A planetary system is a star or other central astronomical object together with the planets, dwarf planets, moons, asteroids, comets, dust, gas and other bodies gravitationally associated with it. The term is broader than the word planetary because a planetary system can contain many kinds of objects besides planets. A planetary system may have one star, two stars or, in some cases, more complicated stellar arrangements.
Planets generally orbit their host star or stars, while moons orbit planets and smaller bodies can follow a wide variety of gravitational trajectories. The architecture of a planetary system describes the arrangement of its planets and smaller objects, including their masses, sizes, orbital distances, orbital periods, eccentricities, inclinations, compositions and resonances. Some known systems have compact groups of planets packed close to their star, whereas others contain widely separated giant planets or highly eccentric orbits. A planetary system is therefore best understood as a gravitationally organised population of bodies rather than simply a star with a collection of planets.
Most planetary systems form from a rotating disc of gas and dust surrounding a young star or stars. This protoplanetary disc develops from material collapsing under gravity within a molecular cloud. Conservation of angular momentum causes the collapsing material to flatten into a rotating structure. Dust grains within the disc collide, stick and grow into larger aggregates, although the detailed physical mechanisms that carry growth from microscopic grains to kilometre scale planetesimals involve several processes.
Once sufficiently large solid bodies form, gravity becomes increasingly important and can drive runaway and oligarchic growth. Protoplanets can collide, merge and accrete material. In regions where temperatures are low enough for volatile substances to condense as ice, there can be much more solid material available, potentially aiding the formation of giant planet cores. Gas giant planets can then acquire massive gaseous envelopes if their cores become sufficiently large while disc gas remains available.
Planetary systems show enormous diversity. Some contain terrestrial planets that are predominantly rocky and metallic, while others contain planets rich in volatile compounds, gas or thick atmospheres. Gas giants are dominated by hydrogen and helium, whereas ice giants contain substantial quantities of heavier volatile materials mixed with hydrogen and helium. Exoplanet discoveries have revealed classes such as super Earths, mini Neptunes, hot Jupiters and tightly packed systems that have no exact counterpart among the major planets of the Solar System.
Planetary systems can also contain planets with highly eccentric orbits, planets orbiting extremely close to their stars, and planets orbiting binary stars. The existence of these diverse architectures showed that planet formation does not inevitably produce a Solar System like arrangement. Instead, migration, gravitational scattering, collisions, disc interactions and the initial distribution of solid material can substantially reshape a system after planets begin forming.
The central star strongly influences the environment of its planetary system through gravity, radiation, magnetic fields and stellar wind. The star determines the main gravitational potential around which planets orbit and provides energy that can drive atmospheric chemistry, weather and biological processes where suitable conditions exist. Stellar luminosity changes as the star evolves, altering the energy received by planets.
Young stars can be particularly active and can emit strong ultraviolet and X ray radiation and powerful stellar winds, potentially affecting developing planetary atmospheres. The region around a star where liquid water could potentially exist on a planetary surface under suitable atmospheric conditions is often called the habitable zone, although this concept is only one factor in habitability and does not guarantee life or even surface liquid water. Atmospheric composition, pressure, clouds, geological activity, planetary rotation and stellar activity all influence actual conditions.
Planetary systems are studied using both direct and indirect observations. Transit photometry detects small reductions in a star's brightness when a planet passes in front of it. Radial velocity measurements detect the gravitational motion that an orbiting planet induces in its star. Direct imaging can reveal some planets when the star's glare can be sufficiently suppressed, especially for young massive planets far from their stars. Microlensing can reveal planets through their effects on gravitationally lensed background stars.
Astrometry measures small changes in a star's apparent position caused by orbiting bodies. Spectroscopy can sometimes reveal atmospheric constituents through transmission or emission spectra. Combining these observations allows astronomers to estimate planetary radius, mass, density, orbital period and sometimes atmospheric composition. The field of exoplanet science has therefore transformed planetary systems from theoretical objects into a large observational population with measurable diversity.
Orbital dynamics provides the framework for understanding how a planetary system behaves over long periods. A planet's orbit is determined primarily by gravity, but gravitational interactions between planets can cause orbital migration, resonances and long term changes in eccentricity and inclination. Mean motion resonance occurs when orbital periods have a simple ratio, allowing repeated gravitational interactions that can either stabilise or destabilise an arrangement depending on the circumstances.
Planetary migration can occur while a planet interacts gravitationally with a protoplanetary disc or with other planets and smaller bodies. Gravitational scattering can eject planets entirely from a system or place them on distant or eccentric orbits. Some systems are dynamically compact and stable, while others may experience collisions or orbital rearrangements over geological timescales. Understanding these processes helps explain why observed planetary systems can differ greatly from the systems predicted by simple models of planet formation.
The smaller bodies of a planetary system are scientifically important because they preserve information about its formation and evolution. Asteroids are generally rocky or metallic small bodies, while comets contain substantial volatile ices mixed with dust and rock. Dwarf planets are bodies massive enough for their gravity to make them approximately rounded but which have not cleared their orbital neighbourhood of other objects.
Dust and debris can form belts and rings, and collisions between larger bodies can continually generate smaller fragments. These populations provide records of the chemical and dynamical conditions present during planet formation. In the Solar System, the asteroid belt, Kuiper Belt and distant Oort Cloud are important examples of reservoirs of smaller bodies. Comparable debris structures have been observed around other stars. Planetary system architecture is therefore determined not only by its planets but by an entire interconnected population of objects.
A planetary system should not be confused with a galaxy, star cluster or planetary subsystem. A galaxy contains billions of stars and their associated systems, while a star cluster contains many gravitationally related stars. A planetary system is normally centred on one star or a small stellar group and occupies a much smaller region of space.
The term solar system is sometimes used informally to mean any planetary system, but astronomically the Solar System specifically refers to the system centred on the Sun. The study of planetary systems combines astronomy, planetary science, geology, atmospheric science, chemistry and physics. Current research seeks to determine how common different system architectures are, how planets acquire their atmospheres and water, how planetary migration operates, and how the properties of host stars influence the formation and long term evolution of planets.
