Just as it is possible to use interferometry to obtain very detailed images of objects in space, the interference patterns of light can be analyzed to take very detailed spectra. The resulting measurements of Doppler shifts in the spectra—and hence, the motions of the objects producing the shifts—can be extremely precise. With current technology, for example, it is actually possible to measure changes of speed as little as walking-pace level at distances of hundreds of trillions of miles!

As it turns out, this is the level of speed changes that large planets exert on the stars they orbit. If, for example, the gas giant planet Jupiter orbited the Sun at the distance of Mercury, the Sun would wobble back and forth, changing the direction of its motion every few weeks by an amount similar to a running person. By measuring the spectra of nearby Sun-like stars, and using interferometry to detect the minuscule changes in their speed, it is possible to detect and confirm the existence of planets orbiting around them. Hundreds of extrasolar planets have been detected in exactly this way, all of them orbiting faraway stars. 3