Scientists Discover a Planet with a Tail

Author:  Brian Jacobsmeyer
Institution:  University of Colorado - Boulder
Date:  November 2010

A super-hot planet in the constellation Pegasus has blurred the lines between planets and comets: it is the first planet observed to have a tail.

A recent study, led by Jeffrey Linsky of the University of Colorado at Boulder, found that the super-hot HD 209458b (colloquially known as "Osiris") has an evaporating atmosphere that has formed a tail behind the planet. Astronomers used the Cosmic Origins Spectrograph aboard the Hubble Space Telescope to determine the chemical composition and atmosphere of the planet, located about 153 light years from Earth. The findings are detailed in the July 10 issue of The Astrophysical Journal.

Osiris has a history of notable firsts in astronomy: the first planet observed to transit (pass in front of) its host star, the first planet observed with an atmosphere, and now, the only planet known to have a tail.

"Osiris is by far the best studied expolanet," said Kevin France, a member of the team credited with the discovery.

Stellar winds from Osiris' star rip elements such as gaseous carbon and silicon off the planet, resulting in a unique tail of these substances trailing behind it. Astronomers observed the tail as Osiris transited in front of its star. The team noticed that there was a 1.5 percent reduction in visible light that reached Hubble. However, when analyzing the ultraviolet light from the star,light indicative of the planet's atmosphere--the amount increased to 8 percent.

"When you watch certain elements transit the star, you get a much bigger dip," explains France, describing the presence of a bloated atmosphere. "The atmosphere is big and very puffy."

Known as a "Hot Jupiter," Osiris belongs to a class of large planets that orbit close to their parent stars, about 100 times closer than Jupiter does to the sun. Consequently, Osiris' atmosphere reaches temperatures of 2,000 degrees Fahrenheit. In addition, the planet travels at a blistering pace, revolving around its sun in only 3.5 days.

Scientists have discovered other "Hot Jupiters" with "puffy" atmospheres, but further study is needed to discover other potential "cometary planets." Although these planets are certainly uninhabitable, scientists believe signs of water vapor and carbon monoxide in their atmospheres imply that there may exist Earth-like planets nearby.

"What we can learn from special examples like Osiris is what the conditions would be like for terrestrial planets," France explains. Furthermore, he expects we'll be discovering more Earth-like planets in the near future: "Terrestrial planets are probably coming in the next 10 years."

Author: Brian Jacobsmeyer

Reviewed by: Natasha Hochlowski, Selby Cull, and Yangguang Ou

Published by: Maria Huang