The Kepler space telescope confirmed its first exoplanet in the habitable zone, Kepler-22b, in 2011.
NASA’s Kepler mission detected Kepler-22b by watching for tiny, repeated dips in the brightness of its host star, Kepler-22. The planet’s existence was announced on December 5, 2011, after additional observations helped confirm the signal.
Kepler-22b was notable because it was the first known transiting planet found in the habitable zone of a Sun-like star. That region has temperatures where liquid water might exist, although being in the habitable zone does not prove that a planet is habitable or inhabited.
A common mix-up is confusing the discovery telescope with Spitzer. Spitzer contributed follow-up observations, but Kepler made the original discovery. Kepler-22b is about 640 light-years away, has roughly twice Earth’s radius, and completes an orbit in about 290 days.