Of medium size, Earth is the third planet from the Sun. It is unique among the terrestrial planets in having a large natural satellite, a strong magnetic field and, most importantly, life. The circumstances in which life emerged on Earth remain a mystery.

Earth is made up of several layers: a gaseous envelope, consisting mainly of nitrogen and oxygen, which forms its atmosphere; a liquid envelope or hydrosphere of oceans, seas, rivers and glaciers; and a predominant solid component.
This solid part itself divides into 3 layers:

• a rocky crust, which can be more than 20 kilometres thick;
• a mantle of denser rock, about 2,900 kilometres thick;
• a liquid outer core and solid inner core, 3,500 kilometres thick and composed mainly of iron and nickel.

As Earth cools, it solidifies and releases various gases such as water vapour, carbon dioxide and ammonia. Trapped by gravity, these gases go on to form our atmosphere. The liquid oceans covering much of Earth’s surface have earned it its title of the blue planet.

Earth is the largest body in the solar system with a solid surface, and probably the only one with tectonic plate movement, since these two features are linked. Volcanic and seismic activity provide tangible proof of our planet’s constantly shifting crust.

^{Last updated: May 2003}