Hubble detects the ongoing evaporation of an extra-solar planet
An atmosphere made of hydrogen has been observed for the first time around an extra-solar planet. This atmosphere is hot and very extended: the planet is evaporating. This unexpected discovery obtained with the Hubble Space Telescope by a team led by Alfred Vidal-Madjar (Institut d'Astrophysique de Paris, CNRS), raises the issue of the survival of the planets too close to their parent star.


More than 100 planets are known to orbit stars other than the Sun. About 15% of them are very close to their star and orbit in a few days, like the planet around the star HD209458 with a "year" of 3.5 days only! HD209458 is a normal star, similar to our Sun, located at 150 light-years from the Earth. It can be seen with a simple binocular, in the Pegasus constellation. Its planet, named HD209458b, has been discovered in 1999. This planet is massive and gaseous like Jupiter. Located at less than 7 million kilometers from its star HD209458b is intensively heated (the Earth is located at 150 million kilometers from the Sun). HD209458b is thus one of the so-called 'Hot-Jupiters'. It is a prime target for astronomers because, as seen from the Earth, the planet transits in front of the star each 3.5 days. During this 3-hour eclipse, the planet hides a small part of the stellar disk, which thus appears slightly fainter. That allows the atmosphere of the planet to be observed, because it imprints its signature onto the light passing through, like the sunset light is reddened when passing through the Earth's atmosphere.

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.


visit the HD209458b IAP web-site.

Artist view of the evaporating extrasolar planet HD209458b

visit the HD209458b IAP web-site.

Observations compared to our model