Our solar system contains planets, comets and asteroids all of which travel around our star, the Sun. Until 2006 nine main planets were listed, but under a new classification there are now only eight main planets and three 'dwarf planets': Pluto, Ceres and the unnamed 2003 UB313. More than 160 moons are known.
The Sun is the centre of our solar system. The mass of the Sun alone is one thousand times the mass of all the components of the solar system.
Why do the planets orbit the Sun?
The planets move like this because of the gravitational pull of the Sun. Without this force, the planets would move off into space.
Which way do the planets go around the Sun?
The answer to this question depends on where in space you are looking from. We normally imagine ourselves looking at our solar system from above the Earth's north pole. When viewed from this position, the planets move in an anticlockwise direction around the Sun. Our moon also orbits the Earth in an anticlockwise direction. If we viewed the planets from a position below the Earth's south pole, they would be seen moving around the Sun in a clockwise direction.
How long do the planets take to complete an orbit of the Sun?
The further away from the Sun a planet is, the longer it takes. The Earth takes one year to orbit the Sun, but Pluto (the furthest planet), takes about 250 times as long. Mercury (the closest planet), goes around the Sun about four times in the time it takes the Earth to go round once.
Does each planet spin on its axis?
Yes, but some spin much faster than others. All except Venus spin in an anticlockwise direction. The moons orbiting the planets spin too, most of them also in an anticlockwise direction.
How far away are the other planets?
Although most people think of the planets as moving in a circle around the Sun, they actually move in a slightly elongated orbit called an ellipse. Because of this, the distance from the Earth to the Sun varies between 147 and 152 million kilometres.
Pluto is about 40 times further from the Sun than we are. It has a highly elliptical orbit, which sometimes brings it closer to the Sun than Neptune. The Earth's distance from the other planets depends where they are in their orbits. For example, the closest that Earth gets to Mars is about 57 million kilometres. However, when Earth and Mars are on opposite sides of the Sun, the distance between them is about 400 million kilometres.
Has anyone ever been to another planet?
Although American astronauts landed on the moon in 1969 and again in the early 1970s, no one has ever visited any of the other planets. Robot spacecraft have flown past all the major planets, and probes have landed on Venus, Mars and Saturn's moon Titan.
Are there any more planets waiting to be discovered?
Within and outside of our solar system there are plenty of planets and planet-like objects still to be discovered. Since the late 1980s more than 1000 Trans-Neptunian Objects (TNOs) have been found beyond the orbit of Neptune. Some of these are large enough to qualify as 'dwarf planets'. In the same period more than 200 planets have been discovered in orbit around other stars.
What are asteroids?
They are rocky lumps of material, sometimes known as the minor planets. Most of the asteroids lie between Mars and Jupiter. The biggest known asteroid (Ceres – classified as a dwarf planet in 2006) has a diameter of about 1000 kilometres.
What are comets?
Comets are a bit like giant dirty snowballs with diameters between 5 and 50 kilometres. Most of them have highly elliptical orbits. On one side of their orbit they move in close to the Sun. On the opposite side, many move out far beyond the orbit of Pluto. As they get closer to the Sun, the icy layers start to melt and vaporize, leaving behind a trail of material which is seen as a tail.
The tail can be millions of kilometres long. One of the most famous comets is Halley's comet, which reappears every 76 years (the time it takes to orbit the Sun). The most recent spectacular comet was Hale-Bopp, which appeared in our skies in early 1997. Its orbit is so large that it won't be seen from the Earth again for more than 2000 years.