The planet in peril is dubbed HD209458b and orbits a star in the constellation Pegasus in such a way that it passes between the star and Earth once every revolution. This arrangement allowed researchers to develop a new method for examining the planet's atmosphere. By analyzing light from the star as it passes through the atmosphere, astronomers can determine its chemical composition. An international team led by Alfred Vidal-Madjar of the Institute of Astrophysics in Paris examined spectroscopic data collected by the Hubble Space Telescope during three passes of the planet and found a startling drop in the amount of hydrogen detected. Study co-author Alain Lecavelier des Etangs, also of the Institute of Astrophysics, explains that "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." (See the image above for an artist's rendition of HD209458b orbiting its parent star.)
Computer simulations indicate that HD209458b's comet-like tail is about 124,000 miles long, or nearly five times the circumference of Earth. The scientists estimate that the hydrogen loss is about 10 million kilograms each second. Notes co-author Gilda Ballester of the University of Arizona, "The hydrogen will just keep evaporating away during the lifetime of the star."