Scorched Alien Planet Has a Comet Tail
Artist's view of extrasolar planet HD 209458b. Astronomers using NASA's Hubble Space Telescope have confirmed the existence of a baked object that could be called a "cometary planet." The gas giant planet, named HD 209458b, is orbiting so close to its star that its heated atmosphere is escaping into space. Observations taken with Hubble's Cosmic Origins Spectrograph (COS) suggest powerful stellar winds are sweeping the cast-off atmospheric material behind the scorched planet and shaping it into a comet-like tail.
Credit: NASA, ESA, and G. Bacon (STScI)

A scorched alien planet is flying so close to its parent star that its atmosphere is being swept off it in a glowing tail like some sort of giant comet, NASA announced Thursday.

The existence of the planet and its strange tail, which was suggested in previous studies, was confirmed recently by NASA's Hubble Space Telescope. These new observations of the gas giant planet, called HD 209458b, suggest strong winds from its nearby star are blowing the atmosphere off the scorched world and shaping it into a comet-like tail.

"Since 2003 scientists have theorized the lost mass is being pushed back into a tail, and they have even calculated what it looks like," said astronomer Jeffrey Linsky of the University of Colorado in Boulder, leader of the study. "We think we have the best observational evidence to support that theory. 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."

This planet orbits its star from a distance of about 4 million miles (7 million km) ? 100 times closer than Jupiter circles the sun ? yet the mass of HD 209458b is only slightly less than Jupiter. HD 209458b zips around its star in a short 3.5 days. In contrast, our solar system's fastest planet, Mercury, orbits the sun in 88 days. [Gallery: Strangest Alien Planets]

At such close quarters with its star, HD 209458b's atmosphere is a scorching 2,000 degrees Fahrenheit (1,093 Celsius). Past studies have suggested that it is losing about 10,000 tons of gas every second as material is blown off by its parent star. Eventually, it might be stripped entirely of its gas envelope, leaving behind a liquid core of lava. Even so, it will be a long while before the planet is completely destroyed.

"It will take about a trillion years for the planet to evaporate," Linsky said.

The planet is located 153 light-years from Earth. This world was the first of the few known extrasolar planets that seen to be passing in front of, or transiting, their stars.

The existence of HD 209458b was first suggested in 1999. Subsequent studies found indications of oxygen and carbon in its atmosphere. More recently, it became the first alien world outside our solar system found to have water.

To date, astronomers have found more than 400 extrasolar planets using the transiting method and by studying the wobble induced on stars by planets that orbit them.

Linsky and his team used Hubble's Cosmic Origins Spectrograph to analyze the planet's atmosphere during transiting events. This allows the astronomers to study the structure and chemical makeup of a planet's atmosphere by sampling the starlight that passes through it. The dip in starlight because of the planet's passage, excluding the atmosphere, is very small, only about 1.5 percent. When the atmosphere is added, the dip jumps to 8 percent, indicating a bloated atmosphere.

The data also showed the atmosphere escaping the planet was not all traveling at the same speed.

?"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."

?Editor's note: a typo has been corrected to reflect the fact that astronomers, not astronauts, have discovered exploplanets.