planet
A team of astronomers has discovered an Earth-mass planet that is the lightest to have its mass and physical size measured. C. Pulliam & D. Aguilar (CfA)

It may weigh the same as Earth, but the recently discovered KOI-314c planet has some stark differences.

A team of international astronomers detailed their discovery at the 223rd meeting of the American Astronomical Society. It is both the lightest planet to have its mass and physical size measured and the first Earth-mass planet found that crosses in front of 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), lead author of the discovery, 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 planet, KOI-314c, weighs the same as Earth but is 60 percent larger in diameter, suggesting it has a thick gaseous atmosphere. The team discovered the planet using data from NASA's Kepler spacecraft.

"The really incredible thing that shocked us is that it had exactly the same mass as our home planet of Earth, but it's 60 percent bigger than the Earth, which means it must have a huge atmosphere on top of a rocky core deep down in the middle," Kipping told the Canadian Broadcast Corporation.

While the planet may have the same mass as Earth, it is not as hospitable. Astronomers determined KOI-314c is 220 degrees Fahrenheit, is only 30 percent water, and is enveloped by a significant atmosphere of hydrogen and helium hundreds of miles thick. It is located 200 light-years away.

Rather than measuring the tiny wobbles of the planet’s parent star to determine the planet’s weight – a conventional method of measuring an exoplanet – astronomers relied on a technique known as transit timing variations (TTV), which has proved to work well in studying smaller planets.

"Rather than looking for a wobbling star, we essentially look for a wobbling planet," David Nesvorny of the Southwest Research Institute (SwRI) said. "Kepler 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."

And astronomers say the planet was discovered by chance.

"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," Kipping said.