Worlds Beyond Our Sun

For most of human history, astronomers could only speculate about whether other stars hosted planets of their own. That changed dramatically in 1992 when Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR B1257+12. Three years later, Michel Mayor and Didier Queloz detected 51 Pegasi b, the first exoplanet found around a Sun-like star, using the radial velocity method at the Observatoire de Haute-Provence in France. This discovery earned them the 2019 Nobel Prize in Physics and opened an entirely new field of astrophysics.

The transit method has become the most prolific technique for detecting exoplanets. When a planet passes in front of its host star as seen from Earth, it causes a tiny, periodic dip in the star's brightness. NASA's Kepler Space Telescope, launched in 2009, used this approach to discover more than 2,700 confirmed exoplanets by continuously monitoring the brightness of over 150,000 stars. Its successor, the Transiting Exoplanet Survey Satellite (TESS), launched in 2018, surveys the entire sky for nearby transiting planets.

The radial velocity method detects the subtle gravitational wobble a planet induces in its host star. As the star moves slightly toward and away from Earth, its light is Doppler-shifted. This technique works best for massive planets close to their star and was the first method to successfully detect exoplanets around Sun-like stars. Direct imaging captures actual photographs of exoplanets by blocking the overwhelming glare of the host star, but is currently limited to large, young, bright planets in wide orbits.

Among the most exciting discoveries are planets in the habitable zone -- the region around a star where temperatures allow liquid water to exist. Notable systems include Proxima Centauri b, the nearest known exoplanet at just 4.24 light years; the TRAPPIST-1 system, with seven Earth-sized planets (three in the habitable zone); and Kepler-186f, the first Earth-sized planet confirmed in a habitable zone. The James Webb Space Telescope is now characterizing exoplanet atmospheres, searching for biosignatures like water vapor, carbon dioxide, and potentially even signs of biological activity.