THE EARTH IS THE THIRD of the eight planets that orbit the Sun. It is the largest and densest rocky planets, and the only one known to support life. About 70 per cent of the Earth's surface is covered by water, which is not found in liquid form on the surface of any other planet. There are four main layers: the inner core, the outer core, the mantle, and the crust. At the heart of the planet the solid inner core has a temperature of about 6,600 degree cesius. The heat from this inner core causes material in the molten outer core and mantle to circulate in convection currents.
It is thought that these convection currents generate the Earth's magnetic field, which extends into space as the magnetosphere. The Earth' s atmosphere helps screen out some of the harmful radiation from the Sun, stops most meteoroids from reaching the planet's surface, and traps enough heat to prevent extremes of cold. The Earth has one natural satellite, the Moon, which is thought to have formed when a huge asteroid impacted Earth in the distant past.
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