Oxygen, carbon discovered in exoplanet atmosphere Feb 03, 2004 ( taken from SpaceFlightNow )

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 new class of extrasolar planets - 'the chthonian planets' or 'dead' cores of completely evaporated gas giants - has been proposed.

An artist's impression shows an extended ellipsoidal envelope of oxygen and carbon discovered around the extrasolar planet HD 209458b. Credit: European Space Agency and Alfred Vidal-Madjar (Institut d'Astrophysique de Paris, CNRS, France)

Oxygen and carbon have been detected in the atmosphere of a planetbeyond our Solar System for the first time. Scientists using theNASA/ESA Hubble Space Telescope have observed the famous extrasolarplanet HD 209458b passing in front of its parent star, and found oxygenand carbon surrounding the planet in an extended ellipsoidal envelope -the shape of a rugby-ball. These atoms are swept up from the loweratmosphere 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 ofAstrophysical Journal Letters.

The planet, called HD 209458b, may sound familiar. It is already anextrasolar planet with an astounding list of firsts: the firstextrasolar planet discovered transiting its sun, the first with anatmosphere, the first observed to have an evaporating hydrogenatmosphere (in 2003 by the same team of scientists) and now the firstto have an atmosphere containing oxygen and carbon. Furthermore the'blow-off' effect observed by the team during their October andNovember 2003 observations with Hubble had never been seen before.

In honour of such a distinguished catalogue this extraordinaryextrasolar planet has provisionally been dubbed "Osiris." Osiris is theEgyptian god who lost part of his body - like HD 209458b - after hisbrother killed and cut him into pieces to prevent his return to life.

Oxygen is one of the possible indicators of life that is often lookedfor in experiments searching for extraterrestrial life (such as thoseonboard the Viking probes and the Spirit and Opportunity rovers), butaccording to Vidal-Madjar: "Naturally this sounds exciting - thepossibility of life on Osiris - but it is not a big surprise as oxygenis also present in the giant planets of our Solar System, like Jupiterand Saturn."

What, on the other hand was surprising was to find the carbon andoxygen atoms surrounding the planet in an extended envelope. Althoughcarbon and oxygen have been observed on Jupiter and Saturn, it isalways in combined form as methane and water deep in the atmosphere. InHD 209458b the chemicals are broken down into the basic elements. Buton Jupiter or Saturn, even as elements, they would still remaininvisible low in the atmosphere. The fact that they are visible in theupper atmosphere of HD 209458b confirms that atmospheric 'blow off' isoccurring.

The scorched Osiris orbits 'only' 7 million kilometres from its yellowSun-like star and its surface is heated to about 1,000 degrees Celsius.

Whereas hydrogen is a very light element - the lightest in fact -oxygen and carbon are much heavier in comparison. This has enabledscientists to conclude that this phenomenon is more efficient thansimple evaporation. The gas is essentially ripped away at a speed ofmore than 35,000 km/hour. "We speculate that even heavier elements suchas iron are blown off at this stage as well" says team member AlainLecavelier des Etangs (Institut d'Astrophysique de Paris, CNRS,France).

The whole evaporation mechanism is so distinctive that there is reasonto propose the existence of a new class of extrasolar planets - thechthonian planets, a reference to the Greek God Khtфn, used for Greekdeities from the hot infernal underworld (also used in the French wordautochton). The chthonian planets are thought to be the solid remnantcores of 'evaporated gas giants', orbiting even closer to their parentstar than Osiris. The detection of these planets should soon be withinreach of current telescopes both on the ground and in space.

The discovery of the fierce evaporation process is, according to thescientists, "highly unusual," but may indirectly confirm theories ofour own Earth's childhood. "This is a unique case in which such ahydrodynamic escape is directly observed. It has been speculated thatVenus, Earth and Mars may have lost their entire original atmospheresduring the early part of their lives. Their present atmospheres havetheir origins in asteroid and cometary impacts and outgassing from theplanet interiors," says Vidal-Madjar.

FACTS ABOUT HD 209458b

• Diameter: 1.3 times that of Jupiter.
• Mass: 0.7 Jupiter masses, 220 Earth masses.
• Orbit: One-eighth the size of Mercury's orbit around the Sun (7 million kilometres). 3.5 days.
• Belongs to a type of extrasolar planet known as 'hot Jupiters' - Giant, gaseous planets in low orbits.
• First confirmed transiting extrasolar planet.
• Transit: Every 3.5 days, 3 hours in duration. Eclipses 1.5% of the face of the parent star.
• Surface temperature: About 1,000 degrees Celsius.

THE ATMOSPHERE

• Complex: sodium in the lower atmosphere, evaporating hydrogendetected in upper atmosphere, oxygen and carbon also in the upperatmosphere.
• Extended: During the eclipses the upper atmosphere covers 15% of theface of the parent star.
• What is causing the atmosphere to escape? The planet's outeratmosphere is extended and heated so much by the nearby star that itstarts to escape the planet's gravity. Hydrogen, carbon and oxygenboil off in the planet's upper atmosphere under the searing heat ofthe star.
• Astronomers estimate the amount of hydrogen gas escaping HD 209458bto be at least 10,000 tonnes per second.
• Hydrogen tail is 200,000 kilometres long.
• Evaporation mechanism so tremendous that a whole new class ofextrasolar planets, the chthonian planets - the cores of evaporatedgas giants, is proposed.

FACTS ABOUT THE PARENT STAR

• Name: HD 209458
• Type: Similar to our Sun
• Distance: 150 light-years from Earth in the constellation of Pegasus
• Brightness: 7th magnitude star (visible with binoculars)

HISTORY OF HD 209458b

• 1999: Discovery. Entered the astronomical Hall of Fame when theextrasolar planet passed in front of its parent star and partlyeclipsed it.
• 2001: Hubble detected the element sodium in the lower part of HD209458b's atmosphere.
• 2003: Hydrogen in upper atmosphere detected with Hubble. Signs ofevaporation.
• 2004: Oxygen and carbon detected in upper atmosphere with Hubble.Evaporation mechanism so distinctive that a whole new class ofextrasolar planets, the chthonian planets, being the cores ofevaporated gas giants, is proposed.

THE TEAM
A. Vidal-Madjar, lead author of the discovery paper, J.-M. Desert, A.Lecavelier des Etangs, G. Hebrard (all from Institut d'Astrophysique deParis, France), G. Ballester (University of Arizona, United States), D.Ehrenreich, R. Ferlet (both from 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 OBSERVATIONS
The team observed four transits of the planet in front of the star withHubble in October and November 2003 (with Director's DiscretionaryTime). The observations of structure and chemical make-up of theatmosphere were made in ultraviolet light, using Hubble's spectrographSTIS. Hubble's position above the atmosphere makes it the only telescopethat can currently perform these types of ultraviolet studies.

THE FUTURE
The search and the study of extrasolar planets is the aim of several ofESA's scientific missions.