Definition · Plain-language
Solar System
The Solar System is the Sun together with all the objects that orbit it, bound by its gravity.
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The Sun and everything orbiting it
The Solar System is our cosmic neighbourhood: the Sun and all the bodies held in orbit around it by its gravity. At its heart, the Sun is an ordinary star, but it is so massive that it contains the overwhelming majority of all the matter in the system, and its gravity governs the motion of everything else. The eight planets, their moons, the dwarf planets, and vast numbers of asteroids and comets all circle the Sun. The whole system, in turn, is just one small part of the much larger Milky Way galaxy.
What the Solar System contains
Beyond the Sun, the Solar System holds a varied family of objects. The eight planets, in order from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune — the inner four rocky, the outer four giant. Many planets are circled by moons. Beyond and among them lie the dwarf planets, such as Pluto and Ceres, and enormous numbers of smaller bodies: rocky asteroids, mostly in the belt between Mars and Jupiter, and icy comets from the cold outer regions. The Kuiper Belt and the distant Oort Cloud mark the system’s frozen frontier.
How it formed
The Solar System formed about four and a half billion years ago from a giant cloud of gas and dust. Under gravity, the cloud collapsed and most of its material gathered at the centre to form the Sun, while the rest flattened into a spinning disc. Within that disc, dust grains stuck together and grew, step by step, into the planets, moons and smaller bodies. This shared origin explains why the planets orbit the Sun in the same direction and roughly the same flat plane, and why the rocky planets formed near the warm Sun while the icy giants formed in the cold outer reaches.
Key facts
At a glance
- Definition: the Sun and all the objects bound to it by gravity
- Contains: eight planets, their moons, dwarf planets, asteroids and comets
- Centre: the Sun, holding almost all the system’s mass
- Order of planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
- Formed: about 4.5 billion years ago from a cloud of gas and dust
- Location: within the Milky Way galaxy
Common misconceptions
What people often get wrong
Often heard: The Solar System and the galaxy are the same thing.
Actually: The Solar System is one star (the Sun) and what orbits it. The Milky Way galaxy contains hundreds of billions of stars, of which the Sun is just one — vastly larger.
Often heard: The planets make up most of the Solar System’s mass.
Actually: The Sun holds the overwhelming majority of all the mass in the Solar System. Everything else — all the planets, moons and smaller bodies — adds up to only a tiny fraction by comparison.
Often heard: The Solar System contains only the Sun and the eight planets.
Actually: It also includes moons, dwarf planets like Pluto and Ceres, and vast numbers of asteroids and comets, out to the distant Kuiper Belt and Oort Cloud.
Going deeper
