Earth

In this website we will focus down on one tiny planet: the Earth. Notice from our diagram and model of the solar system how small it is compared to the Sun and the giant planets Jupiter and Saturn. If we didn't live here we probably wouldn't even notice it!

Image of Earth and Moon created by Reto Stöckli, Nazmi El Saleous, and Marit Jentoft-Nilsen, NASA GSFC

Yet there is a very good reason why we should look at this planet and no other.

Water is liquid on Earth

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, traveling 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.

Summary of Earth History

The following diagram summarizes some of the main events in the history of the Earth.

Image courtesy of Woudloper/Woodwalker

Note this diagram uses the geological abbreviations Ga and Ma in place of the bya and mya which are used in this website, and that some of the numbers are slightly different from ours.