The Planet With a Tail

Today there is a new tail to add to the tale of astronomical tails. This one belongs to a planet.

Comet McNaught Credit: Robert McNaught

You're all familiar with the beautiful tails that comets display as the go around the Sun. Energy from our star ionizes particles and strips dust away from the comet's nucleus to form these sometimes magnificent tails.

Mira, one of the most famous variable stars, also sports a tail. Mira is an old evolved red giant star that is losing massive amounts of surface material as it hurdles along through space. This tail material, imaged for the first time in 2007, has been released over the past 30,000 years. Coincidentally, Mira happens to reside inside the tail of the constellation Cetus the Whale.

Image credit: NASA/JPL-Caltech

The common factor in the formation of these tails is a strong stellar wind emanating from the star; our Sun in the case of comets, and the wind from the red giant Mira in the case of the variable star.

Now astronomers have discovered evidence for a planet sporting a tail caused by this same phenomena, the stellar wind of the star  HD 209458. The gas giant planet, named HD 209458b, is orbiting so close to its star that it only takes three and a half days to orbit the star.  As a result of this uncomfortable proximity its super-heated atmosphere is being blown off into space. Observations taken with Hubble's Cosmic Origins Spectrograph (COS) suggest this cast-off atmospheric material is accumulating behind the scorched planet forming a comet-like tail.

 Image credit: NASA, ESA, and G. Bacon (STScI)

"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 COS 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."

The star is not just blowing off the top layers of the atmosphere either. COS detected carbon and silicon in the planet's super-hot, 2,000-degree-Fahrenheit atmosphere. This means the parent star is heating the entire atmosphere, dredging up heavier elements and allowing them to escape the planet along with the lighter gases.

Considering the continuous loss of material into space, how long before this planet simply evaporates? "It will take about a trillion years for the planet to evaporate," answered Linsky.