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Gassy ‘Earth-like’ planet is in fact, nothing like Earth


A team of astronomers has identified a planet with a similar mass to Earth that travels in front of its host star. Despite these similarities though, measurements show it’s also 60 percent larger in diameter than Earth, which suggests its surface will be far different to that of our own planet’s. The find outlines the importance of measuring a planet’s mass and size to get that ratio determined before laying any claims to the term “Earth-like”, which is so often attached to such discoveries in the press — as indeed, it has been with the revelation of this find. 

“This planet might have the same mass as Earth, but it is certainly not Earth-like,” David Kipping, lead scientist working on the investigation and astronomer at Harvard-Smithsonian Centre 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.” 

In a separate statement, Nasa’s Jet Propulsion Laboratory revealed the results of four-year’s worth of research done by the Kepler team into exoplanets the spacecraft has honed in on. It found that 75 percent of all planet candidates it has spotted are somewhere between the size of Earth and Neptune. Although the information confirms the team’s suspicions that the discoveries are planets, it alone provides no indication of whether those planets are rocky, gassy or in any way actually habitable. Examples include Kepler-99b and Kepler-406b, which are rocky planets and have a similar mass, but are 40 percent larger that Earth. On top of that, they orbit their host star within five days, so would be far too hot to host life. 

KOI-314c, the planet identified by Kipping and his team, was also identified 200 light-years from Earth using Kepler while they were looking for exomoons. It circles its star, a red dwarf (a low temperature star commonly found in our own Solar System) every 23 days, leading the team to estimate the surface temperature at 104C. The most starkly differentiating factor though has to be the fact that the entire planet is only 30 percent denser than water, which means it is probably engulfed in gas, considering its size. The team has estimated the clouds of hydrogen and helium covering it surface extend for hundreds of miles.

The team was able to give an estimate for the light planet’s mass because it was accompanied by a second planet, KOI-314b, which is around about the same size but weighs four times more than Earth and orbits the red dwarf every 13 days. Because of these differences, the planets exert forces on each other as they orbit, gradually tweaking their orbit timings as they crossover the star.

“Kepler saw two planets transiting in front of the same star over and over again,” explains co-investigator David Nesvorny of the Southwest Research Institute. “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.” It’s a technique called transit timing variations, which could be helpful for measuring such unusually light planets. Usually Doppler measurements would be taken — a similar technique that relies on the relative wobble of the host star as it is pulled, rather than the planet’s. 

Reiterating the relative limitations for the exoplanet search, as progressed in the past few years using Kepler, NASA’s Ames Research Center mission scientist Natalie Batalha said: “Kepler’s primary objective is to determine the prevalence of planets of varying sizes and orbits. Of particular interest to the search for life is the prevalence of Earth-sized planets in the habitable zone. But the question in the back of our minds is: are all planets the size of Earth rocky? Might some be scaled-down versions of icy Neptunes or steamy water worlds? What fraction are recognisable as kin of our rocky, terrestrial globe?”

Last year another team identified a planet the same mass and size as Earth. Kepler-78b, however, orbits its star every 8.5 hours, making its blistering hot surface uninhabitable.


A paper has been submitted to The Astrophysical Journal detailing the KOI-314c find.