The Kepler Space Telescope

Launched by NASA in March 2009, the Kepler Space Telescope was designed with a single goal: to find Earth-like planets orbiting other stars. Named after Johannes Kepler, the mission relied heavily on his Third Law to characterize the planets it discovered.

The Transit Method

Kepler detected planets by watching for tiny dips in a star's brightness caused by a planet passing (transiting) in front of it. By measuring how often the dip occurred (the orbital period, T), and combining this with knowledge of the star's mass, astronomers used Kepler's Third Law to calculate the planet's orbital distance (a):

The depth of the brightness dip also revealed the planet's size relative to the star. See How Exoplanets Are Detected for more on the transit method and other techniques.

Results

During its primary mission (2009–2013) and extended K2 mission (2014–2018), Kepler observed over 500,000 stars and confirmed the existence of more than 2,600 exoplanets. These included rocky worlds in the habitable zone, giant planets orbiting close to their stars ("hot Jupiters"), and multi-planet systems.

Legacy

Kepler demonstrated that planets are common throughout the galaxy. Statistical analysis of its data suggests that there may be more planets than stars in the Milky Way. The mission was succeeded by TESS (Transiting Exoplanet Survey Satellite) in 2018, which surveys a larger area of sky using the same transit technique.