Earth  

“The Blue Marble” photograph of Earth, taken from Apollo 17
Designations
Pronunciation	i /ˈɜrθ/
Adjective	earthly, tellurian, telluric, terran, terrestrial.
Orbital characteristics
Epoch J2000.0[note 1]
Aphelion	152,098,232 km 1.01671388 AU[note 2]
Perihelion	147,098,290 km 0.98329134 AU[note 2]
Semi-major axis	149,598,261 km 1.00000261 AU[1]
Eccentricity	0.01671123[1]
Orbital period	365.256363004 days[2] 1.000017421 yr
Average orbital speed	29.78 km/s[3] 107,200 km/h
Mean anomaly	357.51716°[3]
Inclination	7.155° to Sun‘s equator 1.57869°[4] to invariable plane
Longitude of ascending node	348.73936°[3][note 3]
Argument of perihelion	114.20783°[3][note 4]
Satellites	1 natural (The Moon) 8,300+ artificial (as of 1 March 2001 (2001 -03-01)[update])[5]
Physical characteristics
Mean radius	6,371.0 km[6]
Equatorial radius	6,378.1 km[7][8]
Polar radius	6,356.8 km[9]
Flattening	0.0033528[10]
Circumference	40,075.017 km (equatorial)[8] 40,007.86 km (meridional)[11]
Surface area	510,072,000 km2[12][13][note 5]148,940,000 km2 land (29.2 %) 361,132,000 km2 water (70.8 %)
Volume	1.08321×1012 km3[3]
Mass	5.9736×1024 kg[3]
Mean density	5.515 g/cm3[3]
Equatorial surface gravity	9.780327 m/s2[14] 0.99732 g
Escape velocity	11.186 km/s[3]
Sidereal rotation period	0.99726968 d[15] 23h 56m 4.100s
Equatorial rotation velocity	1,674.4 km/h (465.1 m/s)[16]
Axial tilt	23°26’21″.4119[2]
Albedo	0.367 (geometric)[3] 0.306 (Bond)[3]
Surface temp.    Kelvin    Celsius	min mean max 184 K[17] 287.2 K[18] 331 K[19] -89.2 °C 14 °C 57.8 °C
Atmosphere
Surface pressure	101.325 kPa (MSL)
Composition	78.08% nitrogen (N2)[3] 20.95% oxygen (O2) 0.93% argon 0.038% carbon dioxide About 1% water vapor (varies with climate)

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,^[20] or by its Latin name, Terra.^{[note 6]}

Home to millions of species including humans, Earth is currently the only astronomical body where life is known to exist.^[21] The planet formed 4.54 billion years ago, and life appeared on its surface within one billion years.^[22] 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.^[23] The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist during this period. The planet is expected to continue supporting life for at least another 500 million years.^[24][25]

Earth’s outer surface 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 with salt water oceans, the remainder consisting of continents and islands which together have many lakes and other sources of water contributing to the hydrosphere. Liquid water, necessary for all known life, is not known to exist in equilibrium on any other planet’s surface.^{[note 7]} 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 8]} The Earth’s axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet’s surface with a period of one tropical year (365.24 solar days).^[26] 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, as well as the products of the biosphere, contribute resources that are used to support a global human population. These 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 the Earth as the center of the universe, and a modern perspective of the world as an integrated environment that requires stewardship.

Chronology

Scientists have been able to reconstruct detailed information about the planet’s past. The earliest dated Solar System material was formed 4.5672 ± 0.0006 billion years ago,^[27] and by 4.54 billion years ago (within an uncertainty of 1%)^[22] the Earth and the other planets in the Solar System had formed out of the solar nebula—a disk-shaped mass of dust and gas left over from the formation of the Sun. This assembly of the Earth through accretion was thus largely completed within 10–20 million years.^[28] Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed shortly thereafter, 4.53 billion years ago.^[29]

The current consensus model^[30] for the formation of the Moon is the giant impact hypothesis, in which the Moon was created when a Mars-sized object (sometimes called Theia) with about 10% of the Earth’s mass^[31] impacted the Earth in a glancing blow.^[32] In this model, some of this object’s mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to coalesce into the Moon.

Outgassing and volcanic activity produced the primordial atmosphere of the Earth. Condensing water vapor, augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects produced the oceans.^[33] The newly formed Sun was only 70% of its present luminosity, yet evidence shows that the early oceans remained liquid—a contradiction dubbed the faint young Sun paradox. A combination of greenhouse gases and higher levels of solar activity served to raise the Earth’s surface temperature, preventing the oceans from freezing over.^[34] By 3.5 billion years ago, the Earth’s magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.^[35]

Two major models have been proposed for the rate of continental growth:^[36] steady growth to the present-day^[37] and rapid growth early in Earth history.^[38] Current research shows that the second option is most likely, with rapid initial growth of continental crust^[39] followed by a long-term steady continental area.^[40][41][42] On time scales lasting hundreds of millions of years, the surface continually reshaped as continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago (Ma), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 Ma, then finally Pangaea, which broke apart 180 Ma.^[43]

Evolution of life

At present, Earth provides the only example of an environment that has given rise to the evolution of life.^[44] Highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago and half a billion years later the last common ancestor of all life existed.^[45] The development of photosynthesis allowed the Sun’s energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed a layer of ozone (a form of molecular oxygen [O₃]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.^[46] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.^[47]

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 Ma, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed “Snowball Earth“, and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.^[48]

Following the Cambrian explosion, about 535 Ma, there have been five major mass extinctions.^[49] The most recent such event was 65 Ma, when an asteroid impact triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared some small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright.^[50] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of the human race. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,^[51] affecting both the nature and quantity of other life forms.

The present pattern of ice ages began about 40 Ma and then intensified during the Pleistocene about 3 Ma. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last continental glaciation ended 10,000 years ago.^[52]