Which detection method finds exoplanets by using a foreground star's gravity as a lens?

The detection method that uses a foreground star’s gravity as a lens is gravitational microlensing.

In this technique, a foreground object passes almost directly between Earth and a more distant background star. Its gravity bends and magnifies the background star’s light, producing a temporary brightening that astronomers can measure. A planet orbiting the foreground star can add a short-lived extra signal to the event.

Microlensing is especially valuable because it can reveal planets that emit little or no light and do not regularly cross in front of their host stars. Unlike the transit method, it does not require repeated dips in a star’s brightness. The alignment is usually a one-time event, so the discovery often cannot be repeated.

The “micro” refers to the tiny, unresolved image displacement and the relatively small lensing mass—not to a different force. Astrometry measures a star’s positional wobble, direct imaging attempts to see a planet’s light, and transit photometry measures periodic dimming. All are distinct from microlensing.