canada, predicted this
phenomenon. He theorized that at such high temperatures, hydrogen atoms in
the upper atmosphere would reach speeds great enough to escape the planet¹s
gravity, which is comparable to Earth¹s gravity. And as the hydrogen atoms
flowed out from the planet at near sonic speed, they would drag heavier
oxygen and carbon atoms in the atmosphere up with them, called blow off.

The team also observed, as French team member Alain Lecavelier des Etangs
predicted, that the planet is so close to its star that the combined gravity
fields of the star and the planet shape its upper atmosphere into the form
of a rugby ball, allowing even more gas to escape.

Vidal-Madjar is among those who speculate that early Venus, Earth, and
perhaps Mars lost their original atmospheres this way.

"It¹s exciting that we found what we proposed we would find in observing the
planet in the UV with the Hubble Space Telescope," Ballester said.

The astronomers are reporting the new discovery in a forthcoming issue of
Astrophysical Journal Letters, in the article, "Detection of oxygen and
carbon in the upper atmosphere of the extrasolar planet HD 209458b."

The Hubble Space Telescope is jointly operated by the National Aeronautics
and Space Administration and the European Space Agency.

* * *Related web sites * * *
Astrophysical Journal Letters article
http://arxiv.org/abs/astro-ph/0401457

European Space Agency news release
http://spacetelescope.org/bin/news.p1?string=heic0402

Science team page on HD 209458b
http://www.iap.fr/exoplanetes/index_en.html
HEIC video clips
http://www.spacetelescope.org/videos/

Gilda E. Ballester
http://vega.lpl.arizona.edu/~gilda