In this artist's conception, a planet spins through a clearing in a nearby star's dusty, planet-forming disc

In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. It was first proposed in 1734 by Emanuel Swedenborg.^[1] While originally applied only to our own Solar System, this method of planetary system formation was subsequently thought to be at work throughout the universe. Image File history File links Size of this preview: 800 × 600 pixelsFull resolution (1600 × 1200 pixel, file size: 109 KB, MIME type: image/jpeg) cellspacing=8 cellpadding=0 style=width:100%; clear:both; margin:0. ... Image File history File links Size of this preview: 800 × 600 pixelsFull resolution (1600 × 1200 pixel, file size: 109 KB, MIME type: image/jpeg) cellspacing=8 cellpadding=0 style=width:100%; clear:both; margin:0. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... The theories concerning the formation and evolution of the Solar System are complex and varied, interweaving various scientific disciplines, from astronomy and physics to geology and planetary science. ... Emanuel Swedenborg, 75, holding the manuscript of Apocalypsis Revelata (1766). ... This article is about the Solar System. ... For other uses, see Universe (disambiguation). ...

Stars form in massive dense clouds of molecular hydrogen—giant molecular clouds (GMC). Such clouds are gravitationally unstable and prone to fragmentation into even denser clumps. The clumps inside the GMC then collapse and produce stars. Star formation is a complicated multistage process, which always produces a gaseous protoplanetary disk around the young star. This disk may give birth to planets under some circumstances. Thus the formation of planetary systems is thought to be a natural result of star formation process. The formation of a sun-like star usually takes around one hundred million years.^[2] Molecular hydrogen, H2, is a molecule formed from two atoms of hydrogen. ... A dark nebula is a large cloud which appears as star-poor regions where the dust of interstellar medium seems to be concentrated. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ...

The protoplanetary disk is an accretion disk—it continues to feed the central star. Initially the disk is very hot. However later at the T tauri star stage it cools making formations of small dust grains made of rocks and ices possible. The grains may eventually coagualate into kilometer sized planetesimals. If the disk is massive enough the runaway accretions begins, which results in quick (about 100,000–300,000 years) formations of Moon to Mars sized planetary embryos. Near the star the planetary embryos go through the stage of violent mergers, producing a few terrestrial planets. The last stage takes around one hundred million (or 10⁷) to a billion (or 10⁸) years.^[2] An accretion disc (or accretion disk) is a structure formed by material falling into a gravitational source. ... Drawing of a T-Tauri star with a circumstellar accretion disk T Tauri stars are a class of variable stars named after their prototype - T Tauri. ... Look up dust in Wiktionary, the free dictionary. ... This article is about the geological substance. ... This article is about the clotting of blood. ... A kilometer (Commonwealth spelling: kilometre), symbol: km is a unit of length in the metric system equal to 1,000 metres (from the Greek words χίλια (khilia) = thousand and μέτρο (metro) = count/measure). ... Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. ... The inner planets, Mercury, Venus, Earth, and Mars, their sizes to scale. ...

The formation of giant planets is a more complicated process. It is thought to occur beyond the so called snow line, where planetary embryos are mainly made of various ices. As a result they are several times more massive than in the inner part of the protoplanetary disk. What follows after the embryo formation is not completly clear. However some embryos appear to continue to grow and eventually reach 5–10 Earth masses—the threshold value, which is necessary to begin accretion of the hydrogen–helium gas from the disk. The accumulation of gas by the core is initially a slow process, which continues for several million years, but after the forming protoplanet reaches about 30 Earth masses it accelerates and proceeds in a runaway manner. The Jupiter and Saturn–like planets are thought to accumulate the bulk of their mass during only 100,000 (or 10⁴) years. The accretion stops when the gas is exhausted. The formed planets can migrate over long distances during or after their formation. The ice giants like Uranus and Neptune are thought to be failed cores, which formed too late when the disk had almost disappeared.^[2] Metroplex (in shadow) and Giant Planet Gigantion, or Giant Planet, is a fictional planet home to giant Transformers in the animated television program, Transformers: Cybertron; it is referred to as Gigalonia in Transformers: Galaxy Force, the Japanese version of the show. ... In astronomy or planetary physics, the frost line refers to a particular distance in the solar nebula from the central protosun where it is cool enough for hydrogen compounds such as water, ammonia, and methane to condense into solid ice grains. ... This article is about the chemistry of hydrogen. ... General Name, symbol, number helium, He, 2 Chemical series noble gases Group, period, block 18, 1, s Appearance colorless Standard atomic weight 4. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... From top: Neptune, Uranus, Saturn, and Jupiter. ... For other uses, see Uranus (disambiguation). ... Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ...

History

The nebular hypothesis was first proposed in 1734 by Emanuel Swedenborg.^[1] In 1755 Immanuel Kant, who was familiar with Swedenborg's work, developed the theory further. He argued that nebulae slowly rotate, gradually collapsing and flattening due to gravity and eventually forming stars and planets, in a process of planetary formation. A similar model was proposed in 1796 by Pierre-Simon Laplace. These can be considered early theories of cosmology. Emanuel Swedenborg, 75, holding the manuscript of Apocalypsis Revelata (1766). ... 1755 was a common year starting on Wednesday (see link for calendar). ... Kant redirects here. ... The Triangulum Emission Nebula NGC 604 lies in a spiral arm of Galaxy M33, 2. ... Gravity is a force of attraction that acts between bodies that have mass. ... This article is about the astronomical object. ... This article is about the astronomical term. ... Year 1796 (MDCCXCVI) was a leap year starting on Friday (link will display the full calendar) of the Gregorian calendar (or a leap year starting on Monday of the 11-day slower Julian calendar). ... Pierre-Simon, marquis de Laplace (March 23, 1749 - March 5, 1827) was a French mathematician and astronomer whose work was pivotal to the development of mathematical astronomy. ...

While originally applied only to our own Solar System, this method of planetary system formation was subsequently believed by theorists to be at work throughout the universe; over 250 extrasolar planets have since been discovered in our galaxy. This article is about the Solar System. ... For other uses, see Universe (disambiguation). ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ... For other uses, see Galaxy (disambiguation). ...

What is known and unknown about planetary formation

Known

The star formation process naturally results in the appearance of accretionary disks around young stellar objects.^[3] At the age of about 1 million years 100% of stars may have such disks.^[4] This conclusion is supported by observations of the gaseous and dusty disks around protostars and T Tauri stars as well as by theoretical considerations.^[5] The observations of the disks show that the dust grains inside them grow in size on the short time scale (over thousands of years) producing 1 cm sized particles.^[6] // CM, cM, Cm or cm may stand for: CM Apollo Command/Service Module (command module is one half) Cameroon, ISO and FIPS country code category management Catholic Memorial center of mass Championship Manager, a series of association football computer games Chelmsford British post code region CM Chessmaster Chief Minister of...

The accretion process, by which 1 km planetesimals grow into 1000 km sized bodies, is well understood now.^[7] This process develops inside any disk, where the number density of planetesimals is sufficiently high, and proceeds in a runaway manner. At the later stage the growth slows and continues as the oligarchic accretion. The end result is formation of planetary embryos with varying sizes, which depend on the distance from the star.^[7]

Various simulations have demonstrated that the merger of embryos in the inner part of the planetary system leads to the formation of a few Earth sized bodies. So the origin of terrestrial planets is now considered to be an almost solved problem.^[8]

Unknown

The physics of accretion disks is not well understood.^[9] The main problem is the mechanism of angular momentum transport from the inner to the outer part of the disk, which is necessary for efficient accretion by the protostar. The process or processes, which are responsible for the disappearance of the disks, are also not well known.^[10][11] This gyroscope remains upright while spinning due to its angular momentum. ...

The formation of planetesimals is the biggest unsolved problem in the theory of planet formation. The precise mechanism by which 1 cm particles coalesce into the 1 km planetesimals is not understood. This mechanism appears to be the key to the question as to why some stars have planets, while others have nothing around them (even dusty belts).^[12]

The formation of giant planets is another unsolved problem. Current theories have serious difficulty explaining how their cores can form fast enough to accrete significant amounts of gas from the quickly disappearing protoplanetary disk.^[7][13] The lifetime of the disks (less than 10⁷ years) appears to be shorter than the timescale of the core formations.^[4] Another problem of giant planet formation is their migration. Some calculations show that interaction with the disk can cause rapid inward migration, which, if not stopped, will result in the planets plunging into the star.^[14]

Formation of stars and protoplanetary disks

Protostars

Main article: protostar

The visible-light and infrared views of the Trifid Nebula—a giant star-forming cloud of gas and dust located 5,400 light-years away in the constellation Sagittarius

Stars are thought to form inside giant clouds of cold molecular hydrogen—giant molecular clouds roughly 300,000 times the mass of the Sun and 20 parsecs in diameter.^[15][2] Over millions of years, giant molecular clouds are prone to collapse and fragmentation.^[16] These fragments then form small, dense cores which in turn collapse into stars.^[15] These cores range in mass from a fraction to several times that of the Sun. They possess diameters of 0.01-01 pc (2000-20,000 AU) and a core particle number density of roughly 10,000 to 100,000 cm^-3.^[17][15][18] These cores are called protostellar nebulae (protosolar in the case of Sun).^[2] A Protostar is an object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. ... The Trifid Nebula (also known as M20 and NGC 6514) is an H II region at right ascension 18h 02. ... This article is about the astronomical object. ... A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2). ... Molecular hydrogen, H2, is a molecule formed from two atoms of hydrogen. ... A dark nebula is a large cloud which appears as star-poor regions where the dust of interstellar medium seems to be concentrated. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ... This article or section does not cite its references or sources. ... Sol redirects here. ...

The initial collapse of a solar-mass protostellar nebula takes around 100,000 years.^[15][2] Every nebula begins its collapse with a certain amount of angular momentum. Gas in the central part of the nebula, whose angular momentum is relatively low, undergoes fast compression and forms a hot hydrostatic (not contracting) core with a fraction of the mass of the original nebula.^[19] This core forms the seed of what will become a star.^[19][2] As the collapse continues, conservation of angular momentum means the rotation of the infalling gas accelerates.^[11][20] This rotation largely prevents the gas from directly accreting onto the central core and instead forces it to spread outwards near its equatorial plane, forming a disk that in turn accretes onto the central core.^[2][11][20] The core gradually grows in mass until it becomes a young hot protostar.^[19] At this stage the protostar and its disk are heavily obscured by the infalling envelope and are not directly observable.^[3] In fact the remaining envelope's opacity is so high that even millimeter-wave radiation has trouble escaping from inside it.^[3][2] Such objects are observed as very bright condensations, which emit mainly millimeter-wave and submillimiter-wave radiation.^[18] They are classified as (spectral) Class 0 protostars.^[3] The collapse is often accompanied by bipolar outflows (jets), which emanate along the rotational axis of the inferred disk. Such outflows are often observed in star-forming regions (see Herbig-Haro (HH) objects).^[21] At this stage the protostar has a high luminosity in the millimeter-wave and submillimiter-wave spectral regions— a protostar of solar mass may radiate at up to one hundred solar luminosities and does not fuse hydrogen.^[19] Their main source of energy is gravitational collapse.^[19] This gyroscope remains upright while spinning due to its angular momentum. ... Fluid pressure is the pressure on an object submerged in a fluid, such as water. ... In physics, angular momentum intuitively measures how much the linear momentum is directed around a certain point called the origin; the moment of momentum. ... In astrophysics, the term accretion is used for at least two distinct processes. ... An accretion disc (or accretion disk) is a structure formed by material falling into a gravitational source. ... A Protostar is an object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. ... Look up opacity in Wiktionary, the free dictionary. ... Radio waves sent at terahertz frequencies, known as terahertz radiation, terahertz waves, T-rays, T-light, T-lux and THz, are in the region of the light spectrum between 10 terahertz and 100 gigahertz, corresponding to the wavelength range 30 micrometres (ending edge of far-infrared light, micrometre wavelength) to... Electromagnetic waves sent at terahertz frequencies, known as terahertz radiation, terahertz waves, terahertz light, T-rays, T-light, T-lux and THz, are in the region of the electromagnetic spectrum between 300 gigahertz (3x1011 Hz) and 3 terahertz (3x1012 Hz), corresponding to the wavelength range starting at submillimeter (<1 millimeter... The Boomerang Nebula is an excellent example of a bipolar outflow. ... A gas jet, fluid jet, or hydro jet is a nozzle intended to eject gas or fluid in a coherent stream into a surrounding medium, or the stream itself that is ejected by such a nozzle. ... This article is about rotation as a movement of a physical body. ... Herbig-Haro object HH47, imaged by the Hubble Space Telescope. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ... This article is about the chemistry of hydrogen. ... This article or section does not cite its references or sources. ...

Infrared image of the molecular outflow from an otherwise hidden newborn star HH 46/47

As the envelope's material infalls onto the disk, it eventually becomes thin and transparent and the young stellar object (YSO) becomes observable; initially in far-infrared light and later in the visible.^[18] Around this time the protostar also begins to fuse deuterium and then ordinary hydrogen.^[19] This event, which can be called the birth of a new star, happens at approximately 100,000 years after the collapse began.^[2] The external appearance of the YSO at this stage corresponds to the spectral class I protostars,^[3] also called young T Tauri stars or evolved protostars.^[3] By this stage the forming star has already accreted much of its mass: the total mass of the disk and remaining envelope does not exceed 10-20% of the mass of the central YSO.^[18] Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen (~154 PPM). ... Drawing of a T-Tauri star with a circumstellar accretion disk T Tauri stars are a class of variable stars named after their prototype - T Tauri. ...

At the next stage of evolution, the envelope disappears, having been accreted by the disk, and the star (or protostar) becomes a classical T Tauri star. This happens after about 1 million years.^[2] It should be noted that a T Tauri star is a young star with mass less than about 2.5 solar masses showing a heightened level of stellar activity. T Tauri stars are divided into two classes: weakly lined and classical. Weakly lined T Tauri stars do not possess accretion disks; in contrast, classical T Tauri stars have accretion disks and continue to accrete hot gas, which manifests itself by strong emission lines in the spectrum. Classical T Tauri stars evolve into weakly lined T Tauri stars.^[22] The mass of the remaining disk is about 1–3% of the stellar mass, and it is accreted onto the star at the rate of between a 10 milionth to one billionth a solar mass per year.^[23] A pair of bipolar jets is usually present as well.^[24] The accretion explains all peculiar properties of the classical T Tauri stars: strong flux in the emission lines (up to 100% of the intrinsic luminosity of the star), magnetic activity, photometric variability and jets.^[25] The emission lines actually form as the accreted gas hits the "surface" of the star, which happens around its magnetic poles.^[25] The jets are byproducts of accretion: they carry away excessive angular momentum. The classical T Tauri stage lasts about 10 million years.^[2] The disk eventually disappears due to accretion onto central star, planet formation, ejection by jets and photoevaporation by UV-radiation from the cental star and nearby stars.^[26] As a result the young star becomes a weakly lined T Tauri star, which slowly, over timeframe of hundreds of millions of years, evolves into an ordinary sun-like star.^[19] Drawing of a T-Tauri star with a circumstellar accretion disk T Tauri stars are a class of variable stars named after their prototype - T Tauri. ... The Boomerang Nebula is an excellent example of a bipolar outflow. ... flux in science and mathematics. ... A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ... This article does not cite any references or sources. ... In physics, magnetism is a phenomenon by which materials exert an attractive or repulsive force on other materials. ... Look up activity in Wiktionary, the free dictionary. ... Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical objects electromagnetic radiation. ... In computer science and mathematics, a variable is a symbol denoting a quantity or symbolic representation. ... Magnetic lines of force of a bar magnet shown by iron filings on paper A magnet is an object that has a magnetic field. ... This gyroscope remains upright while spinning due to its angular momentum. ...

Protoplanetary disks

See also: Protoplanetary disk

See also: planetesimal

A protoplanetary disk forming in the Orion Nebula

Under some circumstances, the disk, which can now be called protoplanetary, gives birth to a planetary system.^[2] As was shown above, protoplanetary disks are ubiquitous around all Sun-like stars.^[27][4] They exist from the beginning of a star's formation, but at the earliest stages are unobservable due to the opacity of the surrounding envelope.^[3] At the stage of Class 0 protostar the disk is thought to be massive and hot. It is an accretion disk, which feeds the central protostar.^[11][20] The temperature can easily exceed 400 K inside 5 AU and 1000 K inside 1 AU.^[28] The heating of the disk is primarily caused by the vicious dissipation of turbulence in it and by the infall of the gas from the nebula.^[11][20] The high temperature in the inner disk causes most of the volatile material (water, organics, and even some rock) to evaporate, leaving only the most refractory elements like iron. Volatile dust can survive only in the outer part of the disk.^[28] A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. ... Image File history File links M42proplyds. ... Image File history File links M42proplyds. ... The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated south of Orions Belt. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... An artists concept of a planetary system A planetary system consists of the various non-stellar objects orbiting a star such as planets, moons, asteroids, meteoroids, comets, and cosmic dust. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... Look up opacity in Wiktionary, the free dictionary. ... A Protostar is an object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. ... In astrophysics, the term accretion is used for at least two distinct processes. ... For other uses, see Kelvin (disambiguation). ... For other uses, see Viscosity (disambiguation). ... A wave that loses amplitude is said to dissipate. ... In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. ... For other uses, see Temperature (disambiguation). ... Volatiles are that group of compounds with low boiling points (see volatile) that are associated with a planets or moons crust and/or atmosphere. ... This article is about the geological substance. ... The term refractory can refer to multiple things: A refractory clergyman is one who refused to swear an oath to the French Revolution-era French state under the Civil Constitution of the Clergy. ... General Name, symbol, number iron, Fe, 26 Chemical series transition metals Group, period, block 8, 4, d Appearance lustrous metallic with a grayish tinge Standard atomic weight 55. ...

The main problem in the physics of accretionary disks is the generation of turbulence and the mechanism responsible for the high effective viscosity.^[2] The turbulent viscosity is thought to be responsible for the transport of the mass to the central protostar and momentum to the periphery of the disk. This transport is vital for accretion, because gas can be accreted by the central protostar only if it losses most of its angular momentum.^[11] Since the momentum is conserved, a part of the gas must drift outward carrying the excesive momentum.^[11][10] The result of this transport is the growth of both the protostar and of the disk radius, which can reach 1000 AU if the initial angular momentum of the nebula is large enough.^[20] Large disks are routinely observed in many star-forming regions^[5] such as the Orion nebula. For other uses, see Viscosity (disambiguation). ... This gyroscope remains upright while spinning due to its angular momentum. ... This article is about an authentication, authorization, and accounting protocol. ... The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated south of Orions Belt. ...

The lifespan of the disk is about 10 million years.^[4] By the time the star reaches the classical T-Tauri stage, the disk becomes thinner^[23] and cools. Less volatile materials start to condense in the inner disk forming dust grains. These grains have size of 0.1-1 μm and contain crystalline silicates.^[6] The transport of the material from the outer disk can mix these newly formed dust grains with primordial ones, which contain organic matter and other volatiles. This mixing can explain some peculiarities in the composition of solar system bodies. I.e. the presence of interstellar grains in the primitive meteorites and comets.^[28] Volatility most frequently refers to the standard deviation of the change in value of a financial instrument with a specific time horizon. ... For other uses, see Condensation (disambiguation). ... Crystal (disambiguation) Insulin crystals A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... In chemistry, a silicate is a compound containing an anion in which one or more central silicon atoms are surrounded by electronegative ligands. ... “Space dust” redirects here. ... The introduction of this article does not provide enough context for readers unfamiliar with the subject. ... This article is about the Solar System. ... The interstellar medium (or ISM) is a term used in astronomy to describe the rarefied gas and dust that exists between the stars (or their immediate circumstellar environment) within a galaxy. ... Willamette Meteorite A meteorite is a natural object originating in outer space that survives an impact with the Earths surface without being destroyed. ...

In the dense disk environment dust particles tend to stick to each other, leading to the formation of larger particles up to several centimeters in size.^[29] The signatures of the dust processing and coagulation are observed in the infrared spectra of the young disks.^[6] Further evolution of the dust particles can lead to the formation of planetesimals with a size of 1 km or larger, which are building blocks of planets.^[29][2] Planetesimal formation is another unsolved problem of disk physics, because simple sticking becomes ineffective as dust particles grow larger.^[12] The favorite hypothesis is formation by the gravitational instability. Particles several centimeters in size or larger slowly settle near the middle plane of the disk. This middle layer can become very thin (less than 100 km) and dense. This layer is gravitationally unstable and will fragment into numerous clumps, which will collapse into planetasimals.^[12][2] This article is about the clotting of blood. ... Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. ... This article is about the astronomical term. ... It has been suggested that Jeans mass be merged into this article or section. ...

Planetary formation can also be triggered by gravitational instability within the disk itself, which leads to its fragmentation into clumps. Some of these clumps, if they are dense enough, can collapse, forming gas giant planets.^[10] Gravitational instability can cause rapid formation of gas giant planets and even brown dwarfs at the timescale of 1000 years.^[30] However it is only possible in massive disks (more massive than 0.3 solar masses). In comparizon typical disk masses are 0.01–0.1 solar masses. Because massive disks are rare, this mechanism of the planet formation is thought to be infrequent.^[9][2] Fragmentation is a term that occurs in several fields and describes a process of something breaking or being divided into pieces (fragments). ... This article or section does not cite its references or sources. ... This article does not cite any references or sources. ... This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. ...

The ultimate dissipation of protoplanetary disks is triggered by a number of different mechanisms. The inner part of the disk is either accreted by the star or ejected by the bipolar jets,^[23][24] whereas the outer part can evaporate under the star's UV radiation during its T Tauri stage^[31] or by nearby stars.^[26] The gas in the central part can either be accreted or ejected by the growing planets, while the small dust particles are ejected by the radiation pressure of the central star. What is finally left is either a planetary system, a remnant disk of dust without planets, or nothing, if planetesimals failed to form.^[2] A wave that loses amplitude is said to dissipate. ... The Boomerang Nebula is an excellent example of a bipolar outflow. ... For other uses, see Ultraviolet (disambiguation). ... For other uses, see Radiation (disambiguation). ... Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. ... An artists concept of a planetary system A planetary system consists of the various non-stellar objects orbiting a star such as planets, moons, asteroids, meteoroids, comets, and cosmic dust. ... Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. ...

Because planetesimals are so numerous, and spread throughout the protoplanetary disk, some survive the formation of a planetary system. Asteroids are understood to be left-over planetesimals, gradually grinding each other down into smaller and smaller bits, while comets are typically planetesimals from the farther reaches of a planetary system. Meteorites are samples of planetesimals that reach a planetary surface, and provide a great deal of information about the formation of our Solar System. Primitive-type meteorites are chunks of shattered low-mass planetesimals, where no thermal differentiation took place, while processed-type meteorites are chunks from shattered massive planetesimals. Only the largest planetesimals survive these high-energy collisions with lower-mass planetesimals, and can continue to grow.^[32] For other uses, see Asteroid (disambiguation). ... Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ... Willamette Meteorite A meteorite is a natural object originating in outer space that survives an impact with the Earths surface without being destroyed. ... In cosmogony, planetary differentiation is a process by which the denser portions of a planet will sink to the center; while less dense materials rise to the surface. ...

Formation of planets

Rocky planets

Rocky planets form in the inner part of the protoplanetary disk, where temperature is high enough to prevent condensation of water and other ices.^[33] This results in coagulation of purely rocky grains and later in the formation of rocky planetesimals.^[33] Such conditions are thought to exist in the inner 3–4 AU part of the disk of a sun-like star.^[2]

After planetesimals (about 1 km in diameter) have formed by one way or another, runaway accretion begins.^[7] It is called runaway because the mass growth rate is proportional to R⁴~M^4/3, where R and M are the radius and mass of the growing body, respectively.^[34] It is obvious that the specific (divided by mass) growth accelerates as the mass increases. Such accretion leads to preferential growth of larger bodies at the expense of smaller ones.^[7] The runaway accretion lasts between ten and a hundred thousand years and ends when the largest bodies exceed approximately 1000 km in diameter.^[7] Slowing of the accretion is caused by gravitational perturbations by large bodies on the remaining planetesimals.^[34][7] In addition, influences of large bodies at this stage prevents further growth of any small bodies.^[7]

The next stage is called oligarchic accretion.^[7] It is characterized by the dominance of several hundred of the largest bodies – oligarchs, which continue to slowly accrete planetesimals.^[7] No bodies other than the oligarchs can grow.^[34] At this stage the rate of accretion is proportional to R², i.e. to the cross-section of an oligarch. ^[34] The specific accretion rate is proportional to M^-1/3, i.e. it declines with the mass of the body. This allows smaller oligarchs to catch up to larger ones. The oligarchs are kept at the distance of about 10·H_r (H_r=(M/3M_s)^1/3 is Hill radius and M_s is the mass of Sun) from each other by the influence of the remaining planetesimals.^[7] Their orbital eccentricities and inclinations remain small. The oligarchs continue to accrete until planetesimals are exhausted in the disk around them.^[7] The final mass of an oligarch depends on the distance from the star and surface density of planetesimals and is called the isolations mass.^[34] For the rocky planets it is up to 0.1 of the Earth mass, or one Mars mass.^[2] The final result of the oligarchic stage is the formation of about 50 planetary embryos uniformly spaced at about 10·H_r.^[8] They are thought to reside inside gaps in the disk and to be separated by rings of remaining planetesimals. This stage is thought to last a few 10⁵ years.^[7][2]

The last stage of rocky planet formation is the merger stage.^[2] It begins when only a small number of planetesimals remains and embryos become massive enough to perturb each other, which causes their orbits to become chaotic.^[8] During this stage embryos expel remaining planetesimals, and collide with each other. The result of this process, which lasts for ten to a hundred million years, is the formation a limited number of Earth sized bodies. Simulation show that the number of surviving planets is on average from 2 to 5.^{[35][32][8][2]} In the Solar System they may be represented by Earth and Venus.^[8] Formation of both planets required merging of approximately 10 embryos, while the equal number of them were thrown out of the Solar System.^[32] Some the embryos, which originated in the asteroid belt, are thought to have brought water to Earth.^[33] Mars and Mercury may be regarded as remaining embryos that survived that rivalry.^[32] Rocky planets that have managed to coalesce settle eventually into more or less stable orbits, explaining why planetary systems are generally packed to the limit; or in other words why they always appear to be at the brink of instability.^[8]

Giant planets

The dust disk around Fomalhaut—the brightest star in Piscis Austrini constelation. Asymmetry of the disk may be caused by a giant planet (or planets) orbiting the star.

The formation of giant planets is an outstanding problem in the planetary sciences.^[9] In principle two possibilities for their formation exist. The first one is a disk instability model, where giant planets form in the massive protoplanetary disks as a result of its gravitational fragmentation (see above).^[30] The disk instability may also lead to the formation of brown dwarfs, which are usually classified as stars. The second possibility is a core accretion model, which is also known as a nucleated instability model.^[9] The latter scenario is thought to be the most promising one, because it can explain the formation of the giant planets in relatively low mass disks (less than 0.1 solar masses). In this model, giant planet formation is divided into two stages: a) accretion of a core of approximately 10 Earth masses and b) accretion of gas from the protoplanetary disk.^[9][2] This article is about the star. ...

Giant planet core formation is thought to proceed roughly along the lines of terrestrial planet formation.^[7] It starts with planetesimals, which then undergo the runaway growth followed by the slower oligarchic stage.^[34] Hypotheses do not predict a merger stage, due to the low probability of collisions between planetary embryos in the outer parts of planetary systems.^[34] An additional difference is the composition of the planetesimals, which in the case of giant planets form beyond the so called snow line and consist mainly of ice (ice to rock ratio is about 4 to 1).^[13] This enhances the mass of planetesimals four times. However the minimum mass nebular, which is capable of terrestrial planet formation, can only form 1-2 Earth mass cores at the distance of Jupiter (5 AU) within ten million years.^[34] The latter number represents an averages lifetime of gaseous disks around sun-like stars.^[4] The proposed solutions include enhanced mass of the disk (a tenfold increase would suffice);^[34] protoplanet migration, which allows the embryo to accrete more planetesimals;^[13] and finally accretion enhancement due to gas drag in the gaseous envelopes of the embryos.^[13][36] Some combination of the above-mentioned ideas may explain the formation of the cores of gas giant planets such as Jupiter and perhaps even Saturn.^[9] The formation of planets like Uranus and Neptune is more problematic, since no theory has been capable of providing for the in situ formation of their cores at the distances of 20-30 AU from the central star.^[2] To resolve this issue an idea has been brought forward that they initially accreted in the Jupiter-Saturn region and then were scattered and migrated to their present location.^[37] In astronomy or planetary physics, the frost line refers to a particular distance in the solar nebula from the central protosun where it is cool enough for hydrogen compounds such as water, ammonia, and methane to condense into solid ice grains. ...

Once the cores are of sufficient mass (5–10 Earth masses), they begin to gather gas from the surrounding disk.^[2] Initially it is a slow process, which can increase the core masses up to 30 earth masses in a few million years.^[13][36] After that the accretion rates increase dramatically and the remaining 90% of the mass is accumulated in 10⁴ years.^[36] The accretion of the gas stops, when it is exhausted, i.e. when a gap opens in the protoplanetary disk.^[14] In this model ice giants – Uranus and Neptune are failed cores that began gas accretion too late, when almost all gas had already disappeared. The post runaway gas accretion stage is characterized by migration of the newly formed giant planets and continued slow gas accretion.^[14] Migration is caused by the interaction of the planet sitting in the gap with the remaining disk. It stops, when the protoplanetary disk disappears or when the end of the disk is attained. The latter case corresponds to the so called hot Jupiters, which are likely to have stopped their migration, when they reached inner hole in the protoplanetary disk.^[14] Artists impression of roaster extrasolar planet HD 209458b (Osiris). ...

Giant planets can significantly influence terrestrial planet formation. The presence of giants tends to increase eccentricities and inclinations of planetesimals and embryos in the terrestrial planet region (inside 4 AU in the Solar System).^[32][35] On the one hand, if giant planets form too early they can slow or prevent inner planet accretion. On the other hand, if giant planets form near the end of the oligarchic stage, as is thought to have happened in the Solar System, they will influence the merges of planetary embryos making them more violent.^[32] As a result the number of terrestrial planets will decrease and they will be more massive.^[38] In addition, the size of the system will shrink, i.e. terrestrial planets will form closer to the central star. In the Solar System the influence of giant planets (particularly of Jupiter) is thought to have been limited because they are relatively remote from the terrestrial planets (inside 2 AU).^[38] The inner planets, Mercury, Venus, Earth, and Mars, their sizes to scale. ... Look up Eccentricity in Wiktionary, the free dictionary. ... Inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit and is the angular distance of the orbital plane from the plane of the reference (usually planets equator or the ecliptic), stated in degrees. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ...

The region of a planetary system adjacent to the giant planets will be influenced in a different way.^[35] In such a region eccentricities of embryos may become so large that they may pass close to a giant planet. As a result they may (and probably will) be thrown out of the planetary system.^[39][32][35] If all embryos are removed then no planets will form in this region.^[35] An additional consequence is that a huge number of small planetesimals will remain, because giant planets along without help from embryos are incapable of clearing them all out. Although the total mass of remaining planetesimals will be small, because cumulative action of the embryos before their ejection and giant planets is still strong enough to remove 99% of the small bodies.^[32] Such a region will eventually evolve in an asteroid belt, which is a full analog of the main asteroid belt in the Solar System, which is located from 2 to 4 AU from the Sun.^[32][35] Metroplex (in shadow) and Giant Planet Gigantion, or Giant Planet, is a fictional planet home to giant Transformers in the animated television program, Transformers: Cybertron; it is referred to as Gigalonia in Transformers: Galaxy Force, the Japanese version of the show. ... Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. ... For other uses, see Asteroid (disambiguation). ...

Meaning of accretion

Use of the term accretion disk for the protoplanetary disk leads to confusion over the planetary accretion process. The protoplanetary disk is sometimes referred to as an accretion disk, because while the young T Tauri-like protosun is still contracting, gaseous material may still be falling onto, accreting on its surface from the disk's inner edge.^[20] An accretion disc (or accretion disk) is a structure formed by material falling into a gravitational source. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... An accretion disc (or accretion disk) is a structure formed by material falling into a gravitational source. ... Drawing of a T-Tauri star with a circumstellar accretion disk T Tauri stars are a class of variable stars named after their prototype - T Tauri. ...

However, that meaning should not to be confused with the process of accretion forming the planets. In this context, accretion refers to the process of cooled, solidified grains of dust and ice orbiting the protostar in the protoplanetary disk, colliding and sticking together and gradually growing, up to and including the high energy collisions between sizable planetesimals.^[7] A Protostar is an object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. ...

In addition, the giant planets probably had accretion disks of their own, in the first meaning of the word. The clouds of captured hydrogen and helium gas contracted, spun up, flattened, and deposited gas onto the surface of each giant protoplanet, while solid bodies within that disk accreted the giant planet's regualar moons.^[40] Metroplex (in shadow) and Giant Planet Gigantion, or Giant Planet, is a fictional planet home to giant Transformers in the animated television program, Transformers: Cybertron; it is referred to as Gigalonia in Transformers: Galaxy Force, the Japanese version of the show. ... An accretion disc (or accretion disk) is a structure formed by material falling into a gravitational source. ... Protoplanets are moon-sized planet embryos within protoplanetary discs. ...

See also

• Formation and evolution of the Solar System
• History of Earth
• Asteroid Belt, Kuiper Belt, and Oort Cloud
• Bok globule, Herbig-Haro object
• T Tauri star

The theories concerning the formation and evolution of the Solar System are complex and varied, interweaving various scientific disciplines, from astronomy and physics to geology and planetary science. ... Geological time put in a diagram called a geological clock, showing the relative lengths of the eons of the Earths history. ... For other uses, see Asteroid (disambiguation). ... The Kuiper belt, derived from data from the Minor Planet Center. ... Artists rendering of the Oort cloud and the Kuiper Belt. ... An image of Bok globules in the H II region IC 2944, taken with the WFPC2 instrument on the Hubble Space Telescope A Bok globule is a dark cloud of dense dust and gas in which star formation is sometimes taking place. ... Herbig-Haro object HH47, imaged by the Hubble Space Telescope. ... Drawing of a T-Tauri star with a circumstellar accretion disk T Tauri stars are a class of variable stars named after their prototype - T Tauri. ...

References