How did the Kepler mission help astronomers find Earth-sized planets?

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Habitable zone The habitable zones (green) for stars that are like the Sun (middle), hotter than the Sun (top), and cooler than the Sun (bottom). The red areas are those in which liquid surface water would be lost as a result of a runaway greenhouse effect, and the blue areas are those in which liquid surface water would be completely frozen.

The Kepler mission identified Earth-sized planets by monitoring stars for the slight, periodic dimming that occurs when a planet passes in front of its host star, a phenomenon known as a transit.1

Because detecting such small planets is extremely difficult—for instance, Earth dims the Sun by only 0.008 percent—Kepler utilized a 95-cm telescope in a heliocentric orbit to provide continuous, high-precision observations free from atmospheric interference. The spacecraft focused on a single patch of sky, using defocused light sensors to monitor over 100,000 stars simultaneously.1

A primary objective was to determine the frequency of planets within the habitable zone, the region where liquid water could potentially exist. Through this method, Kepler successfully discovered thousands of extrasolar planets, including Kepler-186f, which was the first Earth-sized planet confirmed to reside within its star's habitable zone.1