Exoplanets
An exoplanet is a planet orbiting a star other than the Sun (or, rarely, a free-floating planet-mass object); since the first confirmed discoveries in 1992 and 1995, more than six thousand have been confirmed, most found by the transit method (Kepler, TESS) or the radial-velocity method, revealing a diversity of planetary systems very different from our own.
Curious
For most of history, other planetary systems were speculation. The first confirmed exoplanets were found in 1992 around a pulsar (see Pulsars) by Aleksander Wolszczan and Dale Frail. In 1995, Michel Mayor and Didier Queloz found 51 Pegasi b, a planet about half the mass of Jupiter orbiting its Sun-like star in just 4.2 days, which was a surprise. They shared the 2019 Nobel Prize in Physics for this.
The NASA Exoplanet Archive passed 6,000 confirmed planets in 2025. They include “hot Jupiters”, “super-Earths” between Earth and Neptune in size (a type absent from our Solar System) and planets orbiting two stars. Astronomers can now begin analysing the atmospheres of a few of them, but no life has been detected.
Exploring
| Method | Measures | Bias |
|---|---|---|
| Radial velocity | Minimum mass, period | Massive, close-in planets |
| Transit | Radius, period | Close-in planets, aligned orbits |
| Microlensing | Mass ratio, separation | Planets at larger orbits, one-time events |
| Direct imaging | Light from the planet | Young, massive, wide-orbit planets |
Deep Dive
Method details. Radial velocity gives only mp sin i, the mass projected onto the line of sight, because the orbital inclination i is unknown; for transiting systems i is near 90°, so radial velocity plus transit yields both mass and radius, hence density and a first indication of composition. Transit timing variations can reveal additional planets. Transit spectroscopy, with JWST, measures absorption in a planet's atmosphere as starlight filters through it, and has detected species including water, carbon dioxide and sulfur dioxide on several giant planets.
Population results. Kepler (2009–2018) showed that planets are common: most Sun-like stars host at least one planet, and small planets of 1–4 Earth radii are the most frequent type. A radius “gap” near 1.5–2 Earth radii separates rocky super-Earths from gas-rich sub-Neptunes, probably due to atmospheric loss. Hot Jupiters are rare (around 1% of Sun-like stars) and challenge formation theories, with inward migration the leading explanation. Detection biases mean that the observed census is not the true distribution, and the occurrence of Earth-like planets in habitable zones is estimated, with large uncertainty, from extrapolation.
Open questions. How planets form (core accretion versus gravitational instability), why Solar System architecture seems atypical, and whether any exoplanet shows biosignatures are unsettled. Interpretation of atmospheric spectra requires care because stellar surface inhomogeneities and clouds can mimic or hide features. This Codex records detection claims as claims until independently confirmed (see also Gravitational Lensing for the microlensing method).
Concept Relationship Map
Requires: stellar evolution, gravitational lensing (microlensing), classical mechanics (Kepler orbits)
Enables: comparative planetology, searches for biosignatures
Commonly confused with: habitable-zone planets (not the same as inhabited planets)
Sources
- Mayor, M., Queloz, D. (1995). Nature, 378, 355–359.
- Wolszczan, A., Frail, D.A. (1992). “A planetary system around the millisecond pulsar PSR1257+12.” Nature, 355, 145–147.
- NASA Exoplanet Archive, California Institute of Technology / IPAC (exoplanetarchive.ipac.caltech.edu).
- The Nobel Foundation (2019). The Nobel Prize in Physics 2019: Press release.