Too Close For Comfort: Hubble Discovers An Evaporating Planet

The scorched planet, called HD 209458b, orbits only 4 million miles (7 million kilometers) from its yellow, Sun-like star. The Hubble 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.

An international team of astronomers, led by Alfred Vidal-Madjar of the 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 124,000 miles (200,000 kilometers)," says Vidal-Madjar.

HD 209458b is too close to the star for Hubble to photograph directly. 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.

The planet's outer atmosphere is extended and heated so much by the nearby star that it starts to escape the planet's gravity. "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 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 tons per second, but possibly much more.

Hot Jupiters 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 spiraled into their close orbits. This new discovery might help explain why hot Jupiters so often orbit a few million miles from their parent stars. Like HD 209458b, they are not usually found much closer than 4 million miles. Currently, the closest distance is 3.5 million miles (5.7 million kilometers). Hot Jupiters have orbits that are as brief as three days, but not shorter. Perhaps the evaporation of the atmosphere plays a role in setting an inner boundary for orbits of hot Jupiters.

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 Pegasus. In 1999 this star suddenly entered the astronomical "Hall of Fame" when HD 209458b was seen passing in front of the star and partly eclipsing 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, A. Lecavelier des Etangs and J.-M. Désert (Institut d'Astrophysique de Paris, CNRS, France), G. Ballester (University of Arizona), R. Ferlet and G. Hébrard (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. Observations of the atomic hydrogen envelope were made in ultraviolet (Lyman-alpha) light with Hubble's STIS. Hubble's position above the atmosphere makes it the only telescope that can currently perform this type of ultraviolet study.