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First evaporating planet excites European astronomers

First evaporating planet excites European astronomers

11 March 2003

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

The planet called HD209458b 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's tail, trailing far behind the planet. The planet circles the parent star in a tight orbit, each lasting only 3.5 days. Earth also has an extended atmosphere of escaping hydrogen gas, but the loss rate is much lower. Alfred Vidal-Madjar (Institut d'Astrophysique de Paris, CNRS, France), lead author of the discovery, says: "We were astonished to see that the hydrogen atmosphere of this planet extends over 200 000 kilometres."

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 indirectly observe it since the planet 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 faint, but detectable atmospheric signature. In a similar way, our Sun's light reddens as it passes obliquely through the Earth's atmosphere at sunset on Earth.

Using the Space Telescope Imaging Spectrograph (STIS) instrument, astronomers saw how extended the atmosphere of the planet is. They measured how much of the planet's atmosphere filters light from the star and found a startling drop in the star's hydrogen emission. A huge, puffed-up atmosphere would 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 is pulled out from the planet by immense tidal forces from the star. It then 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 HD209458b 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.

HD209458b 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 HD209458b. 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 there are hot Jupiters that orbit even closer to their stars and evaporate so quickly that astronomers cannot observe them.

Notes for editors

HD209458b was among the first extrasolar planets ever discovered. It is gaseous, with a diameter 1.3 times that of Jupiter, and two-thirds the mass. It orbits its parent star at one-eighth the distance of Mercury's orbit around the Sun.

The parent star is called HD209458. It 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 HD209458b passed in front of it and partly eclipsed it. This was the first 'extrasolar planet transit' ever seen. In 2001, Hubble detected the element sodium in the lower part of HD209458b's atmosphere, the first signature of an atmosphere on any extrasolar planet.

The research team was composed of researchers from the Institut d'Astrophysique de Paris, France, the University of Arizona, United States, and the Geneva Observatory, Switzerland. They observed three transits of the planet in front of the star. The observations were made in ultraviolet light, using Hubble's spectrograph STIS. Hubble's position above the atmosphere makes it the only telescope that can perform these types of ultraviolet observations currently.

The team consists 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). This discovery appears in the March 13 issue of Nature.

Image credit: European Space Agency, Alfred Vidal-Madjar (Institut d'Astrophysique de Paris, CNRS, France) and NASA.

For more information, please contact:

Alfred Vidal-Madjar
Institut d'Astrophysique de Paris (IAP/CNRS)
Paris, France
Tel: +33 1 44 32 80 73
E-mail: alfrediap.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: larseso.org

Ray Villard
Space Telescope Science Institute
Baltimore, United States
Tel: +1 410 338 4514
E-mail: villardstsci.edu

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Last Update: 1 September 2019
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