Earth (or the 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 four terrestrial planets. It is sometimes referred to as the world, the Blue Planet,[21] or by its Latin name, Terra.[note 6]
Earth formed approximately 4.54 billion years ago by accretion from the solar nebula, and life appeared on its surface within one billion years.[22] The planet is home to millions of species, including humans.[23] 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 solar radiation, permitting life on land.[24] The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist. The planet is expected to continue supporting life for another 500 million to 2.3 billion years.[25][26][27]
Earth's crust is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with solid ice (Antarctic ice sheet) or sea ice (Arctic ice cap). The planet'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 space, especially the Sun and the Moon. At present, Earth orbits the Sun once every 366.26 times it rotates about its own axis, which is equal to 365.26 solar days, or one sidereal year.[note 7] The Earth's axis of rotation is tilted 23.4° away from theperpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).[28]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 3.8 billion and 4.1 billion years ago, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment.
Both the mineral resources of the planet and the products of the biosphere contribute resources that are used to support a global human population.[29] These inhabitants are grouped into about 200 independent sovereign states (193 United Nations recognized sovereign states), which interact through diplomacy, travel, trade, and military action. Human cultures have developed many views of the planet, including personification as adeity, a belief in a flat Earth or in the Earth as the center of the universe, and a modern perspective of the world as an integrated environment that requires stewardship.
The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next 1.1 Gyr (1.1 billion years) and by 40% over the next 3.5 Gyr.[58] Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the loss of the planet's oceans.[59]
The Earth's increasing surface temperature will accelerate the inorganic CO2 cycle, reducing its concentration to levels lethally low for plants (10 ppm for C4 photosynthesis) in approximately500 million[25] to 900 million years. The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years.[60] After another billion years all surface water will have disappeared[26] and the mean global temperature will reach 70 °C[60] (158 °F). The Earth is expected to be effectively habitable for about another 500 million years from that point,[25] although this may be extended up to 2.3 billion years if the nitrogen is removed from the atmosphere.[27] Even if the Sun were eternal and stable, the continued internal cooling of the Earth would result in a loss of much of its CO2 due to reduced volcanism,[61] and 35% of the water in the oceans would descend to the mantle due to reduced steam venting from mid-ocean ridges.[62]
The Sun, as part of its evolution, will become a red giant in about 5 Gyr. Models predict that the Sun will expand out to about 250 times its present radius, roughly 1 AU (150,000,000 km).[58][63] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 km) from the Sun when the star reaches it maximum radius. The planet was therefore initially expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life would have been destroyed by the Sun's increased luminosity (peaking at about 5000 times its present level).[58] A 2008 simulation indicates that Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be vaporized.[63]
| Compound | Formula | Composition | |
|---|---|---|---|
| Continental | Oceanic | ||
| silica | SiO2 | 60.2% | 48.6% |
| alumina | Al2O3 | 15.2% | 16.5% |
| lime | CaO | 5.5% | 12.3% |
| magnesia | MgO | 3.1% | 6.8% |
| iron(II) oxide | FeO | 3.8% | 6.2% |
| sodium oxide | Na2O | 3.0% | 2.6% |
| potassium oxide | K2O | 2.8% | 0.4% |
| iron(III) oxide | Fe2O3 | 2.5% | 2.3% |
| water | H2O | 1.4% | 1.1% |
| carbon dioxide | CO2 | 1.2% | 1.4% |
| titanium dioxide | TiO2 | 0.7% | 1.4% |
| phosphorus pentoxide | P2O5 | 0.2% | 0.3% |
| Total | 99.6% | 99.9% | |
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