European astronomers observe first evaporating planet

Using the Hubble Space Telescope, for the first time,
astronomers have observed the atmosphere of an extrasolar planet
evaporating off into space. Much of this planet may eventually
disappear, leaving only a dense core. The planet 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.
The 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) is reporting this discovery in
the March 13 NATURE Magazine. “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 very 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 could observe
the planet indirectly since it blocks light from a small part of the
star during transits across the disk 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
working at the Institut d’Astrophysique de Paris, CNRS, France.
Astronomers estimate the amount of hydrogen gas escaping 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, as is the case for HD
209458b. Currently, the current closest distance is 5.7 million
kilometres. Hot Jupiters have orbits that are as brief as 3 days, but
not shorter. Perhaps the evaporation of the atmosphere plays a role in
setting an inner boundary for orbits of hot Jupiters.

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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,
J.-M. Desert (Institut d’Astrophysique de Paris, CNRS, France), G.
Ballester (University of Arizona, United States), R. Ferlet and G.
Hebrard (Institut d’Astrophysique de Paris, France), and M. Mayor
(Geneve Observatory, Switzerland). They observed three transits of the
planet in front of the star with Hubble. The observations of the
atomic hydrogen envelope were made in ultraviolet (Lyman-alpha) light,
using Hubble’s spectrograph STIS. Hubble’s position above the atmosphere
makes it the only telescope currently that can perform these types of
ultraviolet studies.

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

The search and the study of 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.

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:

ESA Communication Department

Media Relations Office, Paris, France

Tel: +33(0)-15369-7155 Fax: +33(0)-15369-7690

Alfred Vidal-Madjar

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

Tel: +33-1-44-32-80-73 E-mail: alfred@iap.fr

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@eso.org

Ray Villard

Space Telescope Science Institute, Baltimore, United States

Tel: +1-410-338-4514

E-mail: villard@stsci.edu