Solar System

What’s in the solar system?

The solar system consists of the Sun and many smaller objects: the planets and dwarf planets, their moons and rings, and “debris” such as asteroids, comets and dust.^[1] By current counts, there are eight planets, five dwarf planets, hundreds of moons, about 1.4 million asteroids and about 4,000 comets.^[2]

The Sun dominates it all, holding about 99.8% of the solar system’s total mass. Jupiter, the biggest planet, holds only about 0.1%.^[1]

Two families of planets

• The terrestrial planets (Mercury, Venus, Earth and Mars) are relatively small worlds, made mostly of rock and metal, with solid surfaces.^[1]
• The giant planets (Jupiter, Saturn, Uranus and Neptune) are much larger and made mostly of lighter ices, liquids and gases. About 1,300 Earths could fit inside Jupiter.^[1]

All eight planets orbit the Sun in the same direction and in roughly the same plane, “like cars traveling on concentric tracks”.^[1] Why they move the way they do is the subject of Orbits and Gravity.

Beyond Neptune

Past Neptune lie thousands of small icy bodies called trans-Neptunian objects. About 6,000 have been discovered so far.^[1] Among them are dwarf planets such as Pluto, which has five known moons, and Eris, which is about the same size as Pluto.^[1]

Leftovers from the beginning

Most asteroids are remnants of the original population of small bodies that existed before the planets themselves formed.^[1] Studying them is like studying the solar system’s building blocks.

Where are we in the galaxy?

The Sun sits in a small, partial arm of the Milky Way called the Orion Arm, or Orion Spur. The solar system orbits the centre of the galaxy at about 515,000 miles per hour, taking about 230 million years to complete one lap.^[2]