Exoplanets
Planets orbiting stars other than the Sun, now numbering in the thousands and revealing a diversity of worlds far exceeding the Solar System.
Definition
Exoplanets are planets that orbit stars other than the Sun, or in some cases drift freely through space, studied through indirect detection and remote characterization.
Scope
This area covers the detection, characterization, and statistics of planets around other stars. It spans the techniques used to find exoplanets, the measurement of their masses, radii, atmospheres, and orbits, the demographics of the planet population, the architectures of multi-planet systems, and the assessment of which worlds might be habitable. Exoplanet science connects directly to planet-formation theory, providing the population that any formation model must reproduce.
Sub-topics
Core questions
- How do we detect planets around other stars and measure their properties?
- How common are planets, and what types are most abundant in the galaxy?
- How are multi-planet systems arranged, and how do they compare to the Solar System?
- Which exoplanets could host habitable conditions, and how would we tell?
Key theories
- Diversity of planetary systems
- Exoplanet surveys reveal that planetary systems are far more varied than the Solar System, including hot Jupiters, super-Earths, and compact resonant chains, reshaping formation theory.
- Statistical occurrence of planets
- Transit and radial-velocity surveys show that planets, especially small ones, are extremely common, with most stars hosting at least one planet.
Clinical relevance
Exoplanet science places the Solar System in a galactic context, tests theories of planet formation against a vast population of worlds, and underpins the search for life-bearing planets beyond Earth.
History
The first planets around other stars were found around a pulsar in 1992 and, decisively, around a Sun-like star with the 1995 discovery of 51 Pegasi b by Mayor and Queloz, recognized with a Nobel Prize. Space missions such as Kepler and TESS, and ground-based surveys, expanded the catalogue to thousands of planets and enabled population studies and atmospheric characterization.
Debates
- Origin of hot Jupiters
- Whether close-in giant planets arrive by smooth disk migration or by high-eccentricity scattering and tidal circularization is an ongoing debate informed by their orbital alignments.
Key figures
- Michel Mayor
- Didier Queloz
- Sara Seager
- Joshua Winn
Related topics
Seminal works
- mayorqueloz1995
- winnfabrycky2015
- perryman2018
Frequently asked questions
- How many exoplanets have been found?
- Thousands of confirmed exoplanets are known, with many more candidates, and the count keeps rising as surveys continue.
- Can we see exoplanets directly?
- Most are detected indirectly through their effects on their star, but a small number of large, young, widely separated planets have been imaged directly.