The discovery of planets orbiting other stars has been one of the most transformative achievements in modern astronomy. Since the first confirmed detection in 1992, astronomers have cataloged thousands of exoplanets ranging from scorching gas giants to potentially habitable rocky worlds, fundamentally reshaping our understanding of planetary systems.
The first confirmed exoplanets were discovered in 1992 orbiting a pulsar — a rapidly spinning neutron star — not a Sun-like star. This was completely unexpected, as astronomers had assumed planets could not survive the supernova explosion that creates pulsars, yet these worlds somehow endured and reformed from the debris.
The transit method detects exoplanets by measuring the tiny dip in a star's brightness as a planet passes in front of it. NASA's Kepler space telescope used this technique to discover over 2,600 confirmed planets by staring continuously at a single patch of sky containing roughly 150,000 stars for four years.
Proxima Centauri b, discovered in 2016, is the closest known exoplanet to Earth at just 4.2 light-years away. It orbits in the habitable zone of the nearest star to our Sun, Proxima Centauri, though intense stellar flares from its red dwarf host star may limit its potential to support life.
The TRAPPIST-1 system contains seven Earth-sized rocky planets, three of which orbit within their star's habitable zone. Discovered in 2017, these worlds are so close together that a person standing on one planet's surface could see the other planets as clearly as we see the Moon from Earth.
Direct imaging of exoplanets remains extraordinarily difficult because a planet's light is typically millions to billions of times fainter than its host star. Specialized instruments called coronagraphs block starlight to reveal planets, and fewer than 30 exoplanets have been directly imaged as of the mid-2020s.
Hot Jupiters — gas giant planets orbiting extremely close to their stars — were the first type of exoplanet discovered around Sun-like stars in 1995. Their existence overturned planetary formation theories, which predicted gas giants could only form far from their stars in cold regions where ices and gases could accumulate.
The James Webb Space Telescope has begun analyzing the atmospheres of exoplanets in unprecedented detail. Its spectrographs can detect molecules such as water vapor, methane, carbon dioxide, and potentially biosignature gases in planetary atmospheres dozens of light-years away.
Rogue planets drift through interstellar space without orbiting any star, having been ejected from their home systems by gravitational interactions. Estimates suggest our galaxy may contain billions of such worlds, and some could potentially retain subsurface oceans heated by internal radioactivity.
The radial velocity method detects exoplanets by measuring the tiny wobble a planet's gravitational pull induces in its host star. This technique has achieved precision capable of detecting stellar motions as slow as walking speed — just one meter per second — from hundreds of light-years away.
Kepler-452b, sometimes called Earth's older cousin, orbits a Sun-like star in the habitable zone and is about 60 percent larger than Earth. At roughly 6 billion years old, it has spent 1.5 billion years longer than Earth in its habitable zone — ample time for life to potentially develop if conditions permitted.
Astrometry, the oldest exoplanet detection method, involves tracking the tiny sideways motion of a star as an orbiting planet tugs it slightly. After decades of false claims, ESA's Gaia mission has finally begun confirming exoplanets through astrometry by measuring stellar positions with microarcsecond precision.
TOI-1452 b, discovered in 2022, is a super-Earth located about 100 light-years away that may be an ocean planet with water comprising up to 30 percent of its mass. For comparison, Earth's oceans account for less than one percent of our planet's total mass, making TOI-1452 b a candidate for a truly water-dominated world.