News release:

Oxygen and carbon discovered in exoplanet atmosphere ‘blow-off’

04-Feb-2004 The well-known extrasolar planet HD 209458b, provisionally nicknamed Osiris, has surprised astronomers again. Oxygen and carbon have been found in its atmosphere, evaporating at such an immense rate that the existence of a whole new class of extrasolar planets – ‘the chthonian planets’ or ‘dead’ cores of completely evaporated gas giants - has been proposed.

Oxygen and carbon have been detected in the atmosphere of a planet beyond our Solar System for the first time. Scientists using the NASA/ESA Hubble Space Telescope have observed the famous extrasolar planet HD 209458b passing in front of its parent star, and found an extended envelope of oxygen and carbon surrounding it believed to have the shape of a rugby-ball. These atoms are swept up from the lower atmosphere with the flow of the escaping atmospheric atomic hydrogen, like dust in a supersonic whirlwind.

The team led by Alfred Vidal-Madjar (Institut d’Astrophysique de Paris, CNRS, France) reports this discovery in a forthcoming issue of Astrophysical Journal Letters.

The planet, called HD 209458b, may sound familiar. It is already an extrasolar planet with an astounding list of firsts: the first extrasolar planet discovered transiting its sun, the first with an atmosphere, the first observed to have an evaporating hydrogen atmosphere (in 2003 by the same team of scientists) and now the first to have an atmosphere containing oxygen and carbon. Furthermore the ‘blow-off’ effect observed by the team during their October and November 2003 observations with Hubble has also never been seen before.

In honour of such a distinguished catalogue this extraordinary extrasolar planet has provisionally been dubbed “Osiris”. Osiris is the Egyptian God who lost part of his body – like HD 209458b - after having been killed and cut into pieces by his brother to prevent his return to life.

Oxygen is one of the possible indicators of life that is often looked for in experiments searching for extraterrestrial life (such as those onboard the Viking probes and the Spirit and Opportunity rovers), but according to Vidal-Madjar: “Naturally this sounds exciting - the possibility of life on Osiris - but it is not a big surprise as oxygen is also present in the giant planets of our Solar System, Jupiter and Saturn”.

What on the other hand did surprise the scientists was to find the carbon and oxygen atoms surrounding the planet in an extended envelope. Although carbon and oxygen have been observed on Jupiter and Saturn, it is always in combined form as methane and water deep in the atmosphere. In HD 209458b the chemicals are broken down into basic elements. But on Jupiter or Saturn they would still remain invisible low in the atmosphere. On HD 209458b the fact that they are visible confirms that atmospheric ‘blow off’ is occurring.

The scorched Osiris orbits ‘only’ 7 million kilometres from its yellow Sun-like star and its surface is heated to 1,100 degrees Celsius.

Whereas hydrogen is a very light element - the lightest in fact - oxygen and carbon are much heavier in comparison. This has enabled scientists to calculate that the evaporation is proceeding at a much greater rate than thought so far. The gas is essentially ripped away at a speed of more than 35,000 km/hour. “We speculate that even heavier elements such as iron are blown off at this stage as well” says Vidal-Madjar.

It is obviously not possible to see what happens to all the evaporating carbon and oxygen, but calculations show that these elements may be trapped by the magnetic field of the star and move towards the poles of the star.

The whole evaporation mechanism is so distinctive that there is reason to propose the existence of a whole new class of extrasolar planets - the chthonian planets, a reference to the Greek God Khtôn, used for Greek deities from the hot infernal underworld. The chthonian planets are thought to be the solid remnant cores of ‘evaporated gas giants’, orbiting even closer to their parent star than Osiris. The detection of these planets should soon be within reach of current telescopes both on the ground and in space.

The discovery of the fierce evaporation process is, according to the scientists, “highly unusual”, but may indirectly confirm theories of our own Earth’s childhood. “This is a unique case in which such a hydrodynamic escape is directly observed. It has been speculated that Venus, Earth and Mars may have lost their entire original atmospheres during the early part of their lives. Their present atmospheres have their origins in asteroid and cometary impacts and outgassing from the planet interiors”, says Vidal-Madjar.

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Notes for editors






A. Vidal-Madjar, lead author of the discovery paper, (Institut d’Astrophysique de Paris, CNRS, France), J.-M. Désert, A. Lecavelier des Etangs (Institut d'Astrophysique de Paris, CNRS, France), G. Hébrard (Institut d'Astrophysique de Paris, France), G. Ballester (University of Arizona, United States), D. Ehrenreich (Institut d’Astrophysique de Paris, CNRS, France), R. Ferlet (Institut d’Astrophysique de Paris, CNRS, France), J. C. McConnell (York University, Toronto, Canada), M. Mayor (Geneve Observatory, Switzerland) and C.D. Parkinson (Caltech/JPL, USA).


The team observed four transits of the planet in front of the star with Hubble in October and November 2003 (with Director’s Discretionary Time). The observations of structure and chemical make-up of the atmosphere were made in ultraviolet light, using Hubble’s spectrograph STIS. Hubble’s position above the atmosphere makes it the only telescope that can currently perform these types of ultraviolet studies.


The search and the study of extrasolar planets is the aim of several of ESA’s scientific missions.

This material is being co-released with NASA/STScI/OPO.

Image credit: European Space Agency, Alfred Vidal-Madjar (Institut d’Astrophysique de Paris, CNRS, France) and NASA.

For more information, please contact:

Alfred Vidal-Madjar

Institut d'Astrophysique de Paris (IAP/CNRS), Paris, France

Tel: +33-1-44-32-80-73


Lars Lindberg Christensen

Hubble European Space Agency Information Centre, Garching, Germany

Tel: +49-89-3200-6306 (089 within Germany)

Cellular (24 hr): +49-173-3872-621 (0173 within Germany)


Gilda Ballester

Tel: +1-520-621-4073


Lori Stiles

University of Arizona

Tel: +1-520-626-4402


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