It can change the brightness of a background star (photometric) or its apparent position (astrometric). Einstein predicted that the photometric brightening would never be observed, due to the very close alignment required. But a few decades later, technology had improved enough that detecting such events which happen for 1 in a million stars became achievable.

The photometric effect, leading to a temporary brightening of a background star, has been used to find planets around other stars than the Sun, as well as black holes, brown dwarfs, and other low-mass objects.

The astrometric effect, consisting of a temporary apparent deflection in the position of the background star, provides much more information about the massive object causing the microlensing effect. In 2014, the nearby white dwarf Stein 2051B passed very close to a background star, deflecting the background star’s image. A team led by astronomers at STScI in Baltimore was able to make the first observation of this light bending caused by a star other than our own Sun. Thanks to these observations, they were able to calculate the mass of this white dwarf. This measurement provided essential information on white dwarfs, the last evolutionary stage for the majority of stars.

There are an estimated 100 million black holes with masses only a few times that of the Sun floating around the Milky Way. When they pass in front of stars, these black holes can also deflect light from these stars, leading to both a photometric and an astrometric effect. If detected, these signals would enable us to calculate the mass of the black holes, as well as how far away they are, and how fast they move in space. This enables us to understand the fate of stars that are too massive to become white dwarfs, and instead collapse and end their lives as black holes.

The mass of planets detected by microlensing cannot always be determined very precisely with current observations. This is because photometry usually does not allow us to gather all necessary information to make these calculations. With astrometry, however we get much more precise information as to the nature of the planets and their host stars. In the future, the WFIRST mission will make routine photometric and astrometric measurement for millions of stars, leading to better mass measurements for thousands of exoplanets and their host stars. This will truly place our own Earth and Solar System into perspective.