Earth (or the Earth) is the third
planet from the
Sun, and the fifth-largest of the eight planets in the
Solar System. It is also the largest, most massive, and densest of the Solar System's four
terrestrial planets. It is sometimes referred to as the
World, the Blue Planet, or
Terra.
Home to millions of
species, including humans, Earth is the only place in the
universe where life is known to exist. The planet formed
4.54 billion years 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. The physical properties of the Earth, as well as its geological history and orbit, allowed life to persist during this period. The world is expected to continue supporting life for another 1.5 billion years, after which the rising luminosity of the Sun will eliminate the biosphere.
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. 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.
[note 7] The Earth's axis of rotation is
tilted 23.4° away from the
perpendicular to its
orbital plane,
[15] 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. Between approximately 4.1 and 3.8 billion years ago,
asteroid impacts during the
Late Heavy Bombardment caused significant changes to the surface environment.
Both the
mineral resources of the planet, as well as the products of the biosphere, contribute resources that are used to support a global human population. The inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade and military action. Human cultures have developed many views of the planet, including personification as a deity, a belief in a
flat Earth or in
Earth being the center of the universe, and a modern perspective of the world as an integrated environment that requires stewardship.
