Earth

Earth is a planet in the solar system that revolves around its star, the Sun, in the third innermost orbit. It is the densest and fifth largest of the eight planets of the solar system. It is also the largest of the four terrestrial or rocky planets.

The Earth was formed approximately 4550 million years ago and life emerged about a billion years later. It is home to millions of species, including humans, and is currently the only astronomical body where life is known to exist. The atmosphere and other abiotic conditions have been significantly altered by the planet's biosphere, favoring the proliferation of aerobic organisms, as well as the formation of an ozone layer that together with the Earth's magnetic field blocks harmful solar radiation, thus allowing life on Earth. The Earth's physical properties, geological history and orbit have allowed life to continue to exist. It is estimated that the planet will continue to be able to support life for another 500 million years, since according to current projections, after that time the increasing luminosity of the Sun will eventually cause the extinction of the biosphere.

The Earth's surface or crust is divided into several tectonic plates that slide over magma during periods of several million years. The surface is covered by continents and islands; these have several lakes, rivers and other sources of water, which together with the saltwater oceans that account for about 71 % of the surface constitute the hydrosphere. No other planet is known with this balance of liquid water, which is indispensable for any known type of life. The Earth's poles are mostly covered with solid ice or ice shelves. The interior of the planet is geologically active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid inner core composed of approximately 88% iron.

The Earth interacts gravitationally with other objects in space, especially the Sun and the Moon. Currently, the Earth completes one orbit around the Sun each time it makes 366.26 revolutions on its axis, which is equivalent to 365.26 solar days or one sidereal year. The Earth's axis of rotation is tilted 23.4° with respect to the perpendicular to its orbital plane, which produces seasonal variations on the surface of the planet with a period of one tropical year. The Earth has only one natural satellite, the Moon, which began orbiting the Earth 4530 million years ago; it produces the tides, stabilizes the tilt of the Earth's axis and gradually reduces the planet's rotational speed. Approximately 3800 to 4100 million years ago, during the so-called late heavy bombardment, numerous asteroids impacted the Earth, causing significant changes to most of its surface.

Both the planet's minerals and the products of the biosphere provide resources that are used to sustain the world's human population. Its inhabitants are grouped into some 200 independent sovereign states, which interact through diplomacy, travel, trade and military action. Human cultures have developed many ideas about the planet, including the personification of a deity, belief in a flat Earth or Earth as the center of the universe, and a modern perspective of the world as an integrated environment requiring stewardship.