They measure the shift in a star's emission spectrum.
Explanation:
Exoplanets are planets found outside the solar system.
Planets orbiting far enough from stars to be resolved reflect very little starlight, so planets are detected through their thermal emission instead. These thermal emission is gotten by observing the shift in the stars emission spectrum.
Coronagraphs are also used to block light from the star which results to leaving the planet visible. Direct imaging of an Earth-like exoplanet requires extreme use of optothermal stability.
They can also be found by using space based telescopes.
