One of the newest planets to be discovered using NASA's Kepler Space Telescope has been identified as the closest in size and composition to Earth that astronomers have ever seen. But if this alien world is to be described as 'Earth-like,' conditions there would make it a hellish twin.
Kepler-78b is mentioned in two papers published in the journal Nature today, October 30th. The planet circles a star a little smaller and slightly cooler than our sun, roughly 400 light years away, in the constellation Cygnus. It orbits so close to that star, and goes around so quickly (in just 8.5 hours) that the automated detection programs used by the Kepler team initially missed finding the planet's transits. It was only found later, in a specific search for short-transit planets.
This video shows an artist's impression of the orbit of Kepler-78b:
Although there have been plenty of claims already of this planet being 'the most Earth-like' we've found, that only counts when you consider its size, density and composition. Kepler-78b is about 1.2 times the size of Earth, and roughly 1.7 times the mass of Earth, giving it a density that is very close to our planet's. That means that it is very likely composed of rock and iron, as Earth is. This makes it the very first planet that we've found that's within 20 per cent of our planet's size and mass, and is also rocky.
However, since the planet is close enough to its star that it's tidally-locked — having one side constantly baked under the 2,000-3,000 degree fury of the star and the other side in perpetual darkness — the surface conditions make it a hellish, lava-strewn wasteland on one side and a barren, airless wasteland on the other. Definitely not 'Earth-like.'
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One more aspect of this discovery that's important is what it means for finding more Earth-like planets.
"The existence of Kepler-78b shows that, at the very least, extrasolar planets of Earth-like composition are not rare," astronomer Drake Deming, of the University of Maryland, wrote in an article in Nature today.
As for the actual 'most Earth-like' planet we know of, currently that's Kepler-62e, which has an Earth Similarity Index of 0.83. There's one other planet that might be even higher on that index, Gliese 581 g, but it's existence hasn't yet been confirmed.
(Image courtesy: David A. Aguilar/Harvard-Smithsonian Center for Astrophysics)
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