Size, mass, compositionEdit
Kepler-78b is about 20% larger than Earth, and is 69% more massive. 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. Although 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. So, 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. Another team used data from the Keck 1 Telescope at the W.M. Keck Observatory in Hawaii to estimate the mass is about 1.69 times that of Earth, and a radius of about 1.2 times that of Earth. 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 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 atmosphere, thoug liquid water and solid portions on the planet would 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.