Image: HD 209458b
Alfred Vidar-Madjar  /  CNRS - ESA
This artist’s impression shows an extended ellipsoidal envelope — the shape of a rugby ball — around the well-known extrasolar planet HD 209458b. Researchers say they have detected oxygen and carbon molecules in the envelope.
updated 2/2/2004 4:49:27 PM ET 2004-02-02T21:49:27

Astronomers have detected the first presence of oxygen and carbon in the atmosphere of an extrasolar planet, a world already known to be venting massive amounts of gas into space.

The find is evidence of an atmospheric "blowoff" in action, where energetic hydrogen gas drags heavier elements along for a supersonic ride into space.

"If you imagine a wind so efficient that it takes everything with it, sand particles for instance, you get the idea," said the study's leader, Alfred Vidal-Madjar of the Institut d'Astrophysique de Paris. The planet "is really losing a lot of material even more efficiently than we thought before."

Despite the oxygen, the faraway planet is not one that would support life.

The Jupiterlike planet is officially called HD 209458b, though Vidal-Madjar's team has nicknamed it Osiris after an Egyptian god who was dismembered by his brother Seth. It orbits a sunlike star 150 light-years from Earth.

Astronomers already knew the planet was rapidly losing its atmosphere after a previous study led by Vidal-Madjar found it spewing out enough hydrogen gas to create an envelope that extended and trailed the extrasolar world.

The planet was thought to be losing at least 10,000 tons of material each second, but researchers weren't sure the process was powerful enough to dredge up heavier elements.

Carbon and oxygen atoms are 10 times heavier than those of hydrogen, and therefore would normally lie low in a planet's atmosphere, explained Gilda Ballester, a University of Arizona astronomer who took part in the study. "So a force stronger than gravity is driving them up along with the hydrogen gas into the extended envelope around this planet," she said.

The cause
The venting process has been attributed to a pair of reasons, namely the intense gravitational forces between the planet and its parent star, as well as the super-hot temperature of the planet's atmosphere. HD 209458b circles its stellar parent every 3.5 days from a distance of just 4.4 million miles (7 million kilometers), which is closer than Mercury's orbit around the sun.

The tight orbit causes intense gravitational tiding that stretches the planet's atmosphere into an oval shape, not unlike a rugby ball, which can allow gas to escape. The upper atmosphere itself is baked at up to 18,000 degrees Fahrenheit (10,000 degrees Celsius), which forces hydrogen atoms to expand outward at supersonic speeds.

The hydrogen wind erupts away from the planet like a geyser and is powerful enough to sweep up carbon and oxygen with it.

The planet may eventually shed its entire atmosphere, leaving behind only a solid core remnant of its once massive self. The unique nature of this process has led Vidal-Madjar's team to propose the existence of a new class of extrasolar planets, one which may be populated by the remains of worlds that have shed their atmospheric skins and orbit even closer to their suns than HD 209458b.

The process is similar to one that may have eventually produced the atmospheres around more local planets, such as Venus and Earth, astronomers said.

"The composition of Earth's atmosphere today is so peculiar, that there must exist an efficient process that blew out much of the original material," Vidal-Madjar told "Now we are directly observing it in Osiris."

More to learn
The next step, Vidal-Madjar says, is to search for even heavier elements, such as iron, in the envelope around HD 209458b, which would go further in confirming the blowout process.

Vidal-Madjar's team used the Hubble Space Telescope to observe HD 209458b between October and November in 2003. Since the planet partially eclipses its parent star — HD 209458 — during each orbit, researchers can to probe its atmospheric makeup during the transit. The new research will appear in an upcoming issue of the Astrophysical Journal Letters.

HD 209458b orbits a star in the constellation Pegasus, which can be seen with binoculars from the ground. The planet was first detected in 1999 using the wobble method of planet hunting. A separate team of astronomers previously detected sodium in the planet's atmosphere as well.

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