March 12, 2003: Astronomers using NASA's Hubble Space Telescope have observed for the first time the atmosphere of a planet beyond our solar system evaporating into space. Most of the planet may eventually disappear, leaving only a dense core. The evaporating planet is a member of a type of planet called a "hot Jupiter," a giant gaseous planet that orbits very closely around its parent star, drawn to it like a moth to a flame. The scorched planet, called HD 209458b, orbits only 4 million miles (7 million kilometers) from its yellow, Sun-like star. The planet circles the parent star in a tight 3.5-day orbit. The Hubble observations reveal a hot and bloated hydrogen atmosphere, which is evaporating off the planet. This huge envelope of hydrogen resembles a comet with a tail trailing behind the planet.See the rest:
The bloated outer atmosphere is heated so much by the nearby star that it starts to escape the planet's gravity. Once the gaseous atmosphere escapes, it is pushed away by the starlight, fanning out in a large tail behind the planet, astronomers said. Astronomers estimate the amount of hydrogen gas escaping HD 209458b to be at least 10,000 tons per second, but possibly much more.
HD 209458b is too close to the star for Hubble to photograph directly. Instead, astronomers observed the planet indirectly by measuring how much starlight is blocked when it passes in front of the star. Astronomers used the Hubble telescope's Space Telescope Imaging Spectrograph 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, an indication that the planet's atmosphere of hydrogen had inflated so much that it was evaporating into space.