The Solar System

The Solar System is the Sun and everything held in orbit by the Sun’s gravity. It includes the eight planets, their moons, dwarf planets, asteroids, and comets.

What it contains

• The Sun: a medium-sized star made mostly of hydrogen and helium. It contains most of the mass of the Solar System, so its gravity keeps everything in orbit.
• Planets (in order from the Sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
• Minor planets: dwarf planets (e.g. Pluto, Ceres) and asteroids (many in the asteroid belt between Mars and Jupiter).
• Moons (natural satellites) that orbit planets.
• Comets: icy bodies with long, elliptical orbits; near the Sun they grow tails that point away from the Sun.

Rocky inner, gaseous outer

The four inner planets (Mercury to Mars) are small, rocky, and dense. The four outer planets (Jupiter to Neptune) are large and mostly gas/ice with many moons and rings.

Accretion model (simple view): Long ago, a rotating cloud of gas and dust collapsed under gravity. Most material formed the Sun. The rest became a flat accretion disc. Close to the hot Sun, only rock and metal could stick together, forming rocky planets. Farther out, colder regions kept ices and gases, forming giant planets.

Orbits and gravity

• Planets, minor planets, and comets follow elliptical paths (ellipses are squashed circles). The Sun sits at one focus, not at the exact centre. Some orbits are almost circular.
• The farther a planet is from the Sun, the slower its average orbital speed and the longer its year.
• In an elliptical orbit, an object moves faster when it is closer to the Sun and slower when it is farther away (energy swaps between gravitational and movement energy).
• Gravitational field strength at a planet’s surface is stronger for more massive planets, and gravity gets weaker as you move farther away.

Average orbital speed can be found with $v = \frac{2\pi r}{T}$, where $r$ is average orbital radius and $T$ is the orbital period.

Common misconceptions

• Orbits are not perfect circles; they are ellipses.
• Pluto is a dwarf planet, not one of the eight main planets.
• A comet’s tail points away from the Sun, not behind its direction of travel.
• There is gravity in space; objects orbit because gravity acts on them.