Why is microlensing useful for finding planets far from their stars?

Gravitational microlensing is a powerful technique for detecting planets because it relies on the gravitational bending of light, a phenomenon predicted by Albert Einstein’s general theory of relativity.12

Unlike methods such as radial velocity or transit photometry, which are most sensitive to large planets orbiting very close to their host stars, microlensing is uniquely effective at identifying planets located far from their stars—often hundreds of millions of kilometers away.2

The technique works when an intervening star and its planets pass between a more distant source star and an observer on Earth. The gravity of the foreground star and its planets acts as a gravitational lens, focusing and brightening the light from the distant star. Because this method does not depend on detecting light from the planet itself or measuring the host star's orbital motion, it can even be used to discover a population of free-floating giant planets that do not orbit any star at all.12