The Solar System is our immediate family in space – the Sun and the planets and other natural objects that circle it, such as comets and asteroids. Relatively speaking, they can all be said to be on our cosmic doorstep.
We now know that the planets and the Sun itself all condensed from a spinning cloud of dust and gas more than four and a half billion years ago.
In recent years, space telescopes have revealed that, far from being unique, our solar system is probably a pretty common phenomenon. More than 200 planets have already been detected around other stars.
Heat-seeking telescopes, observing in the infrared part of the spectrum, have even spotted the disks of dust from which planets are born around others.
There are eight main planets in our own solar system – one fewer than there used to be. That is not because any body has vanished. Instead it is because Pluto, which used to be the ninth planet, Pluto, was relegated to a second division of worlds, called dwarf planets, in 2006.
The planets were named after Roman gods by early astronomers who knew of the existence of all of them as far out as Saturn. The tradition was kept as further planets, too faint to have been known about by the ancients, were discovered.
To the unaided eye, planets appear as no more than bright points of light. But if you watch them carefully, you will notice that they do not twinkle like the stars. This is because they have a size larger than a point source, even if that is not immediately obvious.
Earth is part of the Solar System. It is the third rock from the Sun, as the TV comedy would have it. Between us and our home star are Mercury and Venus. They are known as Inferior Planets – not because there is anything wrong with them but because of their positions inside our own orbit. Both are rocky worlds like the Earth.
Because they lie between us and the Sun, the Inferior Planets show phases like the Moon. When further away from us than the Sun, they can look disk-like, but they show as crescents when between us and the Sun. The point when Mercury or Venus lies between us and the Sun is called Inferior Conjunction and the moment when each lies at its furthest position beyond the Sun is called Superior Conjunction.
Beyond the Earth lie five major worlds, called Mars, Jupiter, Saturn, Uranus and Neptune. They are known as Superior Planets, and once again, this is because they lie outside our own orbit, not because they are any more important than us. Of the five worlds, Mars is solid and rocky. The rest are giant balls of gas with no solid surface.
When planets reach the far side of the Sun with respect to the Earth, they are said to be in Conjunction. They reach a point called Opposition when the Earth lies between them and the Sun. Because they lie beyond the Earth, they can never show a full range of phases like the Moon or inner planets.
Planets and other bodies such as comets are said to be at perihelion when at their closest to the Sun and at aphelion when furthest away.