How Astronomers Find Exoplanets

Planets around other stars used to be a hypothesis. Today astronomers have confirmed thousands of them. Most of these worlds were not seen directly — they were inferred from careful measurements of the light coming from their host stars.

The transit method

When a planet passes in front of its star from our point of view, it blocks a tiny fraction of the star's light. Telescopes like Kepler and TESS have found thousands of planets by watching for this regular, tiny dip in brightness. The bigger the planet compared to its star, the deeper the dip.

The radial velocity method

Every planet tugs on its star as it orbits. The star wobbles slightly, and this wobble shifts its light towards blue and then red in a rhythmic pattern. Astronomers use very precise spectrographs to detect these shifts. This method works especially well for large planets close to their stars.

Direct imaging

In rare cases, a planet is bright enough — and far enough from its star — that a telescope can capture its light after carefully blocking the starlight. Direct imaging works best for young, hot, giant planets on wide orbits.

Microlensing and other techniques

Sometimes a distant planet aligns just right so that its gravity briefly acts as a lens for the light of an even more distant background star. When astronomers see the resulting brightening, they can infer the planet's presence. Other techniques include astrometry and timing variations of pulsars.

Each method has strengths and blind spots. Together they let us begin sorting the strange variety of worlds that exist around other stars.