The short answer
We cannot see most exoplanets directly — they are too faint next to their stars. Instead, four indirect methods dominate: transit photometry, radial velocity, direct imaging, and gravitational microlensing. Each reveals different properties, and the best-characterised worlds have been studied with several methods combined.
Method 1 — Transit photometry
When a planet's orbit is edge-on relative to Earth, it periodically crosses the face of its star. This transit dims the star's light by a tiny but measurable fraction — roughly 1% for a Jupiter-size planet, 0.01% for an Earth-size planet. By measuring the depth of the dip (planet radius), the spacing of dips (orbital period), and combining with the star's distance, we can derive whether the planet is in the habitable zone. The Kepler space telescope used this method to confirm over 2,600 planets. Its successor, TESS, continues the survey.
Method 2 — Radial velocity (the Doppler wobble)
A planet does not orbit its star — both orbit their common centre of mass. For a massive planet, this causes the star to wobble slightly, periodically moving toward and away from Earth. This motion shifts the star's spectral lines via the Doppler effect, allowing ground-based spectrographs to measure the wobble with metre-per-second precision. Radial velocity reveals the planet's minimum mass and orbital period. The first confirmed exoplanet around a Sun-like star — 51 Pegasi b — was found this way in 1995. ESO's HARPS spectrograph is the benchmark instrument.
Method 3 — Direct imaging
Blocking the star's light with a coronagraph reveals nearby planets in infrared. Currently only large, young planets far from their star are detectable this way — they glow with residual heat from formation. The James Webb Space Telescope has pushed the limits dramatically: it can analyse the atmospheric composition of transiting planets by comparing the star's spectrum during and outside of transit, revealing molecules like water vapour, carbon dioxide, and methane.
Method 4 — Gravitational microlensing
When a foreground star drifts in front of a more distant star, the foreground star's gravity acts as a lens, briefly brightening the background star. If the foreground star has a planet, the planet adds a short additional spike to the brightening curve. Microlensing is sensitive to planets at distances from their star where other methods struggle — including planets further out than Earth's orbit. The Nancy Grace Roman Space Telescope will conduct a large microlensing survey targeting thousands of exoplanets.
Method comparison