4.1 Dissecting the Universe
Solar System
Our solar system contains the Sun and the eight planets that orbit the Sun. Also in our solar system are more than 60 moons, millions of asteroids, and billions of comets.
A planet is currently defined as a body that orbits the Sun, has a large enough mass to retain a “round” shape, and dominates its neighbourhood. The planets in our solar system are, in order from the Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Until 2006, Pluto was the ninth planet. There was always some debate about Pluto's classification as a planet. In 2006, astronomers demoted Pluto to the class of dwarf planet. The debate continues, as some astronomers don't agree with the decision that was made.
A moon can be defined as a large body that orbits a planet. Asteroids are small rocky bodies that revolve around the sun, and are sometimes called minor planets or planetoids. A comet is a ball of rock and ice that orbits the sun. Comets have a characteristic glowing tail formed when the sun heats the centre of the comet, and vapours are released.