The Solar System consists of the Sun and everything gravitationally bound to it: eight planets, their moons, dwarf planets, asteroids, comets and interplanetary dust.
The Sun
The Sun contains the overwhelming majority of the mass of the system. It generates energy by nuclear fusion, converting hydrogen into helium in its core, and that energy sustains conditions for life on Earth.
The planets
Inner, rocky planets: Mercury, Venus, Earth and Mars. They are relatively small, dense, and have few or no moons.
Outer, giant planets: Jupiter and Saturn are gas giants composed mainly of hydrogen and helium; Uranus and Neptune are ice giants with larger proportions of water, ammonia and methane. All four have ring systems and many moons.
Smaller bodies
- Asteroid belt — between Mars and Jupiter, remnants that never formed a planet.
- Kuiper belt — icy bodies beyond Neptune, including Pluto.
- Comets — icy bodies that develop a visible tail when heated near the Sun.
Formation
The current model describes a cloud of gas and dust collapsing under gravity about 4.6 billion years ago. Most material formed the Sun; the remainder flattened into a disc where particles collided and gradually accreted into larger bodies.
How we know
Knowledge comes from telescopic observation, spectroscopy, meteorite analysis and direct exploration by spacecraft. Missions to every planet have corrected earlier assumptions repeatedly — a reminder that planetary science advances mainly through measurement rather than theory alone.