An exoplanet is a planet orbiting a star other than the Sun. The first ones were confirmed in the 1990s, and the NASA Exoplanet Archive now lists more than 6,000. Most are never seen directly: they are detected through their effects on their star.
The transit method
When a planet passes in front of its star, the star dims slightly. A Jupiter-sized planet dims a Sun-like star by about 1 percent, an Earth-sized one by less than 0.01 percent. Repeated dips reveal the orbit, and the depth gives the planet’s size.
NASA’s Kepler telescope (2009 to 2018) found thousands of planets this way, and TESS, launched in 2018, surveys bright nearby stars. Transits are also ideal for studying atmospheres, see how astronomers study planetary atmospheres.
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The radial velocity method
A planet’s gravity makes its star wobble slightly. The star’s light shifts toward blue as it moves toward us and toward red as it moves away, see spectroscopy. In 1995, Michel Mayor and Didier Queloz used this method to find 51 Pegasi b, the first planet around a Sun-like star, earning the 2019 Nobel Prize in Physics. The wobble gives the planet’s minimum mass.
Other methods
Direct imaging: blocking the star’s light with a coronagraph to photograph the planet itself. Works best for large, young, hot planets far from their stars. JWST has imaged several.
Gravitational microlensing: a star with a planet passing in front of a background star magnifies its light, see how gravity bends light.
Astrometry: measuring the star’s tiny side-to-side motion in the sky. ESA’s Gaia mission is expected to reveal many giant planets this way.
Pulsar timing: the very first confirmed exoplanets, announced in 1992 by Aleksander Wolszczan and Dale Frail, orbit a pulsar and were found through changes in its pulses, see pulsars.
What the discoveries showed
Planetary systems are extremely diverse: “hot Jupiters” orbiting their stars in a few days, “super-Earths” and “mini-Neptunes” with no equivalent in our solar system, planets around two stars, and compact systems like TRAPPIST-1. Many stars have planets, so planets probably outnumber stars in our galaxy.
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Confirmation matters
Signals can be mimicked by starspots, binary stars or instrument noise. Candidates are usually confirmed with a second method or careful statistical checks. For the search for habitable worlds, see are there other planets like Earth.
