The team, composed of researchers from the Institut d'astrophysique de Paris
(IAP), the University of Arizona, and the Geneva Observatory, did observe
three transits of this planet in front of its star. The observations have been
secured in the ultraviolet using the Hubble Space Telescope, with the
spectrograph STIS that has been installed by the Discovery Space Shuttle
astronauts in February, 1997. The ultraviolet light allows the signature of
the upper atmospheric hydrogen to be observed. Indeed, hydrogen is the
lightest and the most abundant element. Thus, it easily rises to the upper
atmosphere. As seen in the so-called "Lyman-alpha" hydrogen line at 121.6
nanometer, the shadow of the planet appears huge. "We were astonished to see
that the hydrogen atmosphere of this planet extends over 200,000 kilometers"
Alfred Vidal-Madjar said. The gas is detected well beyond the gravitational
influence of the planet; it is seen escaping at more than 100 kilometers per
second (360,000 kilometers per hour), pushed away by the star light.
These observations required a detailed analysis and a particular data
reduction in order to reveal the atmospheric signature of hydrogen. "The most
difficult task was to correct for the thermal effects in the detector, and to
subtract the emission from the upper atmosphere of the Earth, which is also
visible with the Space Telescope" explained Jean-Michel Désert, engineer at
the Institut d'Astrophysique de Paris.
A simple model allows this observation to be explained.
In the upper
atmosphere, the gas temperature increases due to the heating of the star.
Moreover, the gravitational attraction of the planet is reduced by the
attraction of the star, which creates a tidal force similar to the one from
the Sun and the Moon that deforms up and down the Earth oceans. "The
atmosphere is thus stretched, then hydrogen is pushed away by the star light
and strewn out in a large tail similar to those of comets", said Alain
Lecavelier des Etangs (IAP). The amount of gas escaping HD209458b can be
estimated to be at least 10,000 ton of hydrogen per second. But this flow is
likely to be much higher; the planet may loose a significant fraction of its
mass. This evaporation process of the planets which are too close to their
parent star could explain the very few detections of planets orbiting at less
than 7 million kilometers from their star. Those planets should quickly
evaporate, or become hydrogen-poor Neptune-mass planets.