The exoplanet was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured.

Kepler-20f is very likely (>95% chance) a rocky planet because of its radius, which is notable as being the closest to Earth yet: 1.004 R🜨,[2] though a newer estimate suggest a value of 0.952+0.047−0.087 R🜨.

[1] Kepler-20 is a Sun-like star in the constellation Lyra with a mass of 0.91±0.03 M☉ and a radius of 0.94±0.06 R☉, and is thought to be 8.8 billion years old, though there is a large uncertainty in its age.

[6] This gives the planet an insolation flux (i.e. the amount of radiation it receives from the star) 35.9 times that of the Earth, hence its high temperature.

[1] In 2009, NASA's Kepler spacecraft was completing observing stars on its photometer, the instrument it uses to detect transit events, in which a planet crosses in front of and dims its host star for a brief and roughly regular period of time.