Distant, Earth-like Lava World Should Not Even Exist, Scientists Say

Astronomers have discovered a doomed and distant Earth-like world that is currently unexplainable within the current theories of planet formation.

Kepler-78b has a size and density similar to Earth's and orbits a Sun-like star. But the similarities stop there. Kepler-78b is a fiery lava world so close to its parent star that it completes an orbit every eight hours, one of the tightest planetary orbits known.

"This planet is a complete mystery," astronomer David Latham, of the Harvard-Smithsonian Center for Astrophysics (CfA), said. "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."

CfA astronomer Dimitar Sasselov concurs. "Kepler-78b is going to end up in the star very soon, astronomically speaking."

Located about 400 light years away from Earth in the constellation Cygnus, the lava world is about 20 percent larger than Earth and weighs about twice as much, according to the astronomers' calculations, which would suggest the planet has an Earth-like composition of iron and rock.

Yet the astronomers seem more occupied with figuring out how Kepler-78b even exists than they are with the conditions on the distant world. According to their calculations, the planet's star has been there long before the planet, which is mystifying because the currently accepted theories of planet formation suggest that a distance of less than 1 million miles, which is the position of the planet in relation to its star, is too close for a planet to form. Astrophysics have also ruled out that the planet could have moved there later on as well.

When the planet system around Kepler-78b's star was forming, the star was even larger than it is today, which would have resulted in the planet being engulfed by the star.

"It couldn't have formed in place because you can't form a planet inside a star. 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 explained.

The strange properties of Kepler-78b, along with planets with similar sizes and equally quick orbits, have led way to a new class of planets. Kepler-78b is the first of the new class to have its mass measured.

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

While Kepler-78b's origins are a mystery, its end has already been foreseen. The planet is doomed, the astronomers say. It's orbit is so close to its sun that gravitational tides will eventually draw the planet so close to its sun that the star's gravity will rip it apart. Theorists predict the world will see its end within 3 billion years.