"It couldn't have formed in place because you can't form a planet inside a star," Dimitar Sasselov, of the Harvard-Smithsonian Center for Astrophysics and a member of the Pepe-led team, said in a statement. "It couldn't have formed further out and migrated inward, because it would have migrated all the way into the star. This planet is an enigma."
What is clear, however, is that Kepler-78b's days are numbered. The planet will continue circling lower and lower until the immense gravity of its host star tears it apart, likely within 3 billion years or so.
"Kepler-78b is going to end up in the star very soon, astronomically speaking," Sasselov said.
The search for another Earth
The hellishly hot Kepler-78b is not a good place to hunt for alien life. But the determination of its density marks a milestone in the ongoing search for a true "Earth twin" — a planet very much like Earth in size, composition and surface temperature.
"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, writes in an accompanying commentary article today in the same issue of Nature.
Deming points to NASA's upcoming Transiting Exoplanet Survey Satellite mission, or TESS, which is slated to launch in 2017 to hunt for transiting planets around nearby stars (as contrasted with Kepler, whose gaze was more distant).
"By focusing particularly on small stars cooler than the sun, TESS should find exo-Earths whose mass can be measured by trading the close-in orbit of Kepler-78b for more distant orbits around low-mass stars, approaching orbital zones where life is possible," Deming writes. "That trade-off probably cannot be pushed to the point of measuring an Earth twin orbiting once per year around a sun twin, but it will allow future scientific teams to probe habitable planets orbiting small stars."
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