Structure of the Solar System

The solar system consists of the Sun and all the objects that orbit it, held together by gravitational forces.

Structure

Object type	Description	Examples
Star	Massive body that produces energy by nuclear fusion	The Sun
Planets	Large bodies orbiting the Sun	Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Dwarf planets	Smaller bodies orbiting the Sun that have not cleared their orbital neighbourhood	Pluto
Moons (natural satellites)	Bodies orbiting planets	Earth’s Moon, Jupiter’s Ganymede
Asteroids	Rocky bodies, mostly between Mars and Jupiter (the asteroid belt)	Ceres (also a dwarf planet)
Comets	Icy bodies with highly elliptical orbits	Halley’s Comet

Order of planets from the Sun

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

The first four are rocky (terrestrial) planets: small, dense, solid surfaces. The outer four are gas giants (Jupiter, Saturn) and ice giants (Uranus, Neptune): large, low density, no solid surface.

The Sun

• The Sun is a star: a massive ball of hot gas (plasma) that produces energy by nuclear fusion (hydrogen fusing to helium).
• It contains about 99.8% of the mass of the solar system.
• Its gravitational pull holds the planets in their orbits.

Comets

Comets have highly elliptical orbits. They speed up as they approach the Sun (gravitational attraction increases) and slow down as they move away. Near the Sun, ice and dust vaporise to form a visible tail that always points away from the Sun.

Common errors and how to correct them

Including Pluto as a planet. Pluto is classified as a dwarf planet because it has not cleared its orbital neighbourhood of other objects.

Saying the Sun is “on fire.” The Sun produces energy by nuclear fusion, not combustion. There is no burning in the chemical sense.

How to apply this in an exam

Know the order of planets and the difference between rocky and gas/ice giants. When describing comets, mention the elliptical orbit and the formation of the tail near the Sun.