Why was Kepler-186f significant in the search for Earth-sized planets?

Kepler-186f is significant in the search for extrasolar planets because it was the first Earth-sized planet discovered within the habitable zone of its star.12

While other planets had been found in habitable zones previously, such as Kepler-22b, they were significantly larger than Earth. Kepler-186f, which has a radius 1.11 times that of Earth, represents a critical milestone in identifying worlds that could potentially possess liquid water on their surfaces and support life. Its discovery by the Kepler satellite confirmed that Earth-sized planets can exist in orbital regions where conditions might be conducive to life, even around stars different from our own Sun.213