Solar System

The Solar System is the gravitationally bound planetary system centred on the Sun and comprising all objects that orbit it, from the major planets to smaller bodies such as dwarf planets, asteroids, comets, and interplanetary dust. It formed approximately 4.568 billion years ago from the collapse of a dense region within a molecular cloud, giving rise to the Sun and a rotating protoplanetary disc from which the planets accreted. Although scientific conventions vary in the capitalisation of the term, the International Astronomical Union (IAU) capitalises the names of individual astronomical systems while many dictionaries use lower case.

Structure and Major Components

The Sun, a G-type main-sequence star, dominates the system by contributing more than 99.86 per cent of its total mass. Its stability is maintained by hydrostatic equilibrium as hydrogen is fused into helium in its core. Orbiting the Sun are eight major planets: the inner terrestrial planets—Mercury, Venus, Earth, and Mars—with solid surfaces and relatively low mass; and the giant planets—Jupiter and Saturn, classed as gas giants, and Uranus and Neptune, classed as ice giants, composed largely of volatile compounds and lacking definite surfaces.
Beyond the major planets lie abundant smaller bodies. These include asteroids in the region between Mars and Jupiter, collectively forming the asteroid belt, and trans-Neptunian objects (TNOs) in the Kuiper belt. Further out still, the distant Oort cloud is hypothesised to extend to roughly 100,000 astronomical units, serving as the reservoir for long-period comets. The Solar System is enveloped by the heliosphere, a vast bubble of solar wind, whose boundary—the heliopause—is situated approximately 75–90 astronomical units from the Sun.
Six planets, at least seven recognised dwarf planets, and numerous small bodies possess natural satellites. Jupiter and Saturn contain nearly 90 per cent of the mass of all objects orbiting the Sun aside from the Sun itself.

Dwarf Planet Classification and Candidates

Under IAU definitions, dwarf planets are celestial bodies orbiting the Sun that are massive enough for their self-gravity to shape them into a near-spherical form but have not cleared their orbital neighbourhood. The IAU recognises several dwarf planets, including Pluto, Eris, Haumea, Makemake, and Ceres, with others under consideration as research progresses.
There is wide agreement that the Solar System hosts at least nine dwarf planets, though the exact number depends on observational data and interpretation of shape and mass. Many candidate dwarf planets lie in the Kuiper belt or beyond, where faintness and great distance complicate direct measurement. Improved telescopes and surveys continue to refine the list of possible dwarf planets.

Formation and Evolution

The formation of the Solar System began when a region of a molecular cloud collapsed under gravity. This region was composed mostly of hydrogen and helium with traces of heavier elements produced by earlier generations of stars. As the nebula contracted, conservation of angular momentum caused it to flatten into a disc. Material at the hot, dense centre formed the protostar that became the Sun, while solid particles in the disc grew by accretion.
In the inner Solar System, where temperatures were high, only refractory materials such as metals and silicates could condense, forming the terrestrial planets. These materials constituted only a small proportion of the original nebula, limiting the size of the inner planets. Beyond the frost line, where temperatures were low enough for ices to remain solid, volatile compounds were abundant. This allowed the outer planets to accumulate far more mass and eventually capture thick hydrogen- and helium-rich atmospheres.
Leftover debris that never coalesced into planets became small Solar System bodies. Regions of concentration include the asteroid belt, the Kuiper belt, and the distant Oort cloud.
Within about 50 million years of its formation, pressures and temperatures in the Sun’s core rose sufficiently for hydrogen fusion to commence. The young Sun expelled remaining gas from the disc via its solar wind, clearing the environment for the planets to settle into stable orbits.
Models of Solar System evolution suggest that the giant planets migrated from their original positions. According to the Nice model, gravitational interactions between forming planets and planetesimals caused shifts in planetary orbits, contributing to the Late Heavy Bombardment. The “grand tack” hypothesis proposes a temporary inward migration of Jupiter, which may have influenced the distribution of asteroids and the shaping of the inner Solar System.

Present Stability and Future Changes

The Solar System today is in a stable arrangement of gravitationally bound orbits. Although generally long-lived, the system exhibits chaotic characteristics over very long periods due to the complex interactions between planets. Encounters with passing stars, while statistically rare, could potentially perturb orbits.
The Sun will remain in its main-sequence phase for roughly another five billion years. After core hydrogen is exhausted, the Sun will expand into a red giant, increasing to around 260 times its present diameter. Mercury and Venus are expected to be engulfed, and conditions on Earth and Mars will become uninhabitable. Eventually, the Sun will fuse helium for a comparatively short period before shedding its outer layers and leaving behind a dense white dwarf. These ejected layers may form a planetary nebula, returning enriched material to the interstellar medium.

Regional Divisions and Composition

Astronomers often distinguish an inner and outer Solar System. The inner region comprises the terrestrial planets and the asteroid belt. The outer region contains the giant planets and the Kuiper belt. Beyond Neptune, the distribution of trans-Neptunian objects forms a distinct realm of icy bodies, including several dwarf planets.
The mass distribution within the system is highly skewed. The giant planets contain most of the mass not in the Sun, with Jupiter alone representing the bulk. All remaining moons, asteroids, comets, and dwarf planets account for a very small portion of the overall mass.
The Solar System continues to evolve through minor gravitational interactions, collisional processes, and the gradual ageing of the Sun. Despite this, its structure has remained recognisably consistent for billions of years and is expected to do so until major stellar changes occur deep in the future.