Kepler Provides An Estimate Of Eta_Earth*

The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data
“We present occurrence rates for rocky planets in the habitable zones (HZ) of main-sequence dwarf stars based on the Kepler DR25 planet candidate catalog and Gaia-based stellar properties. … We estimate with 95% confidence that, on average, the nearest HZ planet around G and K dwarfs is about 6 pc away, and there are about 4 HZ rocky planets around G and K dwarfs within 10 pc of the Sun.”

*”The term eta-Earth (also written as ZE) is defined as the mean number per star of rocky planets with between 1 and 1.5-2 Earth-radii that reside in the optimistic habitable zone (HZ) of their host star. Eta-Earth enters one formulation of the Drake equation, which endeavors to estimate the occurrence of intelligent life in the Galaxy; at the present time, it is usually calculated separately for each stellar spectral type. Thus, eta-Earth represents the occurrence rate of rocky planets in the optimistic HZ of different stars. The References present some values for eta Earth based on different statistical analyses of the data from the Kepler space telescope.”
Source: Encyclopedia of Astrobiology