Kepler-78b: Mystery Exoplanet Shouldn't Even Exist

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Kepler-78b may be an exoplanet notable for being approximately Earth-sized and likely possessing a rocky surface plus iron core, but that’s where any similarity to our planet ends. It has an extremely tight orbit around sun-like star Kepler-78, completing one ‘year’ in only 8.5 hours. It orbits so close in fact that the alien world’s surface temperature soars to 2,000 degrees hotter than Earth’s. Referring to Kepler-78b as a “rocky” world is therefore a misnomer — it’s a hellish lava world.

But this is just a side-show to the real conundrum behind Kepler-78b: It shouldn’t exist at all.

“This planet is a complete mystery,” said astronomer David Latham of the Harvard-Smithsonian Center for Astrophysics (CfA) in a press release. “We don’t know how it formed or how it got to where it is today. What we do know is that it’s not going to last forever.”

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To current planetary formation theories, this lava world is an abomination. There’s no physical way a small world, only 20 percent larger than Earth, could have evolved in that location and there’s no known mechanism that could have transported it there. But one thing that is certain, it can’t stay roasting in that hellish orbit for long; it’s destined to get swallowed by its star “very soon, astronomically speaking,” said Dimitar Sasselov, also a CfA astronomer.

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.

“Kepler-78b is the poster child for this new class of planets,” said Latham.

Although Kepler originally detected Kepler-78b, the team used data from the high-precision spectrometer HARPS-North at the Roque de los Muchachos Observatory on La Palma and coordinated with a second, independent team using the HIRES spectrograph at the Keck Observatory, Hawaii. Whereas Kepler detects exoplanetary “transits” — as a world passes in front of its star, Kepler detects a slight dip in brightness — ground-based spectrometers measure the “wobble” of a star that is being gravitationally tugged by an orbiting world. A transit can be used to gain a measure of the physical size of the exoplanet, but the “radial velocity method” is required to reveal the mass of the world — using both measures, the planet’s density can be derived.

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The astronomers note that it is entirely possible that a similar, star-skimming world existed inside the orbit of Mercury in our own solar system, only for it to have been swallowed long ago, leaving no trace.

While their origin, so far, is a mystery, perhaps this strange class of explanets should be called “Icarus Worlds,” as, like in Greek mythology, they are flying too close to their suns; a cosmic dance that will ultimately lead to their demise.

Image: This diagram illustrates the tight orbit of Kepler-78b, which orbits its star every 8.5 hours at a distance of less than a million miles. It is only 2.7 stellar radii from the center of the star, or 1.7 stellar radii from the star’s surface. Credit: David A. Aguilar (CfA)

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