Artist's conception of a protoplanetary disc

The formation and evolution of the Solar System is estimated to have begun 4. A protoplanetary disk (or proplyd) is a rotating Circumstellar disk of dense gas surrounding a young newly formed star a T Tauri star or Herbig star The Solar System consists of the Sun and those celestial objects bound to it by Gravity. 6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Gravitation is a natural Phenomenon by which objects with Mass attract one another See also Solar nebula A molecular cloud, sometimes called a stellar nursery if Star formation is occurring within is a type of Interstellar Most of the collapsing mass collected in the centre, forming the Sun, while the rest flattened into a protoplanetary disc out of which the planets, moons, asteroids, and other small Solar System bodies formed. The Sun (Sol is the Star at the center of the Solar System. A protoplanetary disk (or proplyd) is a rotating Circumstellar disk of dense gas surrounding a young newly formed star a T Tauri star or Herbig star A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but Small Solar System Body ( SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that

This widely accepted model, known as the nebular hypothesis, was first developed in the 18th century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace. In Cosmogony, the nebular hypothesis is the most widely accepted model explaining the Formation and evolution of the Solar System. (born Emanuel Swedberg; February 8 1688–March 29 1772 was a Swedish Scientist, Philosopher, Christian mystic, and Theologian Immanuel Kant (ɪmanuəl kant 22 April 1724 12 February 1804 was an 18th-century German Philosopher from the Prussian city of Königsberg Its subsequent development has interwoven a variety of scientific disciplines including astronomy, physics, geology, and planetary science. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit Planetary science, also known as planetology and closely related to planetary astronomy, is the Science of Planets or Planetary systems Since the dawn of the space age in the 1950s and the discovery of extrasolar planets in the 1990s, the models have been both challenged and refined to account for new observations. The Space Age is a contemporary period encompassing the activities related to the Space Race, Space exploration, space technology and the cultural developments An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312

The Solar System has evolved considerably since its initial formation. Many moons have formed from circling discs of gas and dust around their parent planets, while other moons are believed to have been bodies captured by their planets or, as in the case of the Earth's Moon, to have resulted from giant collisions. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Collisions between bodies have occurred continually up to the present day and have been central to the evolution of the solar system. The positions of the planets often shifted, and planets have switched places. ^[1] This planetary migration now is believed to have been responsible for much of the Solar System's early evolution. Planetary migration occurs when a Planet or other stellar satellite interacts with a disk of gas or Planetesimals resulting in the alteration of the satellite's orbital

In roughly 5 billion years, the Sun will cool and expand outward to many times its current diameter (becoming a red giant), before casting off its outer layers as a planetary nebula, and leaving behind a stellar corpse known as a white dwarf. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 A planetary nebula is an Emission nebula consisting of a glowing shell of Gas and plasma formed by certain types of Stars when they die A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. In the far distant future, the gravity of passing stars gradually will whittle away at the Sun's retinue of planets. Some planets will be destroyed, others ejected into interstellar space. Ultimately, over the course of trillions of years, it is likely that the Sun will be left alone with no bodies in orbit around it. ^[2]

History

Main article: History of Solar System formation and evolution hypotheses

Pierre-Simon Laplace, one of the originators of the nebular hypothesis

Ideas concerning the origin and fate of the world date from the earliest known writings; however, for almost all of that time, there was no attempt to link such theories to the existence of a "Solar System", simply because it was not generally known that the Solar System, in the sense we now understand it, existed. "Capture theory" redirects here For the solar capture theory see this article's section on Solar System formation The first step toward a theory of Solar System formation and evolution was the general acceptance of heliocentrism, the model which placed the Sun at the centre of the system and the Earth in orbit around it. In Astronomy, heliocentrism is the theory that the Sun is at the center of the Solar System. This conception had been gestating for millennia, but was widely accepted only by the end of the 17th century. The first recorded use of the term "Solar System" dates from 1704. ^[3]

The current standard theory for Solar System formation, the nebular hypothesis, has fallen into and out of favour since its formulation by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace in the 18th century. In Cosmogony, the nebular hypothesis is the most widely accepted model explaining the Formation and evolution of the Solar System. (born Emanuel Swedberg; February 8 1688–March 29 1772 was a Swedish Scientist, Philosopher, Christian mystic, and Theologian Immanuel Kant (ɪmanuəl kant 22 April 1724 12 February 1804 was an 18th-century German Philosopher from the Prussian city of Königsberg The most significant criticism of the hypothesis was its apparent inability to explain the Sun's relative lack of angular momentum when compared to the planets. In Physics, the angular momentum of a particle about an origin is a vector quantity equal to the mass of the particle multiplied by the Cross product of the position ^[4] However, since the early 1980s studies of young stars have shown them to be surrounded by cool discs of dust and gas, exactly as the nebular hypothesis predicts, which has led to its re-acceptance. ^[5]

Understanding of how the Sun will continue to evolve required an understanding of the source of its power. Arthur Stanley Eddington's confirmation of Albert Einstein's theory of relativity led to his realisation that the Sun's energy comes from nuclear fusion reactions in its core. Sir Arthur Stanley Eddington, OM (28 December 1882 – 22 November 1944 was an English Astrophysicist of the early 20th century Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical This page is about the scientific concept of relativity for philosophical or sociological theories about relativity see Relativism. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus ^[6] In 1935, Eddington went further and suggested that other elements also might form within stars. ^[7] Fred Hoyle elaborated on this premise by arguing that evolved stars called red giants created many elements heavier than hydrogen and helium in their cores. Sir Fred Hoyle FRS ( 24 June, 1915  &ndash 20 August, 2001) was an English Astronomer primarily A red giant is a luminous Giant star of low or intermediate mass (roughly 0 When a red giant finally casts off its outer layers, these elements would then be recycled to form other star systems. ^[7]

Formation

See also: Nebular hypothesis

Pre-solar nebula

The nebular hypothesis maintains that the Solar System formed from the gravitational collapse of a fragment of a giant molecular cloud which likely was several light-years across. In Cosmogony, the nebular hypothesis is the most widely accepted model explaining the Formation and evolution of the Solar System. See also Solar nebula A molecular cloud, sometimes called a stellar nursery if Star formation is occurring within is a type of Interstellar A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by ^[8] Until a few decades ago, the conventional view was that the Sun formed in relative isolation, but studies of ancient meteorites reveal traces of short-lived isotopes such as iron-60 which only form in exploding, short-lived stars. A meteorite is a natural object originating in Outer space that survives an impact with the Earth 's surface Naturally occurring Iron ( Fe) consists of four Isotopes 5845% of radioactive 54Fe (half-life >3 This indicates that a number of supernovae occurred near the Sun while it was forming. A supernova (plural supernovae or supernovas) is a stellar Explosion. A shock wave from one of these supernovae may have triggered the formation of the Sun by creating regions of over-density within the cloud, causing these regions to collapse. For the music album by Converter see Shock Front For the 1977 horror film see Shock Waves A shock wave (also called Because only massive, short-lived stars produce supernovae, the Sun must have formed in a large star-forming region which produced massive stars, possibly similar to the Orion nebula. The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a Diffuse nebula situated south of Orion 's Belt ^[9][10]

Hubble image of protoplanetary discs in the Orion nebula, a light-years-wide "stellar nursery" likely very similar to the primordial nebula from which our Sun formed

One of these regions of collapsing gas (known as the pre-solar nebula)^[11] would form what became the Solar System. The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a Diffuse nebula situated south of Orion 's Belt This region had a diameter of between 7000 and 20,000 astronomical units (AU)^[8][12][13] and a mass just over that of the Sun. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the Its composition was about the same as that of the Sun today. Hydrogen, along with helium and trace amounts of lithium produced by Big Bang nucleosynthesis, formed about 98% of the mass of the collapsing cloud. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Lithium (ˈlɪθiəm is a Chemical element with the symbol Li and Atomic number 3 In Physical cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than those of H-1 (i The remaining 2% of the mass consisted of heavier elements that were created by nucleosynthesis in earlier generations of stars. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen Nucleosynthesis is the process of creating new atomic nuclei from preexisting Nucleons (protons and neutrons ^[14] Late in the life of these stars, they ejected heavier elements into the interstellar medium. ^[15]

Because of the conservation of angular momentum, the nebula spun faster as it collapsed. In Physics, the angular momentum of a particle about an origin is a vector quantity equal to the mass of the particle multiplied by the Cross product of the position As the material within the nebula condensed, the atoms within it began to collide with increasing frequency, converting their kinetic energy into heat. The kinetic energy of an object is the extra Energy which it possesses due to its motion In Physics, heat, symbolized by Q, is Energy transferred from one body or system to another due to a difference in Temperature The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. ^[8] Over about 100,000 years,^[16] the competing forces of gravity, gas pressure, magnetic fields, and rotation caused the contracting nebula to flatten into a spinning protoplanetary disc with a diameter of ~200 AU^[8] and form a hot, dense protostar (a star in which hydrogen fusion has not yet begun) at the centre. A protoplanetary disk (or proplyd) is a rotating Circumstellar disk of dense gas surrounding a young newly formed star a T Tauri star or Herbig star A protostar is a large object that forms by contraction out of the gas of a Giant molecular cloud in the Interstellar medium. ^[17]

At this point in its evolution, the Sun is believed to have been a T Tauri star. Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime T Tauri stars ( TTS) are a class of variable Stars named after their prototype – T Tauri. Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of 0. 001–0. 1 solar masses. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. ^[18] These discs extend to several hundred AU—the Hubble Space Telescope has observed protoplanetary discs of up to 1000 AU in diameter in star-forming regions such as the Orion Nebula^[19]—and are rather cool, reaching only a thousand Kelvin at their hottest. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into Star Formation is the process by which dense parts of Molecular clouds collapse into a ball of plasma to form a Star. The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a Diffuse nebula situated south of Orion 's Belt The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic ^[20] Within 50 million years, the temperature and pressure at the core of the Sun became so great that its hydrogen began to fuse, creating an internal source of energy which countered the force of gravitational contraction until hydrostatic equilibrium was achieved. Hydrostatic equilibrium occurs when compression due to Gravity is balanced by a Pressure gradient which creates a Pressure gradient force in the opposite ^[21] This marked the Sun's entry into the prime phase of its life, known as the main sequence. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness Main sequence stars are those which derive their energy from the fusion of hydrogen into helium in their cores. The Sun remains a main sequence star today. ^[22]

Formation of planets

See also: Protoplanetary disc

Artist's conception of the solar nebula

The various planets are thought to have formed from the solar nebula, the disc-shaped cloud of gas and dust left over from the Sun's formation. A protoplanetary disk (or proplyd) is a rotating Circumstellar disk of dense gas surrounding a young newly formed star a T Tauri star or Herbig star ^[23] The currently accepted method by which the planets formed is known as accretion, in which the planets began as dust grains in orbit around the central protostar. In Astrophysics, the term accretion is used for at least two distinct processes Through direct contact, these grains formed into clumps between one and ten kilometres (km) in diameter, which in turn collided to form larger bodies (planetesimals) of ~5 km in size. The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal These gradually increased through further collisions, growing at the rate of centimetres per year over the course of the next few million years. ^[24]

The inner Solar System, the region of the Solar System inside 4 AU, was too warm for volatile molecules like water and methane to condense, so the planetesimals which formed there could only form from compounds with high melting points, such as metals (like iron, nickel, and aluminium) and rocky silicates. The Solar System consists of the Sun and those celestial objects bound to it by Gravity. Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Nickel (ˈnɪkəl is a metallic Chemical element with the symbol Ni and Atomic number 28 WikipediaNaming For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI These rocky bodies would become the terrestrial planets (Mercury, Venus, Earth, and Mars). A terrestrial planet, telluric planet or rocky planet is a Planet that is primarily composed of Silicate rocks Within our The VENUS ( V ictoria E xperimental N etwork U nder the S ea project is a cabled sea floor observatory operated by the University EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 These compounds are quite rare in the universe, comprising only 0. 6% of the mass of the nebula, so the terrestrial planets could not grow very large. ^[8] The terrestrial embryos grew to about 0. 05 Earth masses and ceased accumulating matter about 100,000 years after the formation of the Sun; subsequent collisions and mergers between these planet-sized bodies allowed terrestrial planets to grow to their present sizes (see Terrestrial planets below). ^[25]

The gas giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where the material is cool enough for volatile icy compounds to remain solid. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. In Astronomy or Planetary science, the frost line, also known as the snow line, refers to a particular distance in the Solar nebula from the The ices which formed the Jovian planets were more abundant than the metals and silicates which formed the terrestrial planets, allowing the Jovian planets to grow massive enough to capture hydrogen and helium, the lightest and most abundant elements. See also Abundances of the elements (data page The abundance of a Chemical element measures how relatively common the element is or how much of the element ^[8] Planetesimals beyond the frost line accumulated up to four Earth masses within about 3 million years. ^[25] Today, the four gas giants comprise just under 99% of all the mass orbiting the Sun. ^[26] Theorists believe it is no accident that Jupiter lies just beyond the frost line. Because the frost line accumulated large amounts of water via evaporation from infalling icy material, it created a region of lower pressure that increased the speed of orbiting dust particles and halted their motion toward the Sun. In effect, the frost line acted as a barrier that caused material to accumulate rapidly at ~5 AU from the Sun. This excess material coalesced into a large embryo of about 10 Earth masses, which then began to grow rapidly by swallowing hydrogen from the surrounding disc, reaching 150 Earth masses in only another 1000 years and finally topping out at 318 Earth masses. Saturn may owe its substantially lower mass simply to having formed a few million years after Jupiter, when there was less gas available to consume. ^[25]

T Tauri stars like the young Sun have far stronger stellar winds than more stable, older stars. A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a Star. Uranus and Neptune are believed to have formed after Jupiter and Saturn did, when the strong solar wind had blown away much of the disc material. The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. As a result, the planets accumulated little hydrogen and helium—not more than 1 Earth mass each. Uranus and Neptune are sometimes referred to as failed cores. ^[27] The main problem with formation theories for these planets is the timescale of their formation. At the current locations it would have taken a hundred million years for their cores to accrete. This means that Uranus and Neptune probably formed closer to the Sun—near or even between Jupiter and Saturn—and later migrated outward (see Planetary migration below). ^[28][27] Motion in the planetesimal era was not all inward toward the Sun; the Stardust sample return from Comet Wild 2 has suggested that materials from the early formation of the Solar System migrated from the warmer inner Solar System to the region of the Kuiper belt. Stardust is an American interplanetary mission of the NASA Jet Propulsion Laboratory, whose primary purpose was to investigate the makeup of the Comet 81P/Wild, also known as Wild 2, is a Comet named after Swiss astronomer Paul Wild (pronounced Vilt) who discovered it in 1978 ^[29]

After between three and ten million years,^[25] the young Sun's solar wind would have cleared away all the gas and dust in the protoplanetary disc, blowing it into interstellar space, thus ending the growth of the planets. ^[30][31]

Subsequent evolution

The planets were originally believed to have formed in or near the orbits at which we see them now. However, this view has been undergoing radical change during the late 20th and early 21st centuries. Currently, it is believed that the Solar System looked very different after its initial formation: several objects at least as massive as Mercury were present in the inner Solar System, the outer Solar System was much more compact than it is now, and the Kuiper belt was much closer to the Sun. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending ^[32]

Terrestrial planets

Artist's conception of the giant impact event that may have created the Moon, a collision typical of the later stages of the inner Solar System's formation

At the end of the planetary formation epoch the inner Solar System was populated by 50–100 Moon- to Mars-sized planetary embryos. Protoplanets are moon-sized planets or larger embryos within Protoplanetary discs They are believed to form out of kilometer-sized Planetesimals that attract each ^[33][34] Further growth was possible only because these bodies collided and merged, a process which took up to 100 million years. These objects would have gravitationally interacted with one another, tugging at each other's orbits until they collided, growing larger until the four terrestrial planets we know today took shape. ^[25] One such giant collision is believed to have formed the Moon (see Moons below), while another removed the outer envelope of the young Mercury. ^[35]

One unresolved issue with this model is that it cannot explain how the initial orbits of the proto-terrestrial planets, which would have needed to be highly eccentric in order to collide, produced the remarkably stable and near-circular orbits the terrestrial planets possess today. ^[33] One hypothesis for this "eccentricity dumping" is that the terrestrials formed in a disc of gas still not expelled by the Sun. The "gravitational drag" of this residual gas would have eventually lowered the planets' energy, smoothing out their orbits. Dynamical friction is a term in Astrophysics related to loss of Momentum and Kinetic energy of moving bodies through a Gravitational interaction ^[34] However, such gas, if it existed, would have prevented the terrestrials' orbits from becoming so eccentric in the first place. ^[25] Another hypothesis is that gravitational drag occurred not between the planets and residual gas but between the planets and the remaining small bodies. As the large bodies moved through the crowd of smaller objects, the smaller objects, attracted by the larger planets' gravity, formed a region of higher density, a "gravitational wake", in the larger objects' path. As they did so, the increased gravity of the wake slowed the larger objects down into more regular orbits. ^[36]

Asteroid belt

The outer edge of the terrestrial region, between 2 and 4 AU from Sun, is called the asteroid belt. The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. The asteroid belt initially contained more than enough matter to form 2–3 Earth-like planets, and, indeed, a large number of planetesimals formed there. Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal As with the terrestrials, planetesimals in this region later coalesced and formed 20–30 Moon- to Mars-sized planetary embryos;^[37] however, the proximity of Jupiter meant that after this planet formed, 3 million years after the Sun, the region's history changed dramatically. Protoplanets are moon-sized planets or larger embryos within Protoplanetary discs They are believed to form out of kilometer-sized Planetesimals that attract each ^[33] Orbital resonances with Jupiter and Saturn are particularly strong in the asteroid belt, and gravitational interactions with more massive embryos scattered many planetesimals into those resonances. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their Jupiter's gravity increased the velocity of objects within these resonances, causing them to shatter upon collision with other bodies, rather than accrete. ^[38]

As Jupiter migrated inward following its formation (see Planetary migration below), resonances would have swept across the asteroid belt, dynamically exciting the region's population and increasing their velocities relative to each other. ^[39] The cumulative action of the resonances and the embryos either scattered the planetesimals away from the asteroid belt or excited their orbital inclinations and eccentricities. Inclination in general is the Angle between a Reference plane and another plane or axis of direction In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape ^[37][40] Some of those massive embryos too were ejected by Jupiter, while others may have migrated to the inner Solar System and played a role in the final accretion of the terrestrial planets. ^[41][37][42] During this primary depletion period, the effects of the giant planets and planetary embryos left the asteroid belt with a total mass equivalent to less than 1% that of the Earth, composed mainly of small planetesimals. ^[40] This is still 10–20 times more than the current mass in the main belt, which is about 1/2,000 the Earth's mass. ^[43] A secondary depletion period that brought the asteroid belt down close to its present mass is believed to have followed when Jupiter and Saturn entered a temporary 2:1 orbital resonance (see below).

The inner Solar System's period of giant impacts probably played a role in the Earth acquiring its current water content (~6×10²¹ kg) from the early asteroid belt. Water is too volatile to have been present at Earth's formation and must have been subsequently delivered from outer, colder parts of the Solar System. ^[44] The water was probably delivered by planetary embryos and small planetesimals thrown out of the asteroid belt by Jupiter. ^[41] A population of main-belt comets discovered in 2006 has been also suggested as a possible source for Earth's water. Main-belt comets are bodies orbiting within the ( main) asteroid belt which show cometary activity during a part of their orbit ^[44][45] In contrast, comets from the Kuiper belt or farther regions delivered not more than about 6% of Earth's water. A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — ^[46][1] The panspermia hypothesis holds that life itself may have been deposited on Earth in this way, although this idea is not widely accepted. Panspermia ( Gk. πάς/πάν (pas/pan all and σπέρμα ( sperma, seed is the Hypothesis that "seeds" of Life exist already ^[47]

Planetary migration

Main article: Planetary migration

According to the nebular hypothesis, the outer two planets are in the "wrong place". Planetary migration occurs when a Planet or other stellar satellite interacts with a disk of gas or Planetesimals resulting in the alteration of the satellite's orbital Uranus and Neptune (known as the "ice giants") exist in a region where the reduced density of the solar nebula and longer orbital times render their formation highly implausible. Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily The two are instead believed to have formed in orbits near Jupiter and Saturn, where more material was available, but to have migrated outward to their current positions over hundreds of millions of years. Planetary migration occurs when a Planet or other stellar satellite interacts with a disk of gas or Planetesimals resulting in the alteration of the satellite's orbital ^[27]

Simulation showing outer planets and Kuiper belt: a) Before Jupiter/Saturn 2:1 resonance b) Scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune c) After ejection of Kuiper belt bodies by Jupiter^[1]

The migration of the outer planets is also necessary to account for the existence and properties of the Solar System's outermost regions. ^[28] Beyond Neptune, the Solar System continues into the Kuiper belt, the scattered disc, and the Oort cloud, three sparse populations of small icy bodies thought to be the points of origin for most observed comets. A trans-Neptunian object (TNO is any object in the Solar system that Orbits the sun at a greater distance on average than Neptune. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — At their distance from the Sun, accretion was too slow to allow planets to form before the solar nebula dispersed, and thus the initial disc lacked enough mass density to consolidate into a planet. The Kuiper belt lies between 30 and 55 AU from the Sun, while the farther scattered disc extends to over 100 AU,^[28] and the distant Oort cloud begins at about 50,000 AU. ^[48] Originally, however, the Kuiper belt was much denser and closer to the Sun, with an outer edge at approximately 30 AU. Its inner edge would have been just beyond the orbits of Uranus and Neptune, which were in turn far closer to the Sun when they formed (most likely in the range of 15–20 AU), and in opposite locations, with Uranus farther from the Sun than Neptune. ^[28][1]

After the formation of the Solar System, the orbits of all the giant planets continued to change slowly, influenced by their interaction with large number of remaining planetesimals. After 500–600 million years (about 4 billion years ago) Jupiter and Saturn fell into a 2:1 resonance; Saturn orbited the Sun once for every two Jupiter orbits. ^[28] This resonance created a gravitational push against the outer planets, causing Neptune to surge past Uranus and plough into the ancient Kuiper belt. The planets scattered the majority of the small icy bodies inwards, while themselves moving outwards. These planetesimals then scattered off the next planet they encountered in a similar manner, moving the planets' orbits outwards while they moved inwards. ^[49] This process continued until the planetesimals interacted with Jupiter, whose immense gravity sent them into highly elliptical orbits or even ejected them outright from the Solar System. This caused Jupiter to move slightly inward. Those objects scattered by Jupiter into highly elliptical orbits formed the Oort cloud;^[28] those objects scattered to a lesser degree by the migrating Neptune formed the current Kuiper belt and scattered disc. ^[28] This scenario explains the Kuiper belt's and scattered disc's present low mass. Some of the scattered objects, including Pluto, became gravitationally tied to Neptune's orbit, forcing them into mean-motion resonances. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their ^[50] Eventually, friction within the planetesimal disc made the orbits of Uranus and Neptune circular again. ^[28][51]

In contrast to the outer planets, the inner planets are not believed to have migrated significantly over the age of the Solar System, because their orbits have remained stable following the period of giant impacts. ^[25]

Late Heavy Bombardment and after

Main article: Late Heavy Bombardment

Gravitational disruption from the outer planets' migration would have sent large numbers of asteroids into the inner Solar System, severely depleting the original belt until it reached today's extremely low mass. The Late Heavy Bombardment (commonly referred to as the lunar cataclysm, or LHB) is a period of time approximately 3800 to 4100 million years ago ( mya ^[40] This event may have triggered the Late Heavy Bombardment which occurred approximately 4 billion years ago, 500–600 million years after the formation of the Solar System. ^[1][52] This period of heavy bombardment lasted several hundred million years and is evident in the cratering still visible on geologically dead bodies of the inner Solar System such as the Moon and Mercury. ^[1][53] The oldest known evidence for life on Earth dates to 3. 8 billion years ago—almost immediately after the end of the Late Heavy Bombardment. ^[54]

Meteor Crater in Arizona. Created 50,000 years ago by an impactor only 50m across, it is a stark reminder that the accretion of the Solar System is not over.

Impacts are believed to be a regular (if currently infrequent) part of the evolution of the Solar System. That they continue to happen is evidenced by the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994, and the impact feature Meteor Crater in Arizona. Shoemaker-Levy redirects here For other Shoemaker-Levy comets see List of periodic comets. Meteor Crater is a Meteorite Impact crater located approximately 43 miles east of Flagstaff, near Winslow in the northern The State of Arizona ( is a state located in the southwestern region of the United States. The process of accretion, therefore, is not complete, and may still pose a threat to life on Earth. ^[55][56]

The evolution of the outer Solar System appears to have been influenced by nearby supernovae and possibly also passage through interstellar clouds. A supernova (plural supernovae or supernovas) is a stellar Explosion. Interstellar cloud is the generic name given to an accumulation of gas plasma and dust in our and other galaxies. The surfaces of bodies in the outer Solar System would experience space weathering from the solar wind, micrometeorites, and the neutral components of the interstellar medium. Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment ^[57]

The evolution of the asteroid belt after Late Heavy Bombardment was mainly governed by collisions. ^[58] Objects with large mass have enough gravity to retain any material ejected by a violent collision. In the asteroid belt this usually is not the case. As a result, many larger objects have been broken apart, and sometimes newer objects have been forged from the remnants in less violent collisions. ^[58] Moons around some asteroids currently can only be explained as consolidations of material flung away from the parent object without enough energy to entirely escape its gravity. ^[59]

Moons

See also: Giant impact hypothesis

Moons have come to exist around most planets and many other Solar System bodies. These natural satellites originated by one of three possible mechanisms:

• co-formation from a circum-planetary disc (only in the cases of the gas giants);
• formation from impact debris (given a large enough impact at a shallow angle); and
• capture of a passing object. A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary.

Jupiter and Saturn have a number of large moons, such as Io, Europa, Ganymede and Titan, which may have originated from discs around each giant planet in much the same way that the planets formed from the disc around the Sun. TemplateInfobox Planet.--> Io (ˈaɪoʊ, or as Greek TemplateInfobox Planet.--> Europa (jʊˈroʊpə; or as TemplateInfobox Planet.--> Ganymede (ˈgænɨmiːd, or as Greek TemplateInfobox Planet.--> Titan (ˈtaɪtən, or as ^[60] This origin is indicated by the large sizes of the moons and their proximity to the planet. These attributes are impossible to achieve via capture, while the gaseous nature of the primaries make formation from collision debris another impossibility. The outer moons of the gas giants tend to be small and have eccentric orbits with arbitrary inclinations. In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape These are the characteristics expected of captured bodies. ^[61][62] Most such moons orbit in the direction opposite the rotation of their primary. The largest irregular moon is Neptune's moon Triton, which is believed to be a captured Kuiper belt object. TemplateInfobox Planet.--> Triton (ˈtraɪtən, or as in Greek The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending ^[56]

Moons of solid Solar System bodies have been created by both collisions and capture. Mars's two small moons, Deimos and Phobos, are believed to be captured asteroids. TemplateInfobox Planet.--> Deimos (ˈdaɪməs; also /ˈdiːməs/ Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but ^[63] The Earth's Moon is believed to have formed as a result of a single, large oblique collision. ^[64][65] The impacting object likely had a mass comparable to that of Mars, and the impact probably occurred near the end of the period of giant impacts. The collision kicked into orbit some of the impactor's mantle, which then coalesced into the Moon. ^[64] The impact was probably the last in series of mergers that formed Earth. It has been further hypothesized that the Mars-sized object may have formed at one of the stable Earth-Sun Lagrangian points (either L₄ or L₅) and drifted from its position. ^[66] Pluto's moon Charon may also have formed by means of a large collision; the Pluto-Charon and Earth-Moon systems are the only two in the Solar System in which the satellite's mass is at least 1% that of the larger body. ^[67]

Future

Astronomers estimate that the Solar System as we know it today will not change drastically until the Sun has fused all the hydrogen fuel in its core into helium, beginning its evolution off of the main sequence of the Hertzsprung-Russell diagram and into its red giant phase. Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD A red giant is a luminous Giant star of low or intermediate mass (roughly 0 Even so, the Solar System will continue to evolve until then.

Long-term stability

The Solar System is chaotic,^[68] with the orbits of the planets open to long-term variations. In Mathematics, chaos theory describes the behavior of certain dynamical systems – that is systems whose state evolves with time – that may exhibit dynamics that One notable example of this chaos is the Neptune-Pluto system, which lies in a 3:2 orbital resonance. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their Although the resonance itself will remain stable, it becomes impossible to predict the position of Pluto with any degree of accuracy more than 10–20 million years (the Lyapunov time) into the future. In Mathematics, the Lyapunov time is the length of time for a Dynamical system to become chaotic. ^[69] Another example is Earth's axial tilt which, thanks to friction raised within Earth's mantle by tidal interactions with the Moon (see below) will be rendered chaotic at some point between 1. In Astronomy, axial tilt is the Inclination angle of a planet's rotational axis in relation to its orbital plane. 5 and 4. 5 billion years from now. ^[70]

The planets' orbits are chaotic over longer timescales, such that the whole Solar System possesses a Lyapunov time in the range of 2–230 million years. ^[71] In all cases this means that the position of a planet along its orbit ultimately becomes impossible to predict with any certainty (so, for example, the timing of winter and summer become uncertain), but in some cases the orbits themselves may change dramatically. Such chaos manifests most strongly as changes in eccentricity, with some planets' orbits becoming significantly more—or less—elliptical. In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape In Mathematics, an ellipse (from the Greek ἔλλειψις literally absence) is a Conic section, the locus of points in a ^[72]

Ultimately, the Solar System is stable in that none of the planets will collide with each other or be ejected from the system in the next few billion years. ^[71] Beyond this, within five billion years or so Mars's eccentricity may grow to around 0. 2, such that it lies on an Earth-crossing orbit, leading to a potential collision. In the same timescale, Mercury's eccentricity may grow even further, and a close encounter with Venus could theoretically eject it from the Solar System altogether^[68] or send it on a collision course with Venus or Earth. ^[73]

Moon-ring systems

The evolution of moon systems is driven by tidal forces. Characteristics A tide is a repeated cycle of sea level changes in the following stages Over several hours the water rises or advances up a beach in the flood A moon will raise a tidal bulge in the object it orbits (the primary) due to the differential gravitational force across diameter of the primary. The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced If a moon is revolving in the same direction as the planet's rotation and the planet is rotating faster than the orbital period of the moon, the bulge will constantly be pulled ahead of the moon. In this situation, angular momentum is transferred from the rotation of the primary to the revolution of the satellite. In Physics, the angular momentum of a particle about an origin is a vector quantity equal to the mass of the particle multiplied by the Cross product of the position The moon gains energy and gradually spirals outward, while the primary rotates more slowly over time.

The Earth and its Moon are one example of this configuration. Today, the Moon is tidally locked to the Earth; one of its revolutions around the Earth is equal to one of its rotations about its axis, which means that it always shows one face to the Earth. A separate article treats the phenomenon of Tidal resonance in Oceanography. However, as the Moon recedes from Earth, Earth's spin will gradually slow, until, in about 50 billion years, the two worlds will become tidally locked to each other. Each will only be visible from one hemisphere of the other. ^[74] Other examples are the Galilean moons of Jupiter (as well as many of Jupiter's smaller moons)^[75] and most of the larger moons of Saturn. The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei. ^[76]

Neptune and its moon Triton, taken by Voyager 2. TemplateInfobox Planet.--> Triton (ˈtraɪtən, or as in Greek Voyager 2 is an unmanned Interplanetary Spacecraft launched on August 20, 1977. Triton's orbit will eventually take it within Neptune's Roche limit, tearing it apart and possibly forming a new ring system. Determining the Roche limit The Roche limit depends on the rigidity of the satellite

A different scenario occurs when the moon is either revolving around the primary faster than the primary rotates, or is revolving in the direction opposite the planet's rotation. In these cases, the tidal bulge lags behind the moon in its orbit. In the former case, the direction of angular momentum transfer is reversed, so the rotation of the primary speeds up while the satellite's orbit shrinks. In the latter case, the angular momentum of the rotation and revolution have opposite signs, so transfer leads to decreases in the magnitude of each (that cancel each other out). ^[77] In both cases, tidal deceleration causes the moon to spiral in towards the primary until it either is torn apart by tidal stresses, potentially creating a planetary ring system, or crashes into the planet's surface or atmosphere. Tidal acceleration is an effect of the Tidal forces between an orbiting Natural satellite ( i A planetary ring is a ring of Cosmic dust and other small particles Orbiting around a Planet in a flat disc-shaped region Such a fate awaits the moons Phobos of Mars (within 30 to 50 million years),^[78] Triton of Neptune (in 3. TemplateInfobox Planet.--> Triton (ˈtraɪtən, or as in Greek 6 billion years),^[79] Metis and Adrastea of Jupiter,^[80] and at least 16 small satellites of Uranus and Neptune. TemplateInfobox Planet.--> Metis (, or as in Greek Μήτις Adrastea (, or as in Greek Αδράστεια also known as Jupiter XV, is the second by distance and the smallest of the four inner moons of Jupiter Tidal acceleration is an effect of the Tidal forces between an orbiting Natural satellite ( i Uranus' Desdemona may even collide with one of its neighboring moons. There is also a Minor planet called 666 Desdemona. ^[81]

A third possibility is where the primary and moon are tidally locked to each other. A separate article treats the phenomenon of Tidal resonance in Oceanography. In that case, the tidal bulge stays directly under the moon, there is no transfer of angular momentum, and the orbital period will not change. Pluto and Charon are an example of this type of configuration. ^[82]

Prior to the 2004 arrival of the Cassini–Huygens spacecraft, the rings of Saturn were widely thought to be much younger than the Solar System and were not expected to survive beyond another 300 million years. Cassini–Huygens is a joint NASA / ESA / ASI Robotic spacecraft mission currently studying the planet Saturn and its Saturn has the most extensive Planetary ring system of any planet in the Solar System. Gravitational interactions with Saturn's moons were expected to gradually sweep the rings' outer edge toward the planet, with abrasion by meteorites and Saturn's gravity eventually taking the rest, leaving Saturn unadorned. ^[83] However, data from the Cassini mission led scientists to revise that early view. Observations revealed 10 km-wide icy clumps of material that repeatedly break apart and reform, keeping the rings fresh. Saturn's rings are far more massive than the rings of the other gas giants. This large mass is believed to have preserved Saturn's rings since the planet first formed 4. 5 billion years ago, and is likely to preserve them for billions of years to come. ^[84]

The Sun and planetary environments

See also: Stellar evolution

In the long term, the greatest changes in the Solar System will come from changes in the Sun itself as it ages. Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime As the Sun burns through its supply of hydrogen fuel, it gets hotter and burns the remaining fuel even faster. As a result, the Sun is growing brighter at a rate of ten percent every 1. 1 billion years. ^[85] In one billion years' time, as the Sun's radiation output increases, its circumstellar habitable zone will move outwards, and the Earth's surface will be hot enough that liquid water can no longer exist on Earth's surface. In Astronomy a habitable zone ( HZ) is a region of space where conditions are favorable for Life as it may be found on Earth. At this point, all life on land will become extinct. ^[86] Evaporation of water, a potent greenhouse gas, from the oceans' surface could accelerate temperature increase, potentially ending all life on Earth even sooner. Greenhouse gases are gaseous constituents of the atmosphere bothnatural and anthropogenic that absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared ^[87] During this time it is possible that as Mars's surface temperature gradually rises, carbon dioxide and water currently frozen under the surface soil will be liberated into the atmosphere, creating a greenhouse effect which will heat up the planet until it achieves conditions parallel to those on Earth today, providing a potential future abode for life. ^[88] By 3. 5 billion years from now, Earth's surface conditions will be similar to those of Venus today. ^[85]

Relative size of our Sun as it is now (inset) compared to its estimated future size as a red giant

Around 5. 4 billion years from now, all of the hydrogen in the core of the Sun will have fused into helium. The core will no longer be supported against gravitational collapse and will begin to contract, heating a shell around the core until hydrogen begins to fuse within it. ^[86] This will cause the outer layers of the star to expand greatly, and the star will enter a phase of its life in which it is called a red giant. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 ^[89][90] Within 7. 5 billion years, the Sun will have expanded to a radius of 1. 2 AU—256 times its current size. At the tip of the red giant branch, as a result of the vastly increased surface area, the Sun's surface will be much cooler (about 2600 K) than now and its luminosity much higher—up to 2700 current solar luminosities. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 Luminosity has different meanings in several different fields of science For part of its red giant life, the Sun will have a strong stellar wind which will carry away around 33% of its mass. A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a Star. ^[86][91][92] During these times, it is possible that Saturn's moon Titan could achieve surface temperatures necessary to support life. TemplateInfobox Planet.--> Titan (ˈtaɪtən, or as ^[93][94]

As the Sun expands, it will most likely swallow the planets Mercury and Venus. The VENUS ( V ictoria E xperimental N etwork U nder the S ea project is a cabled sea floor observatory operated by the University Earth's fate is less clear; although the Sun will envelop Earth's current orbit, the star's loss of mass (and thus weaker gravity) will cause the planets' orbits to move farther out. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 ^[86] If it were only for this, Earth would probably escape incineration,^[91] but a 2008 study suggests that Earth will likely be swallowed up as a result of tidal interactions with the Sun's weakly bound outer envelope. Determining the Roche limit The Roche limit depends on the rigidity of the satellite ^[86]

Gradually, the hydrogen burning in the shell around the solar core will increase the mass of the core until it reaches about 45% of the present solar mass. At this point the density and temperature will become so high that the fusion of helium into carbon will begin, leading to a helium flash; the Sun will shrink from around 250 to 11 times its present (main sequence) radius. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 A helium flash is the sudden beginning of Helium fusion in the core of intermediate mass Stars of less than about 2 Consequently, its luminosity will decrease from around 3000 to 54 times its current level, and its surface temperature will increase to about 4770 K. The Sun will become a horizontal branch star, burning helium in its core in a stable fashion much like it burns hydrogen today. The horizontal branch ( HB) is a stage of Stellar evolution which immediately follows the Red giant branch in stars whose masses are similar to the Sun's The helium-fusing stage will last only 100 million years. Eventually, it will have to again resort to the reserves of hydrogen and helium in its outer layers and will expand a second time, turning into what is known as an asymptotic giant branch star. The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass Stars This is a period of Stellar evolution Here the luminosity of the Sun will increase again, reaching about 2090 present luminosities, and it will cool to about 3500 K. ^[86] This phase lasts about 30 million years, after which, over the course of a further 100,000 years, the Sun's remaining outer layers will fall away, ejecting a vast stream of matter into space and forming a halo known (misleadingly) as a planetary nebula. A planetary nebula is an Emission nebula consisting of a glowing shell of Gas and plasma formed by certain types of Stars when they die The ejected material will contain the helium and carbon produced by the Sun's nuclear reactions, continuing the enrichment of the interstellar medium with heavy elements for future generations of stars. ^[95]

The Ring nebula, a planetary nebula similar to what the Sun will become

This is a relatively peaceful event, nothing akin to a supernova, which our Sun is too small to undergo as part of its evolution. The famously named " Ring Nebula " is located in the northern Constellation of Lyra, and also catalogued as Messier 57, M57 or A supernova (plural supernovae or supernovas) is a stellar Explosion. Any observer present to witness this occurrence would see a massive increase in the speed of the solar wind, but not enough to destroy a planet completely. However, the star's loss of mass could send the orbits of the surviving planets into chaos, causing some to collide, others to be ejected from the Solar System, and still others to be torn apart by tidal interactions. ^[96] Afterwards, all that will remain of the Sun is a white dwarf, an extraordinarily dense object, 54% its original mass but only the size of the Earth. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. Initially, this white dwarf may be 100 times as luminous as the Sun is now. It will consist entirely of degenerate carbon and oxygen, but will never reach temperatures hot enough to fuse these elements. Degenerate matter is matter which has sufficiently high Density that the dominant contribution to its Pressure rises from the Pauli Exclusion Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Thus the white dwarf Sun will gradually cool, growing dimmer and dimmer. ^[97]

As the Sun dies, its gravitational pull on the orbiting bodies such as planets, comets and asteroids will weaken due to its mass loss. All remaining planets' orbits will expand; if Earth still exists, its orbit will lie at about 1. 85 AU, and Mars' orbit will lie at about 2. 8 AU. They and the other remaining planets will become dark, frigid hulks, completely devoid of any form of life. ^[91] They will continue to orbit their star, their speed slowed due to their increased distance from the Sun and the Sun's reduced gravity. Two billion years later, when the Sun has cooled to the 6000–8000K range, the carbon and oxygen in the Sun's core will freeze, with over 90% of its remaining mass assuming a crystalline structure. ^[98] Eventually, after billions more years, the Sun will finally cease to shine altogether, becoming a black dwarf. A black dwarf is a hypothetical Star, created when a White dwarf becomes sufficiently cool to no longer emit significant Heat or Light ^[99]

Galactic interaction

Location of the Solar System within our galaxy

The Solar System travels alone through the Milky Way galaxy in a circular orbit approximately 30,000 light years from the galactic centre. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply The Galactic Center is the rotational center of the Milky Way Galaxy. Its speed is about 220 km/s. ^[100] The period required for the Solar System to complete one revolution around the galactic centre, the galactic year, is in the range of 220-250 million years. The galactic year, also known as a cosmic year, is the duration of time required for the Solar system to Orbit once around the center of the Milky ^[101] Since its formation, the Solar System has completed at least 18 such revolutions.

A number of scientists have speculated that the Solar System's path through the galaxy is a factor in the periodicity of mass extinctions observed in the Earth's fossil record. An extinction event (also known as mass extinction; extinction-level event, ELE is a sharp decrease in the number of Species in a relatively short period FOSSIL is a standard protocol for allowing serial communication for Telecommunications programs under the DOS Operating system. One hypothesis supposes that vertical oscillations made by the Sun as it orbits the galactic centre cause it to regularly pass through the galactic plane. When the Sun's orbit takes it outside the galactic disc, the influence of the galactic tide is weaker; as it re-enters the galactic disc, as it does every 20–25 million years, it comes under the influence of the far stronger "disc tides", which, according to mathematical models, increase the flux of Oort cloud comets into the Solar System by a factor of 4, leading to a massive increase in the likelihood of a devastating impact. A galactic tide is a Tidal force subjected on objects by the Gravitational field of a Galaxy such as the Milky Way. The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, ^[102]

However, others argue that the Sun is currently close to the galactic plane, and yet the last great extinction event was 15 million years ago. Therefore the Sun's vertical position cannot alone explain such periodic extinctions, and that extinctions instead occur when the Sun passes through the galaxy's spiral arms. A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the Spiral arms are home not only to larger numbers of molecular clouds, whose gravity may distort the Oort cloud, but also to higher concentrations of bright blue giant stars, which live for relatively short periods and then explode violently as supernovae. In Astronomy, a blue giant is a Star with a Spectral type of O or B (thus being noticeably blue in appearance and a luminosity class of III ( giant A supernova (plural supernovae or supernovas) is a stellar Explosion. ^[103]

Galactic collision and planetary disruption

Main article: Andromeda-Milky Way collision

An artist's rendition of the collision of the Milky Way and Andromeda galaxies, as it might be seen from Earth

Although the vast majority of galaxies in the Universe are moving away from the Milky Way, the Andromeda Galaxy, the largest member of our Local Group of galaxies, is heading towards it at about 120 km/s. The Andromeda-Milky Way collision is a predicted Galaxy collision, due to take place in approximately three billion years time between the two largest galaxies in the Local The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda The Local Group is the group of galaxies that includes our galaxy the Milky Way. ^[104] In 2 billion years, Andromeda and the Milky Way will collide, causing both to deform as tidal forces distort their outer arms into vast tidal tails. The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced A galactic tide is a Tidal force subjected on objects by the Gravitational field of a Galaxy such as the Milky Way. When this initial disruption occurs, astronomers calculate a 12% chance that the Solar System will be pulled outward into the Milky Way's tidal tail and a 3% chance that it will become gravitationally bound to Andromeda and thus a part of that galaxy. Gravitation is a natural Phenomenon by which objects with Mass attract one another ^[104] After a further series of glancing blows, during which the likelihood of the Solar System's ejection rises to 30%, the galaxies' supermassive black holes will merge. A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1 Eventually, in roughly 7 billion years, the Milky Way and Andromeda will complete their merger into a giant elliptical galaxy. An elliptical galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble (whose name was dedicated During the merger, if there is enough gas, the increased gravity will force the gas to the centre of the forming elliptical galaxy. This may lead to a short period of intensive star formation called a starburst. A starburst galaxy is a Galaxy in the process of an exceptionally high rate of Star formation, compared to the usual star formation rate seen in most galaxies ^[104] In addition the infalling gas will feed the newly formed black hole transforming it into an active galactic nucleus. An active galactic nucleus ( AGN) is a compact region at the centre of a Galaxy which has a much higher than normal luminosity over some or all of the Electromagnetic The force of these interactions will likely push the Solar System into the new galaxy's outer halo, leaving it relatively unscathed by the radiation from these collisions. ^[104][105]

It is a common misconception that this collision will disrupt the orbits of the planets in the Solar System. While it is true that the gravity of passing stars can detach planets into interstellar space, distances between stars are so great that the likelihood of the Milky Way-Andromeda collision causing such disruption to any individual star system is negligible. While the Solar System as a whole could be affected by these events, the Sun and planets are not expected to be disturbed. ^[106]

However, over time, the cumulative probability of a chance encounter with a star increases, and disruption of the planets becomes all but inevitable. In Probability theory and Statistics, the cumulative distribution function (CDF, also probability distribution function or just distribution function Assuming that the Big Crunch or Big Rip scenarios for the end of the universe do not occur, calculations suggest that the gravity of passing stars will have completely stripped the dead Sun of its remaining planets within 1 quadrillion (10¹⁵) years. In Physical cosmology, the Big Crunch is one possible scenario for the Ultimate fate of the universe, in which the Metric expansion of space eventually The Big Rip is a cosmological Hypothesis first published in 2003 about the Ultimate fate of the universe, in which the matter of the universe This point marks the end of the Solar System. While the Sun and planets may survive, the Solar System, in any meaningful sense, will cease to exist. ^[2]

Chronology

The time frame of the Solar System's formation has been determined using radiometric dating. Radiometric dating (often called radioactive dating) is a technique used to date materials usually based on a comparison between the observed abundance of a naturally occurring Scientists estimate that the Solar System is 4. 6 billion years old. The oldest known mineral grains on Earth are approximately 4. The oldest rock or rocks on Earth, as an aggregate of Minerals that have not undergone subsequent metamorphosis, are from the Archean EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 4 billion years old. ^[107] Rocks this old are rare, as Earth's surface is constantly being reshaped by erosion, volcanism, and plate tectonics. Erosion is the carrying away or displacement of solids ( Sediment, Soil, rock and other particles usually by the agents of currents such as wind Plate tectonics and hotspots Divergent plate boundaries At the Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere To estimate the age of the Solar System, scientists use meteorites, which were formed during the early condensation of the solar nebula. A meteorite is a natural object originating in Outer space that survives an impact with the Earth 's surface Almost all meteorites (see the Canyon Diablo meteorite) are found to have an age of 4. 6 billion years, suggesting that the Solar System must be at least this old. ^[108]

Studies of discs around other stars have also done much to establish a time frame for Solar System formation. Stars between one and three million years old possess discs rich in gas, whereas discs around stars more than 10 million years old have little to no gas, suggesting that gas giant planets within them have ceased forming. ^[25]

Timeline of Solar System evolution

Note: All dates and times in this chronology are approximate and should be taken as an order of magnitude indicator only. An order of magnitude is the class of scale or magnitude of any amount where each class contains values of a fixed ratio to the class preceding it

See also

• Age of the Earth
• History of Earth
• Tidal locking

Notes

References

• Michael A. Zeilik, Stephen A. Gregory (1998). Introductory Astronomy & Astrophysics, 4th ed. , Saunders College Publishing. ISBN 0030062284.  

External links

• 7M animation from skyandtelescope.com showing the early evolution of the outer Solar System.
• Quicktime animation of the future collision between the Milky Way and Andromeda