However, for a world that shouldn't exist, its life expectancy is pretty generous. Astronomers reckon that Kepler-78b will succumb to the star's tidal stresses and get ripped apart in about 3 billion years time.
When Kepler-78, which is located around 400 light-years away in the constellation Cygnus, was in the early stages of stellar development it would have been much larger than it is now. When astronomers calculated how big the young star would have been, Kepler-78b would be orbiting inside the star. "It couldn't have formed in place because you can't form a planet inside a star," said Sasselov.
So could the planet have formed in a wider orbit and migrated inward? This is another improbability, say the researchers. "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," Sasselov added.
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Despite its mysterious nature, Kepler-78b isn't alone. The world represents a new class of exoplanets recently discovered by NASA's Kepler Space Telescope. These worlds are small (approaching Earth-sized) with very compact orbits of less than 12 hours, but Kepler-78b is the first of this class to have its mass and density measured.