How does gravitational microlensing reveal extrasolar planets?

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Extrasolar planet Artist's conception of the extrasolar planet HD 209458b, some 150 light-years from Earth.

Gravitational microlensing reveals extrasolar planets by utilizing the gravitational effects predicted by Albert Einstein in his theory of general relativity.1

When a massive object, such as a star or a planet, passes directly between a distant background star and an observer, it acts as a gravitational lens. The gravity of the foreground object deflects and focuses the light from the more distant star, causing it to brighten temporarily. By observing these specific patterns of brightening, astronomers can detect the presence of planets orbiting the foreground star, or even identify free-floating planets that are not bound to any stellar host.1