Exoplanets
Planets orbiting stars beyond the Sun.
Overview
An exoplanet is a planet orbiting a star other than the Sun. The first confirmed detection around a Sun-like star came in 1995, and more than five thousand are now confirmed — enough that the field has shifted from finding individual objects to characterising populations.
- Every detection method has strong selection biases; the known population is not a fair sample.
- The most common planet sizes found so far — between Earth and Neptune — have no Solar System analogue.
- 'Habitable zone' means liquid water could be stable at the surface given suitable conditions. It is not a claim about life.
- The 2019 Nobel Prize in Physics recognised the first detection around a solar-type star.
How exoplanets are detected
- Transit: a planet crossing its star's disc causes a small periodic dip in brightness. This gives the planet's radius relative to the star and requires a nearly edge-on orbit. Kepler and TESS were built around this method, which has produced the most detections.
- Radial velocity: the planet's gravity makes the star wobble, shifting its spectral lines. This gives a minimum mass, and was the method behind the 1995 detection of 51 Pegasi b.
- Direct imaging: blocking the star's light to photograph the planet directly. It works best for young, massive, widely separated planets that are still hot from formation, and yields spectra of the planet itself.
- Gravitational microlensing: a foreground system briefly magnifies a background star, with the planet adding a characteristic spike. It is uniquely sensitive to planets far from their stars, but events do not repeat.
- Astrometry: measuring the star's positional wobble on the sky. Gaia's precision is expected to make this productive for wide-orbit giant planets.
Selection bias is the central caveat
Transit surveys favour large planets in short orbits, because those transit deeply and often. Radial velocity favours massive planets close to their stars, for the same reason. Direct imaging favours the opposite corner of parameter space. The catalogue is therefore a composite of what each technique can reach, not a census.
The early dominance of hot Jupiters in discovery counts is the clearest illustration: they are rare in absolute terms but were by far the easiest to find first. Any statement about how common a planet type is has to be corrected for detection efficiency, and quoted population fractions always carry that model dependence.
The planets that have no Solar System counterpart
The most abundant planets found so far, once bias is corrected, are between Earth and Neptune in size — super-Earths and sub-Neptunes. The Solar System contains nothing in this range, which is a striking result: the most common kind of planet in the Galaxy is a type we cannot study up close.
A related finding is the radius valley, a deficit of planets around 1.5 to 2 Earth radii, which appears to separate rocky planets from those retaining thick hydrogen envelopes. Whether that gap is carved by photoevaporation from stellar radiation or by heat from the planet's own core is an active question.
Habitable zones and atmospheres
The habitable zone is the range of orbital distances at which liquid water could persist on a planet's surface, given an Earth-like atmosphere. It is a screening tool for target selection, not a statement that a planet is habitable — surface conditions depend on atmospheric composition, magnetic field, geological activity and stellar behaviour, none of which the zone accounts for.
Atmospheric characterisation is where the field is now moving. Transmission spectroscopy, which measures how a planet's atmosphere filters starlight during transit, has detected water vapour, carbon dioxide, methane and other species. JWST has substantially extended what is reachable, but detections at the limit of sensitivity are frequently revised, and Asteria Star records claimed atmospheric detections with their confidence and their source rather than as settled facts.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- How many exoplanets have been found?
- More than five thousand are confirmed, and the number rises continually as surveys report. Because the count changes with each catalogue release, this platform links to the exoplanet catalogue rather than freezing a figure here — the catalogue pages carry the archive-sourced records with their provenance.
- What was the first exoplanet discovered?
- Planets around the pulsar PSR B1257+12 were confirmed in 1992, but the landmark detection around a Sun-like star was 51 Pegasi b in 1995 by Michel Mayor and Didier Queloz — a giant planet in a four-day orbit, which was not what theory expected and forced substantial revision of planet-formation models. That work received the 2019 Nobel Prize in Physics.
- Does 'habitable zone' mean a planet has life?
- No. It means the planet orbits at a distance where liquid water could be stable at the surface if it had a suitable atmosphere. It says nothing about whether it has an atmosphere, what that atmosphere is made of, whether the surface is solid, or whether life exists. It is a target-selection criterion, not a finding.
- Can we see exoplanets directly?
- A small number, yes. Direct imaging requires suppressing the star's overwhelming glare with a coronagraph or starshade, and works best for young, massive planets in wide orbits that are still glowing from the heat of formation. Most confirmed exoplanets have never been seen directly and are known only through their effects on their host star's light.