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Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's fourterrestrial planets.
The average diameter of the reference spheroid is about 12,742 km, which is approximately 40,000 km/π, as the meter was originally defined as1/10,000,000 of the distance from the equator to the North Pole through Paris, France.[61]
Earth is the only place in the universe where life is known to exist. Theplanet formed 4.54 billion years ago,[17] and life appeared on its surface within a billion years
The shape of the Earth is very close to that of an oblate spheroid, asphere flattened along the axis from pole to pole such that there is a bulge around the equator.[59] This bulge results from the rotation of the Earth
Earth's outersurface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of thesurface is covered with salt water oceans, the remainder consisting of continents and islands which together have many lakes and other sources of water contributing tothe hydrosphere.
The Earth's terrain varies greatly from place to place. About 70.8%[88] of the surface is covered by water, with much of the continental shelf belowsea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[60] oceanic trenches,submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.