In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting a pulsar 2,300 light-years away. It was the first confirmed detection of planets outside our solar system. Today, we know of more than 5,500 exoplanets, with thousands more candidates awaiting confirmation.
How we find them
Astronomers use several methods to detect exoplanets. The two most productive are the transit method and the radial velocity method.
The transit method detects planets that happen to pass in front of their star from our perspective. As the planet transits, it blocks a tiny fraction of the star's light — typically between 0.01% and 1%. NASA's Kepler mission, launched in 2009, watched 150,000 stars for four years and discovered more than 2,600 confirmed exoplanets using this technique. The slight dimming of a star's light can reveal the planet's size, orbital period, and even hints about its atmosphere.
The radial velocity method detects the gravitational wobble a planet induces in its star. As a planet orbits, it pulls on its star with a tiny but measurable force. Astronomers detect this wobble as a slight shift in the star's spectrum — its light "blue-shifting" as it moves toward us and "red-shifting" as it moves away. This method is particularly good at finding massive planets close to their stars.
Types of exoplanets
The diversity of exoplanets far exceeds what we see in our solar system. The most common types are:
Hot Jupiters are gas giants orbiting extremely close to their stars — some completing an orbit in just a few days. These were a surprise: Jupiter takes 12 years to orbit the Sun, so how could these planets exist so close to their stars? The answer appears to be migration: they likely formed farther out and gradually spiraled inward.
Super-Earths are planets between 1 and 10 Earth masses. Some are rocky worlds like enlarged versions of Earth; others may be mini-Neptunes with thick atmospheres. This category doesn't exist in our solar system but appears to be the most common type of planet in the galaxy.
Ocean worlds may have global liquid water oceans beneath their surfaces, warmed by tidal heating from their parent planet. Europa and Enceladus in our solar system are suspected ocean worlds.
The habitable zone
The habitable zone is the region around a star where liquid water could exist on a planet's surface — not too hot, not too cold. For our Sun, it extends roughly from Venus to Mars. But the concept is more nuanced: a planet's atmosphere, rotation, and axial tilt all affect its surface temperature.
The TRAPPIST-1 system, discovered in 2017, contains seven Earth-sized planets orbiting a red dwarf star 40 light-years away. Three of them (TRAPPIST-1e, f, and g) are in the habitable zone. These worlds are tidally locked — one side always faces the star — which complicates their habitability.
The James Webb Space Telescope
JWST has revolutionized exoplanet science. By analyzing starlight that filters through a planet's atmosphere during transit, it can detect the chemical fingerprints of molecules: water vapor, carbon dioxide, methane, and more. In 2023, it detected signs of carbon-bearing molecules in the atmosphere of K2-18b, a "Hycean world" that may have a liquid ocean beneath a hydrogen-rich atmosphere.
Eventually, JWST and its successors may detect biosignature gases — chemical signs of life — in the atmospheres of distant worlds. But even if we found one, it would raise profound questions about what we do with that information.