Kepler-78b orbits a Sun-like G-type star located 400 light-years from Earth in the constellation Cygnus. It is a planet that circles its star every eight and a half hours at a distance of less than 1.6 million km (1 million mi). The planet is only 2.7 stellar radii from the center of the star, or 1.7 stellar radii from the star's surface. This orbit is one of the tightest known, with the world approximately 900,000 km (550,000 mi) above the star's surface. The surface temperature of the planet is thought to be close to 2400° C (4300° F).
Its formation is a mystery. According to our current theories of planet formation, the planet should not have formed so close to its host star and neither could it have moved there. The planet will however be consumed by its star very soon, in astronomical terms. Gravitational tides are gradually moving it closer to its host star, Kepler-78. The star’s gravity will eventually rip the planet apart and the planet will likely disappear within three billion years.
Kepler-78b is also the first known Earth-sized world with an Earth-sized density. The planet is around 20 percent larger than Earth and has a diameter of nearly 15,000 km (9,200 mi). The planet also weighs 1.8 times as much as Earth, resulting in it having a density similar to Earth’s. Because of this similar density, Kepler-78b is thought to be composed of iron and rock.
When the planetary system of Kepler-78 was forming, the star was much larger than it is now. The current orbit of Kepler-78b would at that time been within the larger star; planets cannot form inside stars. Likewise, the planet could not have formed further out in the system and then migrated inward, as it would have eventually migrated into the star.
Kepler-78b has been made a member of a new class of planets that were identified from data from NASA’s Kepler spacecraft. These worlds all orbit their host stars with periods of less than 12 hours and all are around the same size of Earth. Kepler-10b has a radius of 1.42 x the radius of the Earth while Kepler-36b has a radius of 1.49 that of Earth. Kepler-78b has an even smaller radius, at 1.16 times that of Earth.
Kepler-78b was studied using a new high-precision spectrograph called HARPS-North, at the Roque de los Muchachos Observatory on La Palma. The work was coordinated with another, independent team who were using the HIRES spectrograph at the Keck Observatory. The measurements from the two teams correlated, which increased confidence in the results.
It is possible our solar system once had a planet like Kepler-78b, though such a planet would have been destroyed eons ago.