Astrometry detects exoplanets by measuring tiny changes in a star’s position on the sky.
Every planet and its host star orbit a shared center of mass, or barycenter. Because the star is much heavier, its movement is far smaller, but a sufficiently massive planet can make the star trace a minute wobble against more distant background stars. Astrometry measures that apparent positional shift over time.
The method is conceptually different from transit photometry, which watches for a planet blocking a small fraction of starlight, and from radial velocity, which measures changes in the star’s spectral lines. Microlensing instead relies on the temporary brightening and distortion produced when a foreground system bends light from a background star.
Astrometry is one of astronomy’s oldest proposed planet-hunting techniques, but it is technically demanding. Atmospheric distortion, instrument calibration, stellar activity and the star’s own proper motion can overwhelm the planetary signal. Space missions and long-baseline surveys have improved its reach, making it especially valuable for detecting massive planets on relatively wide orbits.