Kepler-78b: an impossible planet?

Scientists have this week described Kepler-78b as a planet that shouldn’t exist. Orbiting its star at a distance of under one million miles^[1] and with a year of just eight and a half hours, Kepler-78b defies all current theories of planet formation. The planet, discovered by scientists at the Harvard-Smithsonian Center for Astrophysics (CfA), is simply too close to its star.

When the planetary system was forming, the young star was larger than it is now. As a result, the current orbit of Kepler-78b would have been inside the swollen star. “It [Kepler-78b] 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,” explains CfA astronomer Dimitar Sasselov.

Not only is Kepler-78b a mystery world, it is the first known planet to be discovered with a similar size and density to Earth. This would suggest that it also has an Earth-like composition of iron and rock. Kepler-78b is a member of a new class of planets recently identified in data from NASA’s Kepler spacecraft. These newly found worlds all orbit their stars with periods of less than 12 hours and are all about the size of the Earth. Kepler-78b is the first planet in the new class to have its mass measured.

Ultimately however, Kepler-78b is a doomed world. Gravitational tides will draw it even closer to its star until eventually gravity rips it apart entirely.

^[1] The distance between the Earth and the Sun is over ninety-two million miles.

More information at : http://www.cfa.harvard.edu/news/2013-25