multipleplanet

Multipleplanet is a term used to describe a planetary system that hosts more than one planet orbiting a host star, or, in some cases, planets orbiting common circumbinary or hierarchical stars. While many exoplanet discoveries focus on single planets, the majority of confirmed exoplanet host stars appear to have multiple planets. The Solar System is a canonical example of a multi-planet system.

These systems vary widely in planet size, mass, and orbital spacing. Many show compact configurations with several planets within a fraction of an astronomical unit, and some exhibit orbital resonances where orbital periods are near simple integer ratios, indicating past migration and gravitational interaction. Studies of multi-planet systems inform theories of planet formation, migration, and long-term dynamical stability.

Observationally, multiple-planet systems are identified through transit timing variations in transit surveys, simultaneous transits, radial velocity measurements revealing multiple signals, or, less commonly, direct imaging and microlensing. Large-scale surveys such as Kepler and TESS have substantially increased the known number of multi-planet systems. Data from the NASA Exoplanet Archive and the Extrasolar Planets Encyclopaedia track multiplicity and architecture.

Notable multi-planet systems include the TRAPPIST-1 system with seven planets; Kepler-90 with eight planets; Kepler-11 with six planets in a compact arrangement; and the PSR B1257+12 pulsar system, which hosts several planets. The Solar System itself is a multi-planet arrangement with eight planets and several dwarf planets.

Current research focuses on understanding formation pathways, orbital resonances, and the distribution of planetary spacing to test models of disk evolution and migration. Studying multi-planet systems also helps identify potentially habitable worlds within dynamically stable zones, though habitability depends on many factors beyond multiplicity.