gravitational microlensing
/ MY-kroh-LEN-sing /
Gravity bends light. When a massive object passes almost exactly in front of a more distant star, its gravity acts like a lens, bending and focusing the far star's light so that, from our vantage point, the background star briefly brightens. If the lens is itself a star with a planet, the planet adds its own tiny extra bending — a short, sharp blip riding on top of the main brightening. Gravitational microlensing finds planets by catching that fleeting blip as one star drifts in front of another.
The whole event is a chance alignment that never repeats. As the foreground star drifts across our line of sight to a distant star, the background star smoothly brightens over days or weeks and then fades. A planet around the foreground star, if its position is just right, produces a brief spike — sometimes lasting only hours — superimposed on that smooth curve. The shape and timing of the spike reveal the planet's mass relative to its star and its distance from it. Crucially, the method does not need to see any light from the planet or even the lens star at all; it senses the planet purely by its gravity.
Microlensing has a unique reach: it is most sensitive to planets far from their stars, even free-floating planets bound to no star at all, and it can probe worlds thousands of light-years away, across the galaxy. That makes it a powerful complement to transits and radial velocity, which favor close-in planets near the Sun. Its hard limitation is that each event is a one-time accident of alignment — you cannot go back and observe that planet again, ever — so microlensing builds up statistics about planet populations rather than studying individual worlds in depth.
Microlensing has uncovered planets in the cold outer reaches of distant systems, and even hints of rogue planets wandering the galaxy untethered to any star — worlds no other method could find, glimpsed once in a brightening that will never come again.
A planet betrays itself by a brief extra brightening as its star lenses a more distant one.
Each microlensing event is a one-time alignment that never recurs, so the planet found can never be re-observed; the method yields statistics, not repeat study of individual worlds.