Water, water everywhere -- on an extrasolar planet (Forwarded)

ESA News
http://www.esa.int

11 July 2007

Water, water everywhere -- on an extrasolar planet

Scientists report the first conclusive discovery of the presence of water
vapour in the atmosphere of a planet beyond our Solar System.

The discovery was made by analysing the transit of the gas giant HD 189733b
across its star, in the Infrared.

Giovanna Tinetti, ESA fellow at the Institute d'Astrophysique de Paris, and
colleagues from around the world, used data from NASA's Spitzer Space
Telescope. They targeted planet HD 189733b, 63 light-years away, in the
constellation Vulpecula.

The planet was discovered in 2005 as it dimmed the light of its parent star
by some three percent when transiting in front of it. Using Spitzer, Tinetti
and the team observed the star, which is slightly fainter than the Sun. They
watched its starlight dim at two infrared bands (3.6 and 5.8 micrometres).

Had the planet been a rocky body devoid of atmosphere, both these bands and
a third one (8 micrometres), recently measured by a team at Harvard, would
have shown the same behaviour.

Instead, as the planet's tenuous outer atmosphere slipped across the face of
the star, the starlight absorbed showed a different, distinctive pattern.
The atmosphere absorbed less infrared radiation at 3.6 micrometres than at
the other two wavelengths.

"Water is the only molecule that can explain that behaviour," says Tinetti.

The presence of water vapour does not necessarily make it a good candidate
in the search for planets that harbour life. "This is a far from habitable
world," she adds.

Instead of a rocky world like Earth, HD 189733b is large, about 1.15 times
the mass of Jupiter. Located just 4.5 million km from its star, it orbits it
in 2.2 days. In comparison, Earth is 150 million km from the Sun; even
Mercury, the innermost planet, is 70 million km away.

Astronomers classify such worlds as 'hot jupiters'. These planets tend to
have extensive atmospheres because heat from the nearby star gives them
energy to expand. HD 189733b is no exception; its diameter is 1.25 times
that of Jupiter.

HD 189733b's atmospheric temperature is about 1000 Kelvin (a little more
than 700 C) or higher, implying that the significant amounts of water vapour
in the atmosphere cannot condense to fall as rain or form clouds. The
temperature would have to be about five times lower to form clouds of water
vapour or rain.

That does not mean the atmosphere is sedate, however. The planet is gripped
so tightly by the gravity of its star that one hemisphere constantly faces
the star, heating the planet only on one side. This probably generates
fierce winds sweeping from the day-side to the night-side. "There are a
thousand things to learn about these planets," says Tinetti.

Although, being a gas giant, the planet is an unlikely candidate in the
search for life, these results increase hopes for the detection of water on
other rocky planets, which astronomers hope to discover in the near future.

France's COROT mission, in which ESA participates, is expected to detect
dozens of transiting gas giants, and has been working so well that it may
also detect nearly Earth-sized worlds.

Atmospheres of rocky planets should be much more tenuous, so they will have
to wait for future space telescopes, such as the James Webb Space Telescope,
before they can be investigated.

Further into the future, the mission Darwin is expected to be proposed to
ESA in the context of the Cosmic Vision Programme, its launch proposed
sometime after 2018. A constellation of four spacecraft, Darwin's goal would
be to find and analyse atmospheres of Earth-sized planets, looking for
telltale signs of water vapour and other gases that might betray life on
those worlds.

Notes for editors:

The findings appear in the 12 July 2007 issue of the scientific journal
Nature. The original paper, titled 'Water vapour in the atmosphere of a
transiting extrasolar planet', is by G.Tinetti, A.Vidal-Madjar, M-C. Liang,
J-P. Beaulieu, Y. L. Yung, S. Carey, R. Barber, J. Tennyson, I. Ribas, N.
Allard, G. Ballester, D.K. Sing, F. Selsis.

NASA's Jet Propulsion Laboratory (JPL), Pasadena, California, manages the
Spitzer Space Telescope for NASA's Science Mission Directorate, Washington.
Science operations are conducted at the Spitzer Science Center at the
California Institute of Technology (Caltech), also in Pasadena. Caltech
manages JPL for NASA.

For more information:

Giovanna Tinetti, fomer ESA fellow at Institute d'Astrophysique de Paris,
France; currently at University College London, UK
Email: Giovanna @ apl.ucl.ac.uk

Jean-Philippe Beaulieu, Institute d'Astrophysique de Paris, France
Email: Beaulieu @ iap.fr

Jonathan Tennyson, University College London, UK
Email: J.Tennyson @ ucl.ac.uk

Fabio Favata, ESA Coordinator for Astronomy and Fundamental Physics missions
Email: Fabio.Favata @ esa.int

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