The Solar System

A Solar System is a name we give to a star and its family of planets, moons, asteroids, meteors and comets. At the moment, there are twelve known Solar Systems in our galaxy, the Milky Way. This means that there are twelve stars in the Milky Way known to have planets orbiting (spinning around) them. Below is a picture of a galaxy (the Pinwheel Galaxy), full of stars spinning around its central point (known as the Galactic Central Point).

In a galaxy, a star is one of many millions of stars. These stars orbit the central point of the galaxy. In turn, the stars are orbited by planets. These planets are orbited by moons. Also orbiting the stars are comets, in a more eccentric (not circular) orbit than the planets, and asteroids, which orbit stars in a more circular orbit (similar to planets). Asteroids also seem to gather together in belts. In our Solar System, there is an Asteroid Belt between Mars and Jupiter, and one outside Pluto known as the Edgeworth-Kuiper belt. There is also the possibility that another belt of asteroids orbits the Sun between Mercury and the Sun. They can therefore be seen as rockier, heavier, bigger versions of the rings orbiting Saturn (which are believed to be tiny particles of a shattered moon).

The Sun's Solar System, with the Sun at the centre of it, is orbited by nine planets: four Inner Planets and five Outer Planets. These nine planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The Sun is the main source of light and heat for these nine planets, although Pluto, being so far away from the Sun, receives hardly any heat at all. The Sun also appears only as a bright star from Pluto, probably no brighter than Venus appears to people on Earth. The average distance from the Sun to its most distant planet, Pluto, is 5,912,520,000 kilometres although, with the possibility than there are many thousands of asteroids and rocks of ice and iron orbiting the Sun beyond Pluto, the Solar System could be much bigger!

It is the Sun's strong pull of gravity which keeps the planets in orbit around it. If there was no Sun, planets would move in a straight line. The Sun is constantly preventing these planets from going in a straight line by pulling them towards it. The closer a planet is to the Sun, the harder the Sun's gravity pulls on it, and the faster it orbits the Sun. This explains why it takes Mercury, the closest planet to the Sun, only 88 Earth days to orbit it, but it takes Pluto 248 Earth years! Each planet in the Solar System also has a pull of gravity. The strengths of these gravitational fields depend on the planet's size and mass. This is what causes moons or rings to orbit around them. Gravity also affects the weights of things on planets, so a person on Earth may be lighter on another planet and heavier somewhere else. Also, sports would be played very differently on other planets.