Monday, February 25, 2008


Earth (pronounced /'???/) is the third planet from the Sun and is the major of the terrestrial planets in the Solar System, in both diameter and mass. It is also referred to as the Earth, Planet Earth, and the World, and in several contexts, Gaia and Terra.

Home to millions of species including humans, Earth is the only place in the world where life is known to exist. Scientific evidence indicates that the planet formed 4.54 billion years ago, and life appear on its surface within a billion years. Since then, Earth's biosphere has considerably altered the atmosphere and other abiotic conditions on the planet, enable the proliferation of aerobic organisms as well as the formation of the ozone layer which, jointly with Earth's magnetic field, blocks harmful emission, permitting life on land.

Earth's outer surface is divided into several rigid segments, or tectonic plates, that regularly travel across the surface over periods of many millions of years. About 71% of the surface is enclosed with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet's surface. Earth's interior remains active, with a thick layer of comparatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inside the core.

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