Microlensing is a bit of a forgotten relative of observational exoplanet science — we hear about transit and radial velocity planets all the time, and direct imaging is intuitively simple and makes pretty pictures, so it gets lots of press time too. The microlensing technique, meanwhile, has been quietly churning out planet detections since 2004. And yet, if you asked your average exoplanet scientist how microlensing works, they’d probably panic and mutter something about general relativity before running away as soon as possible.
Microlensing works like this: your system consists of a lens star, which magnifies light from a source star. The source star is moving relative to the lens, and the closer it gets to the lens, the more the light rays are bent, and the more the lens magnifies the source. One could plot contours of equal magnification — they would be circles, traced around the lens star, as I’ve tried to demonstrate in Figure 1 (follow link below). The closer to the lens, the more a source would be magnified. The background source will move across the diagram following the red arrow, and as the flux increases and then decreases a flux peak is observed. … (AAS/NOVA)