How do we find planets around other stars if we can't see them?

We mostly find planets around other stars without seeing them at all. A few have been photographed directly, but the vast majority were detected indirectly, by watching what they do to their star. Two tricks have found the most.

The first is the transit method. If a planet's orbit happens to line up with our view, the planet passes in front of its star once per orbit and blocks a little of its light, so the star dims slightly and regularly. The dip can be tiny: an Earth-size planet crossing a Sun-like star dims it by only 0.008%. NASA's Kepler telescope watched hundreds of thousands of stars for exactly these dips and found thousands of planets.

The second is the wobble, or radial-velocity method. A planet's gravity tugs its star around in a small orbit, so the star moves slightly toward us and away from us. That motion shifts the colours of its light through the Doppler effect, and sensitive instruments can measure it. This is how Michel Mayor and Didier Queloz found the first planet around a Sun-like star, 51 Pegasi b, in 1995. It was a giant circling its star in just four days, and the discovery earned a share of the 2019 Nobel Prize in Physics.