How Astronomers Detect Exoplanets

Exoplanets are extremely difficult to see directly. They are tiny compared to their host stars (Earth is about a millionth the brightness of the sun) and they sit close to those stars from our perspective. Detection usually relies on indirect methods. The transit method watches for tiny dips in a star is brightness when a planet passes in front of it from our perspective; the dip can be as small as 0.01 percent and lasts hours. The radial velocity method (also called the Doppler method) measures tiny periodic changes in a star is light wavelength caused by the gravitational pull of orbiting planets. Both methods have produced thousands of detections.

Each method is biased toward certain kinds of planets. Transit detection only works for planets whose orbits pass between us and their stars (a small fraction of all orbits) and works best for large planets close to their stars. Radial velocity favors massive planets close to their stars because they tug their stars more strongly. Direct imaging, the rarest method, captures actual photons reflected or emitted by the planet itself; it works best for very young, hot, large planets far from their stars. Microlensing detects planets through their gravitational distortion of light from background stars; it picks up rare alignments and is sensitive to planets at intermediate distances. Each method covers different parameter space; combining them gives a fuller picture of what populations of planets actually exist.

The Kepler Space Telescope, operating from 2009 to 2018, made transit detection the dominant method. By staring at one patch of sky and continuously monitoring the brightness of about 150,000 stars, Kepler detected thousands of transit signals that were eventually confirmed as planets. The current TESS mission (Transiting Exoplanet Survey Satellite, launched in 2018) does similar work but covers nearly the entire sky and focuses on brighter, closer stars whose planets can be characterized further. Ground-based programs like HARPS, ESPRESSO, and CHEOPS use radial velocity at very high precision. Future projects like the European PLATO mission and various direct imaging missions will continue to expand what astronomers can find.

Detection methods continue to improve. The next lesson covers what we have learned about the actual diversity of exoplanets the methods have revealed.