New Gassy Exoplanet has Mass of Earth but is 60 Percent Larger

An odd exoplanet orbiting a distant star has the mass of Earth, but is 60 percent larger in diameter than Earth, leading astronomers to believe that the new planet has a thick, gaseous atmosphere.

Astronomers reported the find Monday at the 223rd meeting of the American Astronomical Society in Washington.

The planet, named KOI-314c, is the lightest planet to have both its mass and physical size measured, and it is the first Earth-mass planet to have been located as it passes its host star.

"This planet might have the same mass as Earth, but it is certainly not Earth-like," David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA) said in a statement." It proves that there is no clear dividing line between rocky worlds like Earth and fluffier planets like water worlds or gas giants."

The newfound planet is in close orbit with its host star, a red dwarf, which it orbits every 23 days.

At 220 degrees Fahrenheit, the temperature on the planet, which is about 200 light years away, is too hot for life as we know it to exist.

The planet is only about 30 percent denser than water, the astronomers report, which suggests that the gaseous planet is has an atmosphere of hydrogen and helium that is hundreds of miles thick.

"It might have begun life as a mini-Neptune and lost some of its atmospheric gases over time, boiled off by the intense radiation of its star," the CfA said.

To mass this new planet, astronomers used the technique known as transit timing variations, which is not typically used to learn the mass of planets. With this method - which can only be used when two or more planets orbit a star - astronomers can measure pulling forces that lead to slight changes in how planets transit, or pass in front of, their star.

"Rather than looking for a wobbling star, we essentially look for a wobbling planet," said David Nesvorny of the Southwest Research Institute. "[NASA's Kepler space telescope] saw two planets transiting in front of the same star over and over again. By measuring the times at which these transits occurred very carefully, we were able to discover that the two planets are locked in an intricate dance of tiny wobbles giving away their masses."

Although astronomers are happy to have found another planet, their original mission with the Kepler data was to find moons.

"When we noticed this planet showed transit timing variations, the signature was clearly due to the other planet in the system and not a moon. At first we were disappointed it wasn't a moon but then we soon realized it was an extraordinary measurement," said Kipping.