NASA’s Kepler space telescope has discovered a veritable menagerie of alien worlds orbiting other stars, but this most recent exoplanetary find is perhaps one of the strangest. The world is of the approximate mass of the Earth, yet it has a ‘puffed-up’ atmosphere, making it appear 60 percent larger than our planet.

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Located 200 light-years away, KOI-314c orbits its dim red dwarf star once every 23 days. The close proximity to its star ensures that it has a rather toasty atmosphere of around 220 degrees Fahrenheit (or 104 degrees Celsius) — a little above the boiling point of water at sea-level (on Earth). Any discussion of the possibility for life as we know it is moot, but this new discovery is notable nonetheless; this is the first Earth-mass exoplanet discovered that had its mass measured by using Transit Timing Variations (TTVs).

The easiest way to gauge an exoplanet’s mass is to watch the ‘wobble’ of its host star as it orbits. This may be an effective tool for the more massive exoplanets, but when it comes to planets of Earth-like masses, their featherweight gravitational fields have a minimal effect on the star, making the wobble difficult to discern. Lacking star ‘wobble’ data (otherwise known as radial velocity data), an international team of astronomers used an ingenious technique to measure KOI-314c’s mass.

KOI-314c has company. This exoplanet exists in a multi-planetary system with at least one other exoplanet. The second exoplanet, called KOI-314b, also travels across the face of its star from our perspective (an event known as a ‘transit’), allowing Kepler to make very precise measurements of both planet’s orbital periods. KOI-314b is around the same size as KOI-314c, but it is a lot more dense, ‘weighing-in’ at 4 times the mass of the Earth.

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Like the planets in our solar system — and, indeed, the systems of moons around Saturn and Jupiter — KOI-314b and KOI-314c tug on each other gravitationally. From Kepler’s perspective, this gravitational tugging manifests itself as slight changes in transit timings. TTVs have been used to discover the gravitational presence of undiscovered planets that lurk in unseen multi-planetary systems, tugging on other planets that transit their star.

Fascinatingly, TTVs are also being used in an effort to detect exomoons orbiting transiting exoplanets (as the moons’ gravitational tug should cause their parent exoplanet to wobble very slightly, causing a TTV) and the team of astronomers were actually seeking out exomoons in Kepler TTV data when they serendipitously discovered a unique way of deducing KOI-314c’s mass.

“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,” said David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA) and lead author of the discovery. “At first we were disappointed it wasn’t a moon but then we soon realized it was an extraordinary measurement.”

“Rather than looking for a wobbling star, we essentially look for a wobbling planet,” said co-investigator David Nesvorny, of the Southwest Research Institute (SwRI). “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.”

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KOI-314b orbits the star every 13 days, giving it a 5-to-3 resonance with KOI-314c — for every 5 orbits by KOI-314b, KOI-314c orbits 3 times.

Apart from proving that TTVs can be used to gauge the mass of transiting exoplanets, the discovery only adds to the fascinating variety of strange new worlds Kepler is uncovering in our galaxy.

“This planet might have the same mass as Earth, but it is certainly not Earth-like,” said Kipping. “It proves that there is no clear dividing line between rocky worlds like Earth and fluffier planets like water worlds or gas giants.”

Knowing the mass and physical size of KOI-314c, Kipping’s team were able to deduce the exoplanet’s average density — it is only 30 percent more dense than water. Therefore, the astronomers have deduced, the world must be a rocky world which is covered a dense atmosphere of hydrogen and helium, hundreds of miles thick. It seems likely that the world was once a Neptune-like gas giant, but over time, the close proximity to its star boiled off the vast majority of its atmosphere, shrinking it to a more Earth-like mass.

Kipping and his team presented their findings on Monday at the 223rd meeting of the American Astronomical Society in Washington D.C.