The Kepler mission and its extension, called K2, discovered thousands of exoplanets. It detected them using the transit technique, measuring the dip in light intensity whenever an orbiting planet moved across the face of its host star as viewed from Earth. Transits can not only measure the orbital period, they often can determine the size of the exoplanet from the detailed depth and shape of its transit curve and the host star's properties. The transit method, however, does not measure the mass of the planet. The radial velocity method, by contrast, which measures the wobble of a host star under the gravitational pull of an orbiting exoplanet, allows for the measurement of its mass. Knowing a planet's radius and mass allows for the determination of its average density, and hence clues to its composition.
