Exoplanet detection methods explained
Nearly all of them were found indirectly, and each method is biased toward a different kind of planet.
4 min read · Published 28 August 2026
The short answer
Most exoplanets are detected indirectly. The transit method watches for a star dimming as a planet crosses in front, and has found the large majority. Radial velocity measures the star's wobble as a planet tugs it, and was the first successful method. Both are strongly biased toward large planets in close orbits, which is why early exoplanet catalogues were dominated by hot Jupiters.
The main methods
| Method | What it measures | Biased toward |
|---|---|---|
| Transit | Starlight dimming as a planet crosses the star | Large planets, short orbits, near-edge-on alignment |
| Radial velocity | The star's motion toward and away from us | Massive planets in close orbits |
| Direct imaging | The planet's own light, with the star blocked | Large, young, hot planets far from their star |
| Gravitational microlensing | Light bending as a system passes in front of a background star | Planets at intermediate distances; not repeatable |
| Astrometry | The star's position wobbling on the sky | Massive planets in wide orbits |
Notice that every method has a bias, and they differ. Any statement about how common a type of planet is must account for which method found it — the raw catalogue reflects what is easy to detect, not what exists.
The transit method
If a planet's orbit is oriented so it passes between us and its star, the star dims slightly and repeatedly. Measuring that dip gives the planet's size relative to the star, and the repeat interval gives the orbital period.
The dips are small. Jupiter crossing the Sun would dim it by about 1%; Earth by about 0.008%. Detecting that requires exceptional photometric precision, which is why dedicated space telescopes were built for it.
The severe limitation is geometry. The orbit must be nearly edge-on from our viewpoint, which is true for a small fraction of systems — well under 1% for Earth-like orbits. So the method misses the vast majority of planets, not because they are undetectable but because they never cross.
Its great advantage is that transits also permit atmospheric analysis: starlight filtering through a planet's atmosphere during transit carries the signature of its composition, which is how JWST studies exoplanet air.
Radial velocity
A planet does not orbit its star; both orbit their common centre of mass. The star therefore traces a small circle, moving alternately toward and away from us, which shifts its spectrum by the Doppler effect.
This gives the planet's mass, or rather a minimum mass, since the orbital inclination is usually unknown.
The velocities are minuscule. Jupiter makes the Sun move at about 12 metres per second; Earth at about 9 centimetres per second. Detecting an Earth analogue requires measuring a star's velocity to better than walking pace across light years, which is at the edge of current capability.
Combining both methods is powerful: transit gives radius, radial velocity gives mass, and together they give density — which distinguishes a rocky planet from a gas envelope.
What the biases mean
Early catalogues were dominated by hot Jupiters — massive planets in very short orbits. That was widely reported as surprising, and it was partly a selection effect: those are exactly the planets both main methods find most easily.
Correcting for detection bias suggests small planets are far more common than large ones, and that planets are extremely common generally — most stars appear to have them.
Two categories with no solar system analogue turned out to be among the most common: super-Earths and sub-Neptunes, between Earth and Neptune in size. Our own system, oddly, has nothing in that range.
This is a good discipline to carry into any survey result. The first thing to ask about a catalogue is what it was capable of finding, and the answer usually explains a large part of what it contains.
Common questions
- How are exoplanets detected?
- Mostly indirectly. The transit method watches for a star dimming as a planet passes in front, and has found the large majority. Radial velocity measures the star's motion toward and away from us as a planet tugs it. Direct imaging accounts for only a small fraction.
- Why do most known exoplanets orbit close to their stars?
- Detection bias. Both main methods find large, close-in planets most easily — short orbits repeat quickly and produce larger signals. Correcting for this suggests small planets are far more common than the raw catalogue implies.
- What is the transit method's main limitation?
- Geometry. The orbit must be nearly edge-on from our viewpoint for a transit to occur, which is true for well under 1% of systems with Earth-like orbits. The vast majority of planets simply never cross their star from where we sit.
- How can we measure an exoplanet's atmosphere?
- During a transit, some starlight passes through the planet's atmosphere before reaching us, carrying the spectral signature of its composition. This transmission spectroscopy is how molecules such as water, carbon dioxide and methane have been detected.