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Our solar system

Our solar system contains planets, comets and asteroids all of which travel around our star, the Sun. There are nine main planets, one main planetoid and there are currently thought to be 165 moons.

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.

The solar system
Our solar system, approximatley to scale© Calvin J. Hamilton

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?
To see a useful table.

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 elipse. 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 eliptical 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. The closest that the 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. Un-manned spacecraft have flown past them though, and even landed on some of them. In July 1997, the first of a new series of space probes landed on Mars as part of a search for life there. It carried a buggy which went onto the surface of Mars and sent pictures back to Earth. It also analysed samples of different rocks.

Are there any more planets waiting to be discovered?
Outside of our own solar system there are plenty of planets to be discovered. As new galaxies are continuously being created in clouds of swirling gas and dust, new effective Suns are being produced with planets of their own. As with any cycle, formation and destruction are continually taking place and this is the driving force by which the universe is sustained.

Sedna and Quaoar
Courtesy of NASA
As the first months of 2004 approached researchers at Nasa's Californian Institute of Technology discovered a faint glow far away at the boundaries of our solar system. Telescopes in Chile, Spain, Arizona and Hawaii observed the object and soon after NASA's new Spitzer Space Telescope found the object within days. After careful monitoring, it was announced on the 15 March 2004 that a new planet has been discovered named Sedna, after the Inuit goddess of the Ocean. It is the largest body known to orbit the sun at such a distance. With a diameter of 1000 miles, an orbit 900 times greater than that of Earth and an orbital period of 10,500 years it was one of 2004’s highlights. Another object  in a similar orbit with a red indian name, Quaoar, is smaller with a diameter of 900 miles). 

Outside of our own solar system there are plenty of planets to be discovered. As new galaxies are continuously being created in clouds of swirling gas and dust, new effective suns are being produced with planets of their own. As with any cycle, formation and destruction are continually taking place and this is the driving force by which the universe is sustained.

See the section called The furthest object in the solar system.

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 has a diameter of about 800 kilometres.

What are comets?
Comets are a bit like giant dirty ice-balls with diameters between 5–50 kilometres. They have highly eliptical orbits. On one side of their orbit they move in close to the Sun. On the opposite side, they 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 behind 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.

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