The Solar Sysytem

Pluto used to be regarded as a planet but it was recently 'downgraded' to a dwarf planet.

Planets

The planets are held in their orbits by the gravitational pull of the Sun. They are non-luminous objects and are visible because they reflect the light from the Sun. Each planet has its own gravitational field; this is different for each planet and increases with the mass of the planet. 

Jupiter as the most massive planet has the greatest gravitational field strength at 23 N/Kg whilst Mars and Mercury have the smallest at 4 N/Kg. The field strength on Earth is 10 N/Kg.

The gravity that the Sun exerts on each planet also varies with the distance from the Sun. 

Mercury as the closest planet experiences the greatest pull whilst the pull on Neptune is much weaker.

Moons

Moons are natural objects that orbit a planet and their movements are determined by gravitational forces. Like planets. moons are non-luminous and reflect light from the Sun.

The Earth has one moon, orbiting at a distance of approximately 400, 000 km. The Moon has a mass and surface gravity 1/6 that of the Earth. It takes the Moon 29.5 days, (1 lunar month) to orbit the Earth. This is same time that it takes the Moon to spin once on its axis, (remember it takes the Earth 24 hours); because the rotational period is the same as the orbital period, the Moon always has the same part of its surface facing the Earth.

Some planets have many moons and some have none. Mars has two moons and Jupiter has over 60.

Comets

Comets are made of dust and ice, and are approximately 1 - 30 km in diameter. Their orbits are very elongated so at times they are very close to the Sun and at other times they are right out in the outer reaches of the Solar System.

We recognise comets because of their characteristic tail. This tail is caused by the Sun and always streams away from the Sun, no matter in what direction the comet is moving. 

The most famous comet is Halley's Comet which only visits our part of the Solar System every 76 years. Halley's comet is featured in the Bayeux Tapestry, but it was last visible from the Earth in 1986.

Asteroids

Asteroids are minor planets or rocks that orbit the Sun. They are most commonly found in the Asteroid Belt, a region of space between Mars and Jupiter and vary in size from just a few metres to several hundreds of kilometres across.

It is thought that asteroids are the rocky remains of a planet that broke apart or failed to form.