exo- + planeta

Greek exo means 'outside' or 'external.' Greek planetes (πλανήτης) meant 'wanderer' — applied to the points of light that moved against the fixed stars. A planet was, in the original Greek, simply a wandering thing. An exoplanet is a planet outside our solar system — a wanderer around another star. The word was coined in 1992 when astronomers Aleksander Wolszczan and Dale Frail announced the first confirmed exoplanets: two objects orbiting the pulsar PSR 1257+12.

The first exoplanets were not orbiting a sun-like star. They circled a pulsar — the spinning, radiation-blasting remnant of a supernova. That these planets existed at all, given the catastrophic explosion that created their host star, was astonishing. They had either survived the supernova or coalesced from its debris afterward. Neither scenario seemed plausible; both had to be considered.

The first exoplanet around a sun-like star was confirmed in 1995 by Michel Mayor and Didier Queloz using the radial velocity method (Doppler shifts in the star's light). They found 51 Pegasi b — a 'hot Jupiter,' a gas giant orbiting its star in four days, closer than Mercury orbits the Sun. It upended planetary formation models. Queloz and Mayor received the 2019 Nobel Prize in Physics for the discovery.

As of 2024, over 5,600 exoplanets have been confirmed, with thousands more candidates. The Kepler Space Telescope (2009-2018) found most of them by the transit method — detecting the dimming when a planet passes in front of its star. The James Webb Space Telescope now analyzes exoplanet atmospheres, looking for biosignatures. The wanderers have been found. Now we are reading their atmospheres.