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124-409: Earth is the only astronomical object known to presently have bodies of liquid water on its surface, although subsurface oceans are suspected to exist on Jupiter's moons Europa and Ganymede and Saturn's moons Enceladus and Titan . Several exoplanets have been found with the right conditions to support liquid water. There are also considerable amounts of subsurface water found on Earth, mostly in

248-540: A circumstellar disk , and then the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, and dust (including primordial nuclides ). According to nebular theory , planetesimals formed by accretion , with the primordial Earth being estimated as likely taking anywhere from 70 to 100 million years to form. Estimates of the age of the Moon range from 4.5 Ga to significantly younger. A leading hypothesis

372-407: A disk and migrated inward are more likely to have abundant water. Conversely, planets that formed close to their host stars are less likely to have water because the primordial disks of gas and dust are thought to have hot and dry inner regions. So if a water world is found close to a star , it would be strong evidence for migration and ex situ formation, because insufficient volatiles exist near

496-420: A warmer version of an ice giant instead, like Uranus and Neptune . Important preliminary theoretical work was carried out prior to the planetary missions launched starting in the 1970s. In particular, Lewis showed in 1971 that radioactive decay alone was likely sufficient to produce subsurface oceans in large moons, especially if ammonia ( NH 3 ) were present. Peale and Cassen figured out in 1979

620-399: A byproduct of photosynthesis by life forms, so although encouraging, O 2 is not a reliable biosignature . In fact, planets with high concentration of O 2 in their atmosphere may be uninhabitable. Abiogenesis in the presence of massive amounts of atmospheric oxygen could be difficult because early organisms relied on the free energy available in redox reactions involving

744-492: A common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common orbit around the Sun, the period of the synodic month , from new moon to new moon, is 29.53 days. Viewed from the celestial north pole , the motion of Earth, the Moon, and their axial rotations are all counterclockwise . Viewed from a vantage point above the Sun and Earth's north poles, Earth orbits in

868-548: A counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.44 degrees from the perpendicular to the Earth–Sun plane (the ecliptic ), and the Earth-Moon plane is tilted up to ±5.1 degrees against the Earth–Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses . The Hill sphere , or

992-483: A deep brittle layer, thermal energy from serpentinization may be the primary cause of hydrothermal activity in small ocean planets. The dynamics of global oceans beneath tidally flexing ice shells represents a significant set of challenges which have barely begun to be explored. The extent to which cryovolcanism occurs is a subject of some debate, as water, being denser than ice by about 8%, has difficulty erupting under normal circumstances. Nevertheless, imaging data from

1116-420: A full rotation about its axis so that the Sun returns to the meridian . The orbital speed of Earth averages about 29.78 km/s (107,200 km/h; 66,600 mph), which is fast enough to travel a distance equal to Earth's diameter, about 12,742 km (7,918 mi), in seven minutes, and the distance from Earth to the Moon, 384,400 km (238,900 mi), in about 3.5 hours. The Moon and Earth orbit

1240-421: A globe-spanning mid-ocean ridge system. At Earth's polar regions , the ocean surface is covered by seasonally variable amounts of sea ice that often connects with polar land, permafrost and ice sheets , forming polar ice caps . Earth's land covers 29.2%, or 149 million km (58 million sq mi) of Earth's surface. The land surface includes many islands around the globe, but most of

1364-615: A larger potential population of hot Neptunes in the Milky Way than was previously thought. Hot Neptunes may have formed either in situ or ex situ . Because of their close proximity to their parent stars , hot Neptunes have a much greater rate and chance of transiting their star as seen from a farther outlying point, than planets of the same mass in larger orbits. This increases the chances of discovering them by transit-based observation methods . Transiting hot Neptunes include Gliese 436 b (Awohali) and HAT-P-11b . Gliese 436 b

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1488-425: A liquid outer core that generates a magnetosphere capable of deflecting most of the destructive solar winds and cosmic radiation . Earth has a dynamic atmosphere , which sustains Earth's surface conditions and protects it from most meteoroids and UV-light at entry . It has a composition of primarily nitrogen and oxygen . Water vapor is widely present in the atmosphere, forming clouds that cover most of

1612-703: A nearby super-Earth exoplanet with potential deep oceans, was discovered by the Transiting Exoplanet Survey Satellite . Planetary objects that form in the outer Solar System begin as a comet -like mixture of roughly half water and half rock by mass, displaying a density lower than that of rocky planets. Icy planets and moons that form near the frost line should contain mostly H 2 O and silicates . Those that form farther out can acquire ammonia ( NH 3 ) and methane ( CH 4 ) as hydrates, together with CO , N 2 , and CO 2 . Planets that form prior to

1736-470: A single type of observation or by theoretical modeling, including Ariel , Titania , Umbriel , Ceres , Dione , Mimas , Miranda , Oberon , Pluto , Triton , Eris , and Makemake . Outside the Solar System, exoplanets that have been described as candidate ocean worlds include GJ 1214 b , Kepler-22b , Kepler-62e , Kepler-62f , and the planets of Kepler-11 and TRAPPIST-1 . More recently,

1860-416: A source of energy, and nutrients, and all three key requirements can potentially be satisfied within some of these bodies, that may offer the possibility for sustaining simple biological activity over geological timescales. In August 2018, researchers reported that water worlds could support life. An ocean world's habitation by Earth-like life is limited if the planet is completely covered by liquid water at

1984-597: A subsurface ocean depends on the rate of internal heating compared with the rate at which heat is removed, and the freezing point of the liquid. Ocean survival and tidal heating are thus intimately linked. Smaller ocean planets would have less dense atmospheres and lower gravity; thus, liquid could evaporate much more easily than on more massive ocean planets. Simulations suggest that planets and satellites of less than one Earth mass could have liquid oceans driven by hydrothermal activity , radiogenic heating , or tidal flexing . Where fluid-rock interactions propagate slowly into

2108-454: A thick atmosphere made mainly of hydrogen. Those planets would have a wide range area around their star where they could orbit and have liquid water. However, those models worked on rather simplistic approaches to the planetary atmosphere. More complex studies showed that hydrogen reacts differently to starlight's wavelengths than heavier elements like nitrogen and oxygen. If such a planet, with an atmospheric pressure 10 to 20 heavier than Earth's,

2232-451: A variety of hydrogen compounds; on an O 2 -rich planet, organisms would have to compete with the oxygen for this free energy. Astrobiology mission concepts to water worlds in the outer Solar System: Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life . This is enabled by Earth being an ocean world , the only one in

2356-434: A water layer sitting atop a silicate core . For a small satellite like Enceladus , an ocean will sit directly above the silicates and below a solid icy shell, but for a larger ice-rich body like Ganymede , pressures are sufficiently high that the ice at depth will transform to higher pressure phases, effectively forming a "water sandwich" with an ocean located between ice shells. An important difference between these two cases

2480-417: A well-defined surface. Even on cooler water-dominated planets, the atmosphere can be much thicker than that of Earth, and composed largely of water vapor, producing a very strong greenhouse effect . Such planets would have to be small enough not to be able to retain a thick envelope of hydrogen and helium, or be close enough to their primary star to be stripped of these light elements. Otherwise, they would form

2604-453: Is rounded into an ellipsoid with a circumference of about 40,000 km. It is the densest planet in the Solar System . Of the four rocky planets , it is the largest and most massive. Earth is about eight light-minutes away from the Sun and orbits it , taking a year (about 365.25 days) to complete one revolution. Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes). Earth's axis of rotation

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2728-456: Is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity . The thickness of the crust varies from about 6 kilometres (3.7 mi) under the oceans to 30–50 km (19–31 mi) for the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as

2852-535: Is approximately 9.8 m/s (32 ft/s ). Local differences in topography, geology, and deeper tectonic structure cause local and broad regional differences in Earth's gravitational field, known as gravity anomalies . The main part of Earth's magnetic field is generated in the core, the site of a dynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The field extends outwards from

2976-529: Is contained in 3.45 billion-year-old Australian rocks showing fossils of microorganisms . During the Neoproterozoic , 1000 to 539 Ma , much of Earth might have been covered in ice. This hypothesis has been termed " Snowball Earth ", and it is of particular interest because it preceded the Cambrian explosion , when multicellular life forms significantly increased in complexity. Following

3100-457: Is farthest out from its center of mass at its equatorial bulge, the summit of the volcano Chimborazo in Ecuador (6,384.4 km or 3,967.1 mi) is its farthest point out. Parallel to the rigid land topography the ocean exhibits a more dynamic topography . To measure the local variation of Earth's topography, geodesy employs an idealized Earth producing a geoid shape. Such a shape

3224-516: Is gained if the ocean is idealized, covering Earth completely and without any perturbations such as tides and winds. The result is a smooth but irregular geoid surface, providing a mean sea level (MSL) as a reference level for topographic measurements. Earth's surface is the boundary between the atmosphere, and the solid Earth and oceans. Defined in this way, it has an area of about 510 million km (197 million sq mi). Earth can be divided into two hemispheres : by latitude into

3348-563: Is likely that exoplanets with oceans are common in the Milky Way galaxy , based on mathematical modeling studies . Ocean worlds are of interest to astrobiologists for their potential to develop life and sustain biological activity over geological timescales. Major moons and dwarf planets in the Solar System thought to harbor subsurface oceans are of interest because they can be reached and studied by space probes , in contrast to exoplanets , which are light-years away, beyond

3472-426: Is liquid under normal atmospheric pressure. Differences in the amount of captured energy between geographic regions (as with the equatorial region receiving more sunlight than the polar regions) drive atmospheric and ocean currents , producing a global climate system with different climate regions , and a range of weather phenomena such as precipitation , allowing components such as nitrogen to cycle . Earth

3596-513: Is now slightly longer than it was during the 19th century due to tidal deceleration , each day varies between 0 and 2 ms longer than the mean solar day. Earth's rotation period relative to the fixed stars , called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.0989 seconds of mean solar time ( UT1 ), or 23 56 4.0989 . Earth's rotation period relative to

3720-622: Is rare, though the alternative spelling Gaia has become common due to the Gaia hypothesis , in which case its pronunciation is / ˈ ɡ aɪ . ə / rather than the more classical English / ˈ ɡ eɪ . ə / . There are a number of adjectives for the planet Earth. The word "earthly" is derived from "Earth". From the Latin Terra comes terran / ˈ t ɛr ə n / , terrestrial / t ə ˈ r ɛ s t r i ə l / , and (via French) terrene / t ə ˈ r iː n / , and from

3844-407: Is that for the small satellite the ocean is in direct contact with the silicates, which may provide hydrothermal and chemical energy and nutrients to simple life forms. Because of the varying pressure at depth, models of a water world may include "steam, liquid, superfluid, high-pressure ices, and plasma phases" of water. Some of the solid-phase water could be in the form of ice VII . Maintaining

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3968-487: Is that it was formed by accretion from material loosed from Earth after a Mars -sized object with about 10% of Earth's mass, named Theia , collided with Earth. It hit Earth with a glancing blow and some of its mass merged with Earth. Between approximately 4.1 and 3.8 Ga , numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of

4092-453: Is the basis for the astronomical unit (AU) and is equal to roughly 8.3 light minutes or 380 times Earth's distance to the Moon . Earth orbits the Sun every 365.2564 mean solar days , or one sidereal year . With an apparent movement of the Sun in Earth's sky at a rate of about 1°/day eastward, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a solar day—for Earth to complete

4216-459: Is the first ultra-hot Neptune discovered with an orbital period of 19 hours and an atmospheric temperature of over 1700 degrees Celsius. Being so close to its star and with a mass around twice that of Neptune, its atmosphere should have evaporated into space so its existence requires an unusual explanation. A candidate planet around Vega slightly more massive than Neptune was detected in 2021. It orbits Vega, an A-class star, every 2.43 days, and with

4340-457: Is tied to that of the Sun. Over the next 1.1 billion years , solar luminosity will increase by 10%, and over the next 3.5 billion years by 40%. Earth's increasing surface temperature will accelerate the inorganic carbon cycle , possibly reducing CO 2 concentration to levels lethally low for current plants ( 10  ppm for C4 photosynthesis ) in approximately 100–900 million years . A lack of vegetation would result in

4464-455: Is tilted with respect to the perpendicular to its orbital plane around the Sun, producing seasons . Earth is orbited by one permanent natural satellite , the Moon , which orbits Earth at 384,400 km (1.28 light seconds) and is roughly a quarter as wide as Earth. The Moon's gravity helps stabilize Earth's axis, causes tides and gradually slows Earth's rotation . Tidal locking has made

4588-648: The Hubble Space Telescope , as well as Pioneer , Galileo , Voyager , Cassini–Huygens , and New Horizons missions, strongly indicate that several outer Solar System bodies harbour internal liquid water oceans under an insulating ice shell. Meanwhile, the Kepler space observatory , launched on March 7, 2009, has discovered thousands of exoplanets, about 50 of them of Earth-size in or near habitable zones . Planets of almost all masses, sizes, and orbits have been detected, illustrating not only

4712-527: The Milky Way and orbits about 28,000  light-years from its center. It is about 20 light-years above the galactic plane in the Orion Arm . The axial tilt of Earth is approximately 23.439281° with the axis of its orbit plane, always pointing towards the Celestial Poles . Due to Earth's axial tilt, the amount of sunlight reaching any given point on the surface varies over the course of

4836-759: The Pacific , North American , Eurasian , African , Antarctic , Indo-Australian , and South American . Other notable plates include the Arabian Plate , the Caribbean Plate , the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between 50 and 55 Ma . The fastest-moving plates are

4960-672: The Solar System sustaining liquid surface water . Almost all of Earth's water is contained in its global ocean, covering 70.8% of Earth's crust . The remaining 29.2% of Earth's crust is land, most of which is located in the form of continental landmasses within Earth's land hemisphere . Most of Earth's land is at least somewhat humid and covered by vegetation , while large sheets of ice at Earth's polar deserts retain more water than Earth's groundwater , lakes, rivers and atmospheric water combined. Earth's crust consists of slowly moving tectonic plates , which interact to produce mountain ranges, volcanoes , and earthquakes. Earth has

5084-512: The Voyager 2 , Cassini-Huygens , Galileo and New Horizons spacecraft revealed cryovolcanic surface features on several of the icy bodies in our own solar system. Recent studies suggest that cryovolcanism may occur on ocean planets that harbor internal oceans beneath layers of surface ice as it does on the icy moons Enceladus and Europa in our own solar system. Liquid water oceans on extrasolar planets could be significantly deeper than

Ocean world - Misplaced Pages Continue

5208-419: The asthenosphere , the solid but less-viscous part of the upper mantle that can flow and move along with the plates. As the tectonic plates migrate, oceanic crust is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes recycles the oceanic crust back into

5332-415: The celestial equator , this is equivalent to an apparent diameter of the Sun or the Moon every two minutes; from Earth's surface, the apparent sizes of the Sun and the Moon are approximately the same. Earth orbits the Sun, making Earth the third-closest planet to the Sun and part of the inner Solar System . Earth's average orbital distance is about 150 million km (93 million mi), which

5456-409: The habitable zone (HZ), possess a protective magnetic field , and have the gravitational pull needed to retain an ample amount of atmospheric pressure . If the planet's gravity cannot sustain that, then all the water will eventually evaporate into outer space. A strong planetary magnetosphere , maintained by internal dynamo action in an electrically conducting fluid layer, is helpful for shielding

5580-408: The ocean floor form the top of Earth's crust , which together with parts of the upper mantle form Earth's lithosphere . Earth's crust may be divided into oceanic and continental crust. Beneath the ocean-floor sediments, the oceanic crust is predominantly basaltic , while the continental crust may include lower density materials such as granite , sediments and metamorphic rocks. Nearly 75% of

5704-430: The precessing or moving mean March equinox (when the Sun is at 90° on the equator), is 86,164.0905 seconds of mean solar time (UT1) (23 56 4.0905 ) . Thus the sidereal day is shorter than the stellar day by about 8.4 ms. Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near

5828-413: The sphere of gravitational influence , of Earth is about 1.5 million km (930,000 mi) in radius. This is the maximum distance at which Earth's gravitational influence is stronger than that of the more distant Sun and planets. Objects must orbit Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun. Earth, along with the Solar System, is situated in

5952-464: The "last ice age", covered large parts of the continents, to the middle latitudes, in ice and ended about 11,700 years ago. Chemical reactions led to the first self-replicating molecules about four billion years ago. A half billion years later, the last common ancestor of all current life arose. The evolution of photosynthesis allowed the Sun's energy to be harvested directly by life forms. The resultant molecular oxygen ( O 2 ) accumulated in

6076-484: The Cambrian explosion, 535 Ma , there have been at least five major mass extinctions and many minor ones. Apart from the proposed current Holocene extinction event, the most recent was 66 Ma , when an asteroid impact triggered the extinction of non-avian dinosaurs and other large reptiles, but largely spared small animals such as insects, mammals , lizards and birds. Mammalian life has diversified over

6200-654: The Earth. Terra is also the name of the planet in some Romance languages , languages that evolved from Latin , like Italian and Portuguese , while in other Romance languages the word gave rise to names with slightly altered spellings, like the Spanish Tierra and the French Terre . The Latinate form Gæa or Gaea ( English: / ˈ dʒ iː . ə / ) of the Greek poetic name Gaia ( Γαῖα ; Ancient Greek : [ɡâi̯.a] or [ɡâj.ja] )

6324-473: The Earth’s ocean, which has an average depth of 3.7 km. Depending on the planet’s gravity and surface conditions, exoplanet oceans could be up to hundreds of times deeper. For example, a planet with a 300 K surface can possess liquid water oceans with depths from 30–500 km, depending on its mass and composition. To allow surface water to be liquid for long periods of time, a planet—or moon—must orbit within

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6448-533: The Latin Tellus comes tellurian / t ɛ ˈ l ʊər i ə n / and telluric . The oldest material found in the Solar System is dated to 4.5682 +0.0002 −0.0004 Ga (billion years) ago. By 4.54 ± 0.04 Ga the primordial Earth had formed. The bodies in the Solar System formed and evolved with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into

6572-758: The Moon always face Earth with the same side. Earth, like most other bodies in the Solar System, formed 4.5 billion years ago from gas and dust in the early Solar System . During the first billion years of Earth's history , the ocean formed and then life developed within it. Life spread globally and has been altering Earth's atmosphere and surface, leading to the Great Oxidation Event two billion years ago. Humans emerged 300,000 years ago in Africa and have spread across every continent on Earth. Humans depend on Earth's biosphere and natural resources for their survival, but have increasingly impacted

6696-426: The Moon and, by inference, to that of Earth. Earth's atmosphere and oceans were formed by volcanic activity and outgassing . Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids, protoplanets , and comets . Sufficient water to fill the oceans may have been on Earth since it formed. In this model, atmospheric greenhouse gases kept the oceans from freezing when

6820-458: The Solar System's planetary-sized objects, Earth is the object with the highest density . Earth's mass is approximately 5.97 × 10   kg ( 5.970  Yg ). It is composed mostly of iron (32.1% by mass ), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%), with the remaining 1.2% consisting of trace amounts of other elements. Due to gravitational separation ,

6944-403: The Sun when the star reaches its maximum radius, otherwise, with tidal effects, it may enter the Sun's atmosphere and be vaporized. Earth has a rounded shape , through hydrostatic equilibrium , with an average diameter of 12,742 kilometres (7,918 mi), making it the fifth largest planetary sized and largest terrestrial object of the Solar System . Due to Earth's rotation it has

7068-425: The atmosphere and due to interaction with ultraviolet solar radiation, formed a protective ozone layer ( O 3 ) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes . True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by

7192-601: The atmospheric composition, including but not limited to the ocean fraction for dissolution of CO 2 and for atmospheric relative humidity, redox state of the planetary surface and interior, acidity levels of the oceans, planetary albedo , and surface gravity. The atmospheric structure, as well as the resulting HZ limits, depend on the density of a planet's atmosphere, shifting the HZ outward for lower mass and inward for higher mass planets. Theory, as well as computer models suggest that atmospheric composition for water planets in

7316-450: The center, the temperature may be up to 6,000 °C (10,830 °F), and the pressure could reach 360  GPa (52 million  psi ). Because much of the heat is provided by radioactive decay, scientists postulate that early in Earth's history, before isotopes with short half-lives were depleted, Earth's heat production was much higher. At approximately 3  Gyr , twice the present-day heat would have been produced, increasing

7440-454: The continental crust , particularly during the early stages of Earth's history. New continental crust forms as a result of plate tectonics , a process ultimately driven by the continuous loss of heat from Earth's interior. Over the period of hundreds of millions of years, tectonic forces have caused areas of continental crust to group together to form supercontinents that have subsequently broken apart. At approximately 750 Ma , one of

7564-616: The continental surfaces are covered by sedimentary rocks, although they form about 5% of the mass of the crust. Earth's surface topography comprises both the topography of the ocean surface , and the shape of Earth's land surface. The submarine terrain of the ocean floor has an average bathymetric depth of 4 km, and is as varied as the terrain above sea level. Earth's surface is continually being shaped by internal plate tectonic processes including earthquakes and volcanism ; by weathering and erosion driven by ice, water, wind and temperature; and by biological processes including

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7688-419: The core are chaotic; the magnetic poles drift and periodically change alignment. This causes secular variation of the main field and field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago. The extent of Earth's magnetic field in space defines the magnetosphere . Ions and electrons of the solar wind are deflected by

7812-527: The core is primarily composed of the denser elements: iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements. The most common rock constituents of the crust are oxides . Over 99% of the crust is composed of various oxides of eleven elements, principally oxides containing silicon (the silicate minerals ), aluminium, iron, calcium, magnesium, potassium, or sodium. The major heat-producing isotopes within Earth are potassium-40 , uranium-238 , and thorium-232 . At

7936-463: The core, through the mantle, and up to Earth's surface, where it is, approximately, a dipole . The poles of the dipole are located close to Earth's geographic poles. At the equator of the magnetic field, the magnetic-field strength at the surface is 3.05 × 10 T , with a magnetic dipole moment of 7.79 × 10 Am at epoch 2000, decreasing nearly 6% per century (although it still remains stronger than its long time average). The convection movements in

8060-539: The days shorter. Above the Arctic Circle and below the Antarctic Circle there is no daylight at all for part of the year, causing a polar night , and this night extends for several months at the poles themselves. These same latitudes also experience a midnight sun , where the sun remains visible all day. By astronomical convention, the four seasons can be determined by the solstices—the points in

8184-448: The dissipation of the gaseous circumstellar disk experience strong torques that can induce rapid inward migration into the habitable zone, especially for planets in the terrestrial mass range. Since water is highly soluble in magma , a large fraction of the planet's water content will initially be trapped in the mantle . As the planet cools and the mantle begins to solidify from the bottom up, large amounts of water (between 60% and 99% of

8308-611: The earliest known supercontinents, Rodinia , began to break apart. The continents later recombined to form Pannotia at 600–540 Ma , then finally Pangaea , which also began to break apart at 180 Ma . The most recent pattern of ice ages began about 40 Ma , and then intensified during the Pleistocene about 3 Ma . High- and middle-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating about every 21,000, 41,000 and 100,000 years. The Last Glacial Period , colloquially called

8432-544: The equally large area of land under permafrost ) or deserts (33%). The pedosphere is the outermost layer of Earth's land surface and is composed of soil and subject to soil formation processes. Soil is crucial for land to be arable. Earth's total arable land is 10.7% of the land surface, with 1.3% being permanent cropland. Earth has an estimated 16.7 million km (6.4 million sq mi) of cropland and 33.5 million km (12.9 million sq mi) of pastureland. The land surface and

8556-407: The erosion of planetary atmospheres; photolysis of water vapor, and hydrogen/oxygen escape to space can lead to the loss of several Earth oceans of water from planets throughout the habitable zone, regardless of whether the escape is energy-limited or diffusion-limited. The amount of water lost seems proportional with the planet mass, since the diffusion-limited hydrogen escape flux is proportional to

8680-474: The exoplanets TOI-1452 b , Kepler-138c , and Kepler-138d have been found to have densities consistent with large fractions of their mass being composed of water. Additionally, models of the massive rocky planet LHS 1140 b suggest its surface may be covered in a deep ocean. Although 70.8% of all Earth 's surface is covered in water, water accounts for only 0.05% of Earth's mass. An extraterrestrial ocean could be so deep and dense that even at high temperatures

8804-413: The form of aquifers . For exoplanets, current technology cannot directly observe liquid surface water, so atmospheric water vapor may be used as a proxy. The characteristics of ocean worlds provide clues to their history and the formation and evolution of the Solar System as a whole. Of additional interest is their potential to originate and host life . In June 2020, NASA scientists reported that it

8928-468: The form of upwelling infrared radiation because the greenhouse gases absorb and re-radiate energy from the host star. Ice-rich planets that have migrated inward into orbit too close to their host stars may develop thick steamy atmospheres but still retain their volatiles for billions of years, even if their atmospheres undergo slow hydrodynamic escape . Ultraviolet photons are not only biologically harmful but can drive fast atmospheric escape that leads to

9052-638: The growth and decomposition of biomass into soil . Earth's mechanically rigid outer layer of Earth's crust and upper mantle , the lithosphere , is divided into tectonic plates . These plates are rigid segments that move relative to each other at one of three boundaries types: at convergent boundaries , two plates come together; at divergent boundaries , two plates are pulled apart; and at transform boundaries , two plates slide past one another laterally. Along these plate boundaries, earthquakes, volcanic activity , mountain-building , and oceanic trench formation can occur. The tectonic plates ride on top of

9176-418: The habitable zone (HZ) are expected to have distinct geophysics and geochemistry of their surface and atmosphere. For example, in the case of exoplanets Kepler-62e and -62f, they could possess a liquid ocean outer surface, a steam atmosphere, or a full cover of surface Ice I , depending on their orbit within the HZ and the magnitude of their greenhouse effect . Several other surface and interior processes affect

9300-642: The habitable zone (HZ) should not differ substantially from those of land-ocean planets. For modeling purposes, it is assumed that the initial composition of icy planetesimals that assemble into water planets is similar to that of comets: mostly water ( H 2 O ), and some ammonia ( NH 3 ), and carbon dioxide ( CO 2 ). An initial composition of ice similar to that of comets leads to an atmospheric model composition of 90% H 2 O , 5% NH 3 , and 5% CO 2 . Atmospheric models for Kepler-62f show that an atmospheric pressure of between 1.6 bar and 5 bar of CO 2 are needed to warm

9424-404: The heat in Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges . The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs under the oceans. The gravity of Earth is the acceleration that is imparted to objects due to the distribution of mass within Earth. Near Earth's surface, gravitational acceleration

9548-464: The important role of tidal heating (aka: tidal flexing) on satellite evolution and structure. The first confirmed detection of an exoplanet was in 1992. Marc Kuchner in 2003 and Alain Léger et al figured in 2004 that a small number of icy planets that form in the region beyond the snow line can migrate inward to ~1 AU , where the outer layers subsequently melt. The cumulative evidence collected by

9672-406: The land surface is taken by the four continental landmasses , which are (in descending order): Africa-Eurasia , America (landmass) , Antarctica , and Australia (landmass) . These landmasses are further broken down and grouped into the continents . The terrain of the land surface varies greatly and consists of mountains, deserts , plains , plateaus , and other landforms . The elevation of

9796-513: The land surface varies from a low point of −418 m (−1,371 ft) at the Dead Sea , to a maximum altitude of 8,848 m (29,029 ft) at the top of Mount Everest . The mean height of land above sea level is about 797 m (2,615 ft). Land can be covered by surface water , snow, ice, artificial structures or vegetation. Most of Earth's land hosts vegetation, but considerable amounts of land are ice sheets (10%, not including

9920-399: The lithosphere, which is divided into independently moving tectonic plates. Beneath the lithosphere is the asthenosphere , a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 km (250 and 410 mi) below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath

10044-410: The loss of oxygen in the atmosphere, making current animal life impossible. Due to the increased luminosity, Earth's mean temperature may reach 100 °C (212 °F) in 1.5 billion years, and all ocean water will evaporate and be lost to space, which may trigger a runaway greenhouse effect , within an estimated 1.6 to 3 billion years. Even if the Sun were stable, a fraction of the water in

10168-456: The lowercase when it is preceded by "the", such as "the atmosphere of the earth". It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?" The name Terra / ˈ t ɛr ə / occasionally is used in scientific writing and especially in science fiction to distinguish humanity's inhabited planet from others, while in poetry Tellus / ˈ t ɛ l ə s / has been used to denote personification of

10292-497: The magnetosphere. During magnetic storms and substorms , charged particles can be deflected from the outer magnetosphere and especially the magnetotail, directed along field lines into Earth's ionosphere , where atmospheric atoms can be excited and ionized, causing an aurora . Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time ( 86,400.0025 SI seconds ). Because Earth's solar day

10416-409: The magnetosphere; solar wind pressure compresses the day-side of the magnetosphere, to about 10 Earth radii, and extends the night-side magnetosphere into a long tail. Because the velocity of the solar wind is greater than the speed at which waves propagate through the solar wind, a supersonic bow shock precedes the day-side magnetosphere within the solar wind. Charged particles are contained within

10540-483: The magnetosphere; the plasmasphere is defined by low-energy particles that essentially follow magnetic field lines as Earth rotates. The ring current is defined by medium-energy particles that drift relative to the geomagnetic field, but with paths that are still dominated by the magnetic field, and the Van Allen radiation belts are formed by high-energy particles whose motion is essentially random, but contained in

10664-417: The mantle, an extremely low viscosity liquid outer core lies above a solid inner core . Earth's inner core may be rotating at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year, although both somewhat higher and much lower rates have also been proposed. The radius of the inner core is about one-fifth of that of Earth. The density increases with depth. Among

10788-612: The mantle. Due to this recycling, most of the ocean floor is less than 100 Ma old. The oldest oceanic crust is located in the Western Pacific and is estimated to be 200 Ma old. By comparison, the oldest dated continental crust is 4,030 Ma , although zircons have been found preserved as clasts within Eoarchean sedimentary rocks that give ages up to 4,400 Ma , indicating that at least some continental crust existed at that time. The seven major plates are

10912-548: The modern oceans will descend to the mantle , due to reduced steam venting from mid-ocean ridges. The Sun will evolve to become a red giant in about 5 billion years . Models predict that the Sun will expand to roughly 1  AU (150 million km; 93 million mi), about 250 times its present radius. Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, Earth will move to an orbit 1.7 AU (250 million km; 160 million mi) from

11036-523: The moment of inertia – if the body is in hydrostatic equilibrium (i.e. behaving like a fluid on long timescales). Proving that a body is in hydrostatic equilibrium is extremely difficult, but by using a combination of shape and gravity data, the hydrostatic contributions can be deduced. Specific techniques to detect inner oceans include magnetic induction , geodesy , librations , axial tilt , tidal response , radar sounding , compositional evidence, and surface features. A generic icy moon will consist of

11160-399: The mother of Thor . Historically, "Earth" has been written in lowercase. Beginning with the use of Early Middle English , its definite sense as "the globe" was expressed as "the earth". By the era of Early Modern English , capitalization of nouns began to prevail , and the earth was also written the Earth , particularly when referenced along with other heavenly bodies. More recently,

11284-424: The name is sometimes simply given as Earth , by analogy with the names of the other planets , though "earth" and forms with "the earth" remain common. House styles now vary: Oxford spelling recognizes the lowercase form as the more common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name, such as a description of the "Earth's atmosphere", but employs

11408-425: The newly forming Sun had only 70% of its current luminosity . By 3.5 Ga , Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind . As the molten outer layer of Earth cooled it formed the first solid crust , which is thought to have been mafic in composition. The first continental crust , which was more felsic in composition, formed by

11532-593: The ocean may have covered Earth completely. The world ocean is commonly divided into the Pacific Ocean, Atlantic Ocean, Indian Ocean, Antarctic or Southern Ocean , and Arctic Ocean, from largest to smallest. The ocean covers Earth's oceanic crust , with the shelf seas covering the shelves of the continental crust to a lesser extent. The oceanic crust forms large oceanic basins with features like abyssal plains , seamounts , submarine volcanoes , oceanic trenches , submarine canyons , oceanic plateaus , and

11656-670: The oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/a (3.0 in/year) and the Pacific Plate moving 52–69 mm/a (2.0–2.7 in/year). At the other extreme, the slowest-moving plate is the South American Plate, progressing at a typical rate of 10.6 mm/a (0.42 in/year). Earth's interior, like that of the other terrestrial planets, is divided into layers by their chemical or physical ( rheological ) properties. The outer layer

11780-501: The oceans by rainwater hitting rocks on exposed land, so the mechanism would not work on an ocean world. Simulations of ocean planets with 50 Earth oceans' worth of water indicate the pressure on the sea floor would be so immense that the planet's interior would not sustain plate tectonics to cause volcanism to provide the right chemical environment for terrestrial life. On the other hand, small bodies such as Europa and Enceladus are regarded as particularly habitable environments because

11904-599: The orbit of maximum axial tilt toward or away from the Sun—and the equinoxes , when Earth's rotational axis is aligned with its orbital axis. In the Northern Hemisphere, winter solstice currently occurs around 21 December; summer solstice is near 21 June, spring equinox is around 20 March and autumnal equinox is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with

12028-649: The ozone layer, life colonized Earth's surface. Among the earliest fossil evidence for life is microbial mat fossils found in 3.48 billion-year-old sandstone in Western Australia , biogenic graphite found in 3.7 billion-year-old metasedimentary rocks in Western Greenland , and remains of biotic material found in 4.1 billion-year-old rocks in Western Australia. The earliest direct evidence of life on Earth

12152-474: The partial melting of this mafic crust. The presence of grains of the mineral zircon of Hadean age in Eoarchean sedimentary rocks suggests that at least some felsic crust existed as early as 4.4 Ga , only 140  Ma after Earth's formation. There are two main models of how this initial small volume of continental crust evolved to reach its current abundance: (1) a relatively steady growth up to

12276-501: The past 66 Mys , and several million years ago, an African ape species gained the ability to stand upright. This facilitated tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which led to the evolution of humans . The development of agriculture , and then civilization , led to humans having an influence on Earth and the nature and quantity of other life forms that continues to this day. Earth's expected long-term future

12400-413: The planet surface gravity. During a runaway greenhouse effect , water vapor reaches the stratosphere, where it is easily broken down ( photolyzed ) by ultraviolet radiation (UV). Heating of the upper atmosphere by UV radiation can then drive a hydrodynamic wind that carries the hydrogen (and potentially some of the oxygen) to space, leading to the irreversible loss of a planet's surface water, oxidation of

12524-478: The planet's environment . Humanity's current impact on Earth's climate and biosphere is unsustainable , threatening the livelihood of humans and many other forms of life, and causing widespread extinctions . The Modern English word Earth developed, via Middle English , from an Old English noun most often spelled eorðe . It has cognates in every Germanic language , and their ancestral root has been reconstructed as * erþō . In its earliest attestation,

12648-400: The planet. The water vapor acts as a greenhouse gas and, together with other greenhouse gases in the atmosphere, particularly carbon dioxide (CO 2 ), creates the conditions for both liquid surface water and water vapor to persist via the capturing of energy from the Sun's light . This process maintains the current average surface temperature of 14.76 °C (58.57 °F), at which water

12772-421: The polar Northern and Southern hemispheres; or by longitude into the continental Eastern and Western hemispheres. Most of Earth's surface is ocean water: 70.8% or 361 million km (139 million sq mi). This vast pool of salty water is often called the world ocean , and makes Earth with its dynamic hydrosphere a water world or ocean world . Indeed, in Earth's early history

12896-495: The present day, which is supported by the radiometric dating of continental crust globally and (2) an initial rapid growth in the volume of continental crust during the Archean , forming the bulk of the continental crust that now exists, which is supported by isotopic evidence from hafnium in zircons and neodymium in sedimentary rocks. The two models and the data that support them can be reconciled by large-scale recycling of

13020-401: The pressure would turn the water into ice. The immense pressures of many thousands of bar in the lower regions of such oceans, could lead to the formation of a mantle of exotic forms of ice such as ice V . This ice would not necessarily be as cold as conventional ice. If the planet is close enough to its star that the water reaches its boiling point, the water will become supercritical and lack

13144-493: The rates of mantle convection and plate tectonics, and allowing the production of uncommon igneous rocks such as komatiites that are rarely formed today. The mean heat loss from Earth is 87 mW m , for a global heat loss of 4.42 × 10  W . A portion of the core's thermal energy is transported toward the crust by mantle plumes , a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts . More of

13268-455: The reach of current technology. The best-established water worlds in the Solar System, other than the Earth , are Callisto , Enceladus , Europa , Ganymede , and Titan . Europa and Enceladus are considered compelling targets for exploration due to their thin outer crusts and cryovolcanic features. Other bodies in the Solar System are considered candidates to host subsurface oceans based upon

13392-522: The shape of an ellipsoid , bulging at its Equator ; its diameter is 43 kilometres (27 mi) longer there than at its poles . Earth's shape also has local topographic variations; the largest local variations, like the Mariana Trench (10,925 metres or 35,843 feet below local sea level), shortens Earth's average radius by 0.17% and Mount Everest (8,848 metres or 29,029 feet above local sea level) lengthens it by 0.14%. Since Earth's surface

13516-440: The star for in situ formation. Simulations of Solar System formation and of extra-solar system formation have shown that planets are likely to migrate inward (i.e., toward the star) as they form. Outward migration may also occur under particular conditions. Inward migration presents the possibility that icy planets could move to orbits where their ice melts into liquid form, turning them into ocean planets. This possibility

13640-450: The summer and winter solstices exchanged and the spring and autumnal equinox dates swapped. Hot Neptune A hot Neptune is a type of giant planet with a mass similar to that of Neptune or Uranus orbiting close to its star , normally within less than 1 AU . The first hot Neptune to be discovered with certainty was Gliese 436 b (Awohali) in 2007 , an exoplanet about 33 light years away. Recent observations have revealed

13764-679: The surface temperature above freezing, leading to a scaled surface pressure of 0.56–1.32 times Earth's. It is suggested that strong ocean currents exist in Enceladus , Titan , Ganymede , and Europa . In Enceladus , oceanic heat flux inferred from ice shell thickness suggests the upwelling of warm water at the poles and downwelling of colder water at low latitudes. Europa is predicted to have an equatorial upwelling of warm water with greater heat transfer at low latitudes. Global scale currents are organized into three zonal and two equatorial circulation cells, convecting internal heat toward

13888-444: The surface, and possible accumulation of oxygen in the atmosphere. The fate of a given planet's atmosphere strongly depends on the extreme ultraviolet flux, the duration of the runaway regime, the initial water content, and the rate at which oxygen is absorbed by the surface. Volatile-rich planets should be more common in the habitable zones of young stars and M-type stars . Scientists have proposed Hycean planets , ocean planets with

14012-436: The surface, especially in equatorial regions. Titan and Ganymede are hypothesized to behave as a non-rotating system and have no coherent heat transfer patterns. The characteristics of ocean worlds or ocean planets provide clues to their history, and the formation and evolution of the Solar System as a whole. Of additional interest is their potential to form and host life . Life as we know it requires liquid water,

14136-411: The surface, even more restricted if a pressurized, solid ice layer is located between the global ocean and the lower rocky mantle . Simulations of a hypothetical ocean world covered by five Earth oceans' worth of water indicate the water would not contain enough phosphorus and other nutrients for Earth-like oxygen-producing ocean organisms such as plankton to evolve. On Earth, phosphorus is washed into

14260-647: The theorized locations of their oceans would almost certainly leave them in direct contact with the underlying silicate core , a potential source of both heat and biologically important chemical elements. The surface geological activity of these bodies may also lead to the transport to the oceans of biologically-important building blocks implanted at the surface, such as organic molecules from comets or tholins , formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane , often in combination with nitrogen. Molecular oxygen ( O 2 ) can be produced by geophysical processes, as well as

14384-423: The total amount in the mantle) are exsolved to form a steam atmosphere, which may eventually condense to form an ocean. Ocean formation requires differentiation , and a heat source, either radioactive decay , tidal heating , or the early luminosity of the parent body. Unfortunately, the initial conditions following accretion are theoretically incomplete. Planets that formed in the outer, water-rich regions of

14508-399: The upper atmosphere from stellar wind mass loss and retaining water over long geological time scales. A planet's atmosphere forms from outgassing during planet formation or is gravitationally captured from the surrounding protoplanetary nebula . The surface temperature on an exoplanet is governed by the atmosphere's greenhouse gases (or lack thereof), so an atmosphere can be detectable in

14632-483: The variable nature of planet formation but also a subsequent migration through the circumstellar disc from the planet's place of origin. As of 24 July 2024, there are 7,026 confirmed exoplanets in 4,949 planetary systems , with 1007 systems having more than one planet . In June 2020, NASA scientists reported that it is likely that exoplanets with oceans may be common in the Milky Way galaxy , based on mathematical modeling studies . In August 2022, TOI-1452 b ,

14756-461: The word eorðe was used to translate the many senses of Latin terra and Greek γῆ gē : the ground, its soil , dry land, the human world, the surface of the world (including the sea), and the globe itself. As with Roman Terra /Tellūs and Greek Gaia , Earth may have been a personified goddess in Germanic paganism : late Norse mythology included Jörð ("Earth"), a giantess often given as

14880-620: The year. This causes the seasonal change in climate, with summer in the Northern Hemisphere occurring when the Tropic of Cancer is facing the Sun, and in the Southern Hemisphere when the Tropic of Capricorn faces the Sun. In each instance, winter occurs simultaneously in the opposite hemisphere. During the summer, the day lasts longer, and the Sun climbs higher in the sky. In winter, the climate becomes cooler and

15004-527: Was carried out in 2004. If these planets formed ex situ, i.e., by migrating to their current locations while growing, they may contain large quantities of frozen volatiles and amorphous ices . Otherwise, if they formed in situ, their inventory of heavy elements should be made entirely of refractory materials . Yet, regardless of the mode of formation, hot Neptunes should contain large fractions (by mass) of gases, primarily hydrogen and helium, which also account for most of their volume. LTT 9779 b (Cuancoá)

15128-430: Was first discussed in the astronomical literature by Marc Kuchner in 2003. The internal structure of an icy astronomical body is generally deduced from measurements of its bulk density, gravity moments, and shape. Determining the moment of inertia of a body can help assess whether it has undergone differentiation (separation into rock-ice layers) or not. Shape or gravity measurements can in some cases be used to infer

15252-491: Was located at 1 astronomical unit (AU) from their star their water bodies would boil. Those studies now place the habitable zone of such worlds at 3.85 AU, and 1.6 AU if it had a similar atmospheric pressure to Earth. There are challenges in examining an exoplanetary surface and its atmosphere, as cloud coverage influences the atmospheric temperature, structure as well as the observability of spectral features . However, planets composed of large quantities of water that reside in

15376-404: Was the first hot Neptune to be discovered with certainty in 2007. The exoplanet Mu Arae c (Dulcinea) discovered in 2004 might also be a hot Neptune, but it has not been determined definitively. Another may be Kepler-56b , which has a mass somewhat larger than Neptune's and orbits its star at 0.1 AU, closer than Mercury orbits the Sun. The first theoretical study of how hot Neptunes could form

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