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.

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 oxygen and carbon surrounding the
planet in an extended ellipsoidal envelope –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 had 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 his brother killed
and cut him into pieces 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, like Jupiter and Saturn.”

What, on the other hand was surprising 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 the basic elements. But on
Jupiter or Saturn, even as elements, they
would still remain invisible low in the
atmosphere. The fact that they are visible in
the upper atmosphere of HD 209458b 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 about 1,000 degrees

Whereas hydrogen is a very light element – the
lightest in fact – oxygen and carbon are much
heavier in comparison. This has enabled
scientists to conclude that this phenomenon
is more efficient than simple evaporation.
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 team
member Alain Lecavelier des Etangs (Institut
d’Astrophysique de Paris, CNRS, France).

The whole evaporation mechanism is so distinctive
that there is reason to propose the existence of
a new class of extrasolar planets – the
chthonian planets, a reference to the Greek God
Khton, used for Greek deities from the hot infernal
underworld (also used in the French word autochton).
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-

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Notes for editors


  • 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.


  • Complex: sodium in the lower atmosphere,
    evaporating hydrogen detected in upper
    atmosphere, oxygen and carbon also in the
    upper atmosphere.
  • Extended: During the eclipses the upper
    atmosphere covers 15% of the face of the
    parent star.
  • What is causing the atmosphere to escape?
    The planet’s outer atmosphere is extended
    and heated so much by the nearby star that
    it starts to escape the planet’s gravity.
    Hydrogen, carbon and oxygen boil off in
    the planet’s upper atmosphere under the
    searing heat of the star.
  • Astronomers estimate the amount of
    hydrogen gas escaping HD 209458b to be at
    least 10,000 tonnes per second.
  • Hydrogen tail is 200,000 kilometres long.
  • Evaporation mechanism so tremendous that a
    whole new class of extrasolar planets, the
    chthonian planets – the cores of
    evaporated gas giants, is proposed.


  • 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 the extrasolar planet
    passed in front of its parent star and
    partly eclipsed it.
  • 2001: Hubble detected the element sodium
    in the lower part of HD 209458b’s atmosphere.
  • 2003: Hydrogen in upper atmosphere
    detected with Hubble. Signs of evaporation.
  • 2004: Oxygen and carbon detected in upper
    atmosphere with Hubble. Evaporation
    mechanism so distinctive that a whole new
    class of extrasolar planets, the chthonian
    planets, being the cores of evaporated gas
    giants, is proposed.


    A. Vidal-Madjar, lead author of the discovery
    paper, paper, J.-M. D’esert, A. Lecavelier des
    Etangs, G. Hebrard (all from Institut
    d’Astrophysique de Paris, 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 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.

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

    For more information, please contact:

    Alfred Vidal-Madjar
    Institut d’Astrophysique de Paris (IAP/CNRS),
    Paris, France
    Tel: +33-1-44-32-80-73

    A. Lecavelier des Etangs
    Institut d’Astrophysique de Paris, CNRS,
    Tel: +33-1-44-32-80-77

    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