The first discoveries

The first confirmed planets outside the Solar System were found in 1992 by Aleksander Wolszczan and Dale Frail. They orbit PSR B1257+12, a pulsar: the collapsed core of an exploded star that sends out radio pulses at very regular intervals. The planets pull on the pulsar, so the pulses arrive a tiny bit early and late. Timing the pulses revealed the planets. Nobody expected planets around a dead star, and pulsar planets turned out to be rare.

The first planet found around a Sun-like star came in 1995. Michel Mayor and Didier Queloz used radial velocity to find 51 Pegasi b. Its orbit takes only about 4.23 days, and its minimum mass is about 0.47 Jupiter masses. Kepler’s third law (see the Orbits & Gravity lab) gives its orbit radius: about 0.05 AU, far inside Mercury’s 0.39 AU orbit, which takes 88 days. A gas giant that close was a shock, because everyone expected giants to sit far out, as in our own Solar System. Mayor and Queloz shared the 2019 Nobel Prize in Physics (the other half went to James Peebles for cosmology).

Planets like this are called hot Jupiters: giants on orbits of a few days, heated to 1000 K or more. They are easy to find, but they are rare: only roughly 1 percent of Sun-like stars have one (estimates are somewhere between about half a percent and one percent). The Solar System has none. A leading idea is migration: such a giant may form far from its star, where there is plenty of ice and gas, and later move inward, through interaction with the gas disk or through a gravitational shake-up from other bodies.