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Earth seen from Apollo 17

Earth is the third planet from the Sun and is the largest of the terrestrial planets in the Solar System, in both diameter and mass. It is also referred to as the Earth, Planet Earth, Gaia, Terra,[1] and "the World".

Home to millions of species[2] including humans, Earth is the only place in the universe where life is known to exist. Scientific evidence indicates that the planet formed 4.54 billion years[3][4][5][6] ago, and life appeared on its surface within a billion years. Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful radiation, permitting life on land.

Earth's outer surface 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 the surface is covered 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.[7][8] Earth's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days.[9] The Earth's axis of rotation is tilted 23.4°[10] away from the perpendicular to its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).

Earth's only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation. A cometary bombardment during the early history of the planet played a role in the formation of the oceans.[11] Later, asteroid impacts caused significant changes to the surface environment.

Long term periodic changes in the Earth's orbit, caused by the gravitational influence of other planets, are believed to have given rise to the ice ages that have intermittently covered significant portions of Earth's surface in glacial sheets.


  1. Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. C.f.:
    Blue, Jennifer (July 5, 2007). Descriptor Terms (Feature Types). Gazetteer of Planetary Nomenclature. USGS. Retrieved on 2007-07-05.
  2. May, Robert M. (1999). "How many species are there on earth?". Science 241 (4872): 1441-1449. Retrieved on 2007-08-14.
  3. Dalrymple, G.B. (1991). The Age of the Earth. California: Stanford University Press. ISBN 0-8047-1569-6. 
  4. Newman, William L. (July 9, 2007). Age of the Earth. Publications Services, USGS. Retrieved on 2007-09-20.
  5. Dalrymple, G. Brent (2001). "The age of the Earth in the twentieth century: a problem (mostly) solved". Geological Society, London, Special Publications 190: 205-221. Retrieved on 2007-09-20.
  6. Stassen, Chris (September 10, 2005). The Age of the Earth. The TalkOrigins Archive. Retrieved on 2007-09-20.
  7. Other planets in the solar system are either too hot or too cold to support liquid water. However, it is confirmed to have existed on the surface of Mars in the past, and may still appear today. See: Msnbc (March 2, 2007). "Rover reveals Mars was once wet enough for life". NASA. Retrieved 2007-08-28. Staff (November 7, 2005). "Simulations Show Liquid Water Could Exist on Mars". University of Arkansas. Retrieved 2007-08-08. 
  8. As of 2007, water vapor has been detected in the atmosphere of only one extrasolar planet, and it is a gas giant. See: G. Tinetti et al. (July, 2007). "Water vapour in the atmosphere of a transiting extrasolar planet". Nature 448: 169-171.
  9. The number of solar days is one less than the number of sidereal days because the orbital motion of the Earth about the Sun results in one additional revolution of the planet about its axis.
  10. Ahrens, Global Earth Physics: A Handbook of Physical Constants, p. 8.
  11. Morbidelli, A.; Chambers, J.; Lunine, J. I.; Petit, J. M.; Robert, F.; Valsecchi, G. B.; Cyr, K. E. (2000). "Source regions and time scales for the delivery of water to Earth". Meteoritics & Planetary Science 35 (6): 1309–1320. Retrieved on 2007-03-06.