More than 5,000 exoplanets have been discovered, and almost all of them were found by just three methods: watching a star dim as a planet crosses it, measuring the star's wobble as a planet tugs it, or photographing the planet directly. Each method sees different kinds of planets, and together they have mapped a galaxy full of worlds. This guide explains how each method works and what it can and cannot find.

The Transit Method: Watching for Shadows

When a planet passes in front of its star, it blocks a tiny fraction of the star's light, usually less than 1%, and the dip repeats with each orbit. Telescopes like Kepler and TESS watch thousands of stars for these dips, and the size of the dip reveals the planet's size while the timing reveals its orbit. Transits are the most productive method, responsible for most known exoplanets, but they only work when the planet's orbit is edge-on to us, which is true for a small fraction of systems.

Radial Velocity: Measuring the Wobble

A planet's gravity tugs its star, making the star wobble slightly toward and away from us, and that wobble shifts the star's light by the Doppler effect. Spectrographs measure the shift precisely enough to detect a planet's mass and orbit. Radial velocity found the first exoplanets around Sun-like stars, including Proxima Centauri b, and it works for planets that never transit. Its limit is sensitivity: Earth-mass planets cause a wobble of centimetres per second, at the edge of what current instruments can measure.

Direct Imaging: Photographing the Planet

Direct imaging photographs the planet itself, separate from its star, which is like photographing a firefly next to a searchlight. It works best for young, hot, distant planets, because they are bright in infrared and far enough from their star to be resolved. Coronagraphs block the star's light to reveal the planet, and the method gives us the planet's spectrum and, eventually, its weather. It is the hardest method, but it is the one that will one day image an Earth-like world.

What Each Method Misses

Transits miss planets whose orbits are not edge-on, and they favour large planets close to their stars. Radial velocity misses low-mass planets and struggles with active stars whose surfaces mimic planetary signals. Direct imaging misses small, cool, close-in planets, which are the most common kind. No method sees everything, which is why the true number of exoplanets is far larger than the 5,000 we have found: the galaxy likely holds billions.

The Future of Planet Hunting

The next generation of planet hunters will combine methods: the Nancy Grace Roman Space Telescope will find planets by microlensing, the Extremely Large Telescope will image rocky planets directly, and Webb continues to characterise the atmospheres of planets found by transit. The goal is no longer just to count exoplanets, but to understand them: their climates, their compositions, and whether any of them could support life. The methods that found the first 5,000 will find the next 50,000.

Key Takeaways

- Transits watch a star dim as a planet crosses it; radial velocity measures the star's wobble - Direct imaging photographs the planet itself, but only works for young, hot, distant worlds - Each method misses different planets; the true total is in the billions - More than 5,000 exoplanets are known, most found by transits

Frequently Asked Questions

How many exoplanets have been discovered? More than 5,000, mostly by the transit method. The true number in the galaxy is likely in the billions, because each detection method misses most planets.

We find exoplanets by shadows, wobbles and photographs. Each method sees a different slice of the galaxy, and together they reveal a universe full of worlds.