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Paris, 12 March 2003
Press Release
Nø 15-2003

European astronomers observe first evaporating planet 
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Using the Hubble Space Telescope, astronomers have, for the first 
time, observed the atmosphere of an extrasolar planet evaporating into 
space.  Much of this planet may eventually disappear, leaving only a 
dense core. It is a type of extrasolar planet known as a 'hot 
Jupiter'. These giant gaseous planets orbit their stars very closely, 
drawn to them like moths to a flame.

The scorched planet called HD 209458b orbits 'only' 7 million 
kilometres from its yellow Sun-like star. By comparison, Jupiter, the 
closest gas giant in our Solar System, orbits 780 million kilometres 
from our Sun.  NASA/ESA Hubble Space telescope observations reveal a 
hot and puffed-up evaporating hydrogen atmosphere surrounding the 
planet. This huge envelope of hydrogen resembles a comet with a tail 
trailing behind the planet. The planet circles the parent star in a 
tight 3.5-day orbit. Earth also has an extended atmosphere of escaping 
hydrogen gas, but the loss rate is much lower.

A mainly European team led by Alfred Vidal-Madjar (Institut
d'Astrophysique de Paris, CNRS, France) reports this discovery in the 
13 March edition of Nature. "We were astonished to see that the 
hydrogen atmosphere of this planet extends over 200 000 kilometres," 
says Vidal-Madjar.

Studying extrasolar planets, especially if they are very close to 
their parent stars, is not easy because the starlight is usually too 
blinding.  The planet was also too close to the star for Hubble to 
photograph directly in this case. However, astronomers were able to 
observe the planet indirectly since it blocks light from a small part 
of the star during transits across the disc of the star, thereby 
dimming it slightly.  Light passing through the atmosphere around the 
planet is scattered and acquires a signature from the atmosphere. In a 
similar way, the Sun's light is reddened as it passes obliquely 
through the Earth's atmosphere at sunset. Astronomers used Hubble's 
space telescope imaging spectrograph (STIS) to measure how much of the 
planet's atmosphere filters light from the star. They saw a startling 
drop in the star's hydrogen emission. A huge, puffed-up atmosphere can 
best explain this result.

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 boils off in the 
planet's upper atmosphere under the searing heat from the star. "The 
atmosphere is heated, the hydrogen escapes the planet's gravitational 
pull and is pushed away by the starlight, fanning out in a large tail 
behind the planet - like that of a comet," says Alain Lecavelier des 
Etangs, of the Institut d'Astrophysique de Paris. Astronomers estimate 
the amount of hydrogen gas escaping from HD 209458b to be at least 10 
000 tonnes per second, but possibly much more.  The planet may 
therefore already have lost quite a lot of its mass.

HD 209458b belongs to a type of extrasolar planet known as 'hot 
Jupiters'.  These planets orbit precariously close to their stars. 
They are giant gaseous planets that must have formed in the cold outer 
reaches of the star system and then spiralled into their close orbits. 
This new discovery might help explain why 'hot Jupiters' so often 
orbit a few million kilometres from their parent stars. They are not 
usually found much closer than 7 million kilometres, the distance in 
the case of HD 209458b.  Currently, the closest is 5.7 million 
kilometres. Hot Jupiters have orbits as brief as 3 days, but no less. 
Perhaps the evaporation of the atmosphere plays a role in setting an 
inner boundary for orbits of hot Jupiters.  # # #

Notes for editors

HD 209458b has a diameter 1.3 times that of Jupiter, and two-thirds 
the mass. Its orbit is one-eighth the size of Mercury's orbit around 
the Sun.  The parent star is similar to our Sun and lies 150 
light-years from Earth.  It is visible with binoculars as a seventh 
magnitude star in the constellation of Pegasus. In 1999, this star 
suddenly entered the astronomical Hall of Fame when the extrasolar 
planet HD 209458b passed in front of it and partly eclipsed it. This 
was the first confirmed transiting extrasolar planet ever discovered. 
In 2001, Hubble detected the element sodium in the lower part of HD 
209458b's atmosphere, the first signature of an atmosphere on any 
extrasolar planet.

The team is composed of A. Vidal-Madjar, lead author of the discovery
(Institut d'Astrophysique de Paris, CNRS, France) A. Lecavelier des 
Etangs and J.-M. D‚sert (Institut d'Astrophysique de Paris, CNRS, 
France), G.  Ballester (University of Arizona, United States), R. 
Ferlet and G. H‚brard (Institut d'Astrophysique de Paris, France), and 
M. Mayor (Geneva Observatory, Switzerland). With Hubble they observed 
three transits of the planet in front of the star. The observations of 
the atomic hydrogen envelope were made in ultraviolet (Lyman-alpha) 
light, using Hubble's STIS spectrograph. Hubble's position above the 
atmosphere makes it the only telescope currently able to perform this 
type of ultraviolet study.

* * *

Searching for and studying extrasolar planets is the aim of several of
ESA's scientific missions. Eddington, for instance, due for launch in
2007,  will discover large numbers of transiting planets of all types,
including many transiting 'hot Jupiters' similar to HD 209458b. These 
will be ideal targets for the same type of detailed follow-up studies 
with large space- and ground-based telescopes.

For more information, please contact:

ESA Communication Department
Media Relations Office
Paris, France
Tel: +33(0)15369 7155
Fax: +33(0)1 5369 7690

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)
E-mail: lars{at}eso.org

Alfred Vidal-Madjar, Institut d'Astrophysique de Paris (IAP/CNRS),
 Paris, France
Tel: +33-1-44-32-80-73 
E-mail: alfred{at}iap.fr

Ray Villard, Space Telescope Science Institute,
 Baltimore, United States
Tel: +1-410-338-4514
E-mail: villard{at}stsci.edu

For more information about the ESA Science Programme, visit:
http://sci.esa.int

For more information about the ESA visit:
http://www.esa.int

For broadcasters, computer animations of the discovery plus general 
Hubble Space Telescope background footage is available from the ESA 
Television Service, see http://television.esa.int

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