Oxygen and Carbon Discovered in Exoplanet Atmosphere 'Blow-Off'

Lori Stiles
Feb. 4, 2004

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 d'Astrophysique de Paris, CNRS, France, used the Hubble Space Telescope for observations of the planet, called HD 209458b, in October and November 2003. They used Hubble's sensitive ultraviolet spectrograph to probe the structure and chemical make-up of the planet's atmosphere during the transits. Such observations can only be made from space because Earth's 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 planet's 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 planet's 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 planet's 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 planet's 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 planet's gravity, which is comparable to Earth's 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.

French team member Alain Lecavelier des Etangs found 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.

"It's 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.


Resources for the media

Gilda Ballester
Alfred Vidal-Madjar
Lars Lindberg Christensen
Hubble European Space Agency Information Centre
+49-173-3872-621 (cellular) lars@eso.org