Kepler-78b is an extrasolar planet, in the constellation of Cygnus. It orbits its host star, Kepler-78, every 0.355007 days (8.5 hours). At the time of its discovery, it was notable as it was the most similar to Earth in terms of mass, radius, and mean density.


The planet was discovered in 2013 by analyzing data from the Kepler space telescope. The planet was found not only transiting the star, but its occultation and reflected light from the parent star due to orbital phases were also detected. It did not have Kepler object of interest designation at first as the automatic data analysis missed this planet, due to its short period.


Size, mass, compositionEdit

Kepler-78b is 69% more massive and 20% larger than Earth. Two independent teams were involved in pioneering work to estimate the mass of the planet. The estimates were made possible, as Kepler-78b's gravity causes a "wobble" in the orbit of the host star. While this method is usually used to characterize gas giants, ordinarily it is hard to estimate the mass of Earth-sized exoplanets because their gravity is too weak to produce a visible influence on their stars. In this case, the planets orbit is so close to the star that its gravitational influence actually has a detectable effect.

A team led by Francesco Pepe also estimated that Kepler-78b had a mass 1.86 times that of Earth, and a radius of about 1.16 times more then that of Earth. The other, led by Andrew Howard of the University of Hawaii at Manoa, used data from the High Resolution Eschelle Spectrometer on the Keck 1 Telescope at W.M. Keck Observatory in Hawaii to estimate the mass as 1.69 times that of the Earth and the radius as 1.2 times. Both estimates put the planet's density at about 5.5 grams per cubic centimeter, equivalent to that of the Earth and possibly indicative of a rock-iron composition like Earths.

Kepler-78b is very similar to other hot extrasolar planets with high-density, such as Kepler-10b, Kepler-36b, and CoRoT-7b.


Kepler-78b orbits around its star in an 8.5-hour orbit. The planet reflects around 20% to 60% of the starlight that it receives. As it orbits extremely closely to its star, approximately 40% closer then Mercury to the Sun, the estimated surface temperature is 2330 K (2056 °C, 3734 °F) to 3100 K (2830 °C, 5120 °F). The temperature is high enough to have stripped the planet of any stable atmosphere, though liquid water and solid portions on the planet should be stable. According to Francesco Pepe, one of the astronomers involved in the discovery of this planet, said "the planet may be Earth-sized, but it can be imagined like a lava planet rather than an Earth-like planet."

The acceleration of gravity on the planet surface is estimated to be ~11 m/s2, which is slightly larger than Earth's.


According to CFA astronomer Dimitar Sasselov, "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". It is estimated that the planet will be swallowed by its parent star in about three billion years.