Introduction to Microlensing

What is microlensing?

Normally, light from a distant star travels to us in a straight line. This star is called the "source." But if another star passes between us and the more distant star, its gravity will deflect the light from the distant star. Because it bends light, we call the intermediate star the "lens." If the deflection is strong enough, the distant star appears to be two stars. These are called "images" of the distant star.

Deflection of light by a single star

Deflection of light by a single star

Animation by J.C. Yee

In the animation below, the source star moves behind the lens star. As it does so, the images of the source change as a function of time. The bottom panel shows the measured magnification of the source star's light as a function of time.

Animation of lensing by a single star

Animation of lensing by a single star

Animation by B.S. Gaudi

How does microlensing find planets?

If the lens star is orbited by a planet, the planet will further deflect the light from the source star.

Deflection of light by a star with a planet

Deflection of light by a star with a planet

Animation by J.C. Yee

We can detect the planet by measuring the magnification curve and observing the small bump it produces:

Animation of lensing by a star with a planet

Animation of lensing by a star with a planet

Animation by B.S. Gaudi