An international team discovered the first Earth-mass planet that transits, or crosses in front of, its host star.
KOI-314c is the lightest planet to have both its mass and physical size measured.
Surprisingly, although the planet weighs the same as Earth, it is 60 per cent larger in diameter, meaning that it must have a very thick, gaseous atmosphere.
"This planet might have the same mass as Earth, but it is certainly not Earth-like," said David Kipping of the Harvard-Smithsonian Centre for Astrophysics (CfA).
"It proves that there is no clear dividing line between rocky worlds like Earth and fluffier planets like water worlds or gas giants," said Kipping, lead author of the discovery.
The team gleaned the planet's characteristics using data from NASA's Kepler spacecraft. KOI-314c orbits a dim, red dwarf star located approximately 200 light-years away.
It circles its star every 23 days. The team estimates its temperature to be 104 degrees Celsius, too hot for life as we know it.
KOI-314c is only 30 per cent denser than water. This suggests that the planet is enveloped by a significant atmosphere of hydrogen and helium 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, researchers said.
The team estimates its temperature to be 104 degrees Celsius
Conventionally, astronomers measure the mass of an exoplanet by measuring the tiny wobbles of the parent star induced by the planet's gravity.
This radial velocity method is extremely difficult for a planet with Earth's mass. The previous record holder for a planet with a measured mass (Kepler-78b) weighed 70 per cent more than Earth.
To weigh KOI-314c, the team relied on a different technique known as transit timing variations (TTV). This method can only be used when more than one planet orbits a star. The two planets tug on each other, slightly changing the times that they transit their star.
"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," said second author David Nesvorny of the Southwest Research Institute (SwRI).
The second planet in the system, KOI-314b, is about the same size as KOI-314c but significantly denser, weighing about 4 times as much as Earth. It orbits the star every 13 days, meaning it is in a 5-to-3 resonance with the outer planet.
Kipping presented the discovery at a meeting of the American Astronomical Society.