The path of a light beam is bent by the presence of mass, and a massive body can therefore act like a lens (a “gravitational lens”) to distort the image of an object seen behind it. Scientists first confirmed Einstein’s prediction quantitatively during the now famous total eclipse of 29 May 1919 by observing starlight bent by the mass of the Sun. Microlensing is the name given to a related phenomenon: the brightening of light from a star as a cosmic body, acting as a gravitational lens, passing fortuitously in front of it, the light then dimming to normal as the body moves beyond the line-of sight. About one hundred exoplanets have been discovered to date by the microlensing technique, ranging in masses from about fifty Jupiter-masses to less than a few Earth-masses.

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