Planets found by microlensing

General relativity says mass bends light, so a star passing precisely in front of a more distant star acts as a lens and briefly brightens it. If the foreground star has a planet, the planet adds its own short, sharp spike to that brightening, lasting hours to days. Watch enough stars toward the crowded centre of the galaxy and the alignments happen often enough to find planets nobody could otherwise detect.

Microlensing is the only method that reaches planets far from their stars and stars far from us: it finds cold worlds on Jupiter-like orbits, and free-floating planets bound to no star at all. It is also the only method that works well thousands of light-years away, which makes it the closest thing we have to a galactic census rather than a local one.