The Kepler Space Telescope was launched in 2009. The goal of the Kepler Mission was to find Earth-like planets around Sun-like stars. It did this via the transit method.

Kepler continuously observed a single field for 4 years, until the second of its four reaction wheels broke and the telescope was unable to stay stable. By observing stars continuously, it was able to notice small changes in stars' brightness. A periodic dip in brightness could be a sign that there was a planet orbiting that star.