The Exoplanet With a Comet-Like Tail

An extrasolar planet nearly as big as Jupiter is circling so close to its parent star that its atmosphere is being baked off into space, creating a comet-like tail.

An extrasolar planet nearly as big as Jupiter is circling so close to its parent star - a year passes in just 3.5 days - that its atmosphere is being baked off into space, creating a comet-like tail.

The planet, known as HD 209458b, is located about 153 light-years away.

Scientists had suspected since 2003 that stellar winds would be strong enough to sweep the planet's atmosphere into space and had even modeled what it would look like, says Jeffrey Linsky, with the University of Colorado in Boulder, who led a team that used Hubble Space Telescope's Cosmic Origins Spectrograph to make observations.

"We have measured gas coming off the planet at specific speeds, some coming toward Earth. The most likely interpretation is that we have measured the velocity of material in a tail," Linsky said in a press statement.

HD 209458b was the first extrasolar planet to be discovered transiting its parent star, a blessing of geometry - from Earth's perspective – that allows astronomers to study the structure and chemical makeup of its atmosphere by sampling starlight passing through the gases.

When the planet passes in front of the star it blocks about 1.5 percent of its light, not including the planet's atmosphere. When scientists add in the atmosphere, the dip jumps to 8 percent, indicating a substantial atmospheric bloat.

Chemical analysis turned up carbon and silicon, indicating that parent star is heating the entire atmosphere and allowing heavy elements to escape. Not everything is moving at the same speed, however.

"We found gas escaping at high velocities, with a large amount of this gas flowing toward us at 22,000 miles per hour," Linsky said. "This large gas flow is likely gas swept up by the stellar wind to form the comet-like tail trailing the planet."

The research is published in the July 10 issue of The Astrophysical Journal.