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Yet there is a very good reason why we should look at this planet and no other. The Earth is the only planet on which water forms a liquid, which is essential for life. The reason has to do with its distance from the Sun. A planet further from the Sun, like Mars, is so cold that water freezes into ice. Closer to the Sun, like Venus, water boils and all the molecules fly apart. Only on the Earth can water form that marvelous substance, liquid water. The Earth, like most of the other planets in the Solar System, has an almost perfectly circular orbit. This is unusual. In most of the other planetary systems studied the planets have oval (elliptical) orbits. If the Earth had an oval orbit, travelling sometimes near to the Sun and sometimes far from it, life could not have evolved on the planet. At times the oceans would have boiled and at times they would have frozen, and life as we know it would have been difficult if not impossible.

Because they were made from a spinning disc, all planets spin like tops and they orbit (go round) the Sun. The Earth spins once a day and orbits once a year. The points which the Earth spins round are called the north and south poles.

Earth is the third planet from the Sun.