A planetary disk forming in the Orion Nebula.

In cosmogony, the solar nebula is believed to be a gaseous cloud from which Earth's solar system formed. This 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. A similar model was proposed in 1796 by Pierre-Simon Laplace. These Can be consiDered early theories of cosmology. 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. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... This article or section does not cite its references or sources. ... Cumulus mediocris clouds, as seen from a plane window. ... Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... The planets of the solar system are believed to have formed out of a collapsed and spinning cloud of gas and dust. ... Emanuel Swedenborg, 75, holding the manuscript of Apocalypsis Revelata (1766). ... 1755 was a common year starting on Wednesday (see link for calendar). ... Immanuel Kant (22 April 1724 – 12 February 1804), was a German philosopher from Königsberg in East Prussia (now Kaliningrad, Russia). ... 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. ... STAR is an acronym for: Organizations Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astronomy club. ... The eight planets and three dwarf planets of the Solar System. ... 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). ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

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 200 extrasolar planets have since been discovered in our galaxy. Major features of the Solar System (not to scale; from left to right): Pluto, Neptune, Uranus, Saturn, Jupiter, the asteroid belt, the Sun, Mercury, Venus, Earth and its Moon, and Mars. ... The Universe is defined as the summation of all particles and energy that exist and the space-time in which all events occur. ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. ...

Overview of the solar nebula hypothesis

The original nebula

The hypothesis maintains that a planetary system begins as a large (typically ~10,000 AU in diameter), roughly spherical cloud of very cold interstellar gas, part of a larger molecular cloud. Such a nebula is just dense enough to begin contracting under the force of its own gravity, and its collapse may have been initiated by a pressure wave from a nearby event (such as a shock wave from a supernova) compressing the molecular cloud. The composition of such a nebula will reflect the composition of the resulting star; for our own Solar System the Solar Nebula is believed to have been comprised of about 98% (by mass) hydrogen and helium present since the Big Bang, and 2% heavier elements created by earlier generations of stars which died and ejected them back into interstellar space (see nucleosynthesis). The fraction of heavier elements is known as the cloud's metallicity; statistically, stars with larger metallicities (i.e. that formed from a cloud with more heavy elements) are more likely to possess planets. Once begun, the gravitational contraction of the solar nebula accelerates slowly but inevitably. The astronomical unit (AU or au or a. ... This article or section does not cite its references or sources. ... A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2). ... Gravity is a force of attraction that acts between bodies that have mass. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Standard atomic weight 4. ... According to the Big Bang model, the universe emerged from an extremely dense and hot state. ... Nucleosynthesis is the process of creating new atomic nuclei from preexisting nucleons (protons and neutrons). ... The globular cluster M80. ...

As it collapses, three physical processes shape the nebula: it heats up, its spin increases, and it flattens. The nebula heats up because atoms move more quickly as they fall deeper into the gravitational well and become denser, colliding more frequently: gravitational potential energy is converted to kinetic energy of the atoms, or thermal energy. Second, while initially imperceptible, the solar nebula had some small amount of net rotation (angular momentum), and because angular momentum is conserved, the nebula must rotate more quickly as it shrinks in size. Finally, the nebula must also flatten into a disk, called a protoplanetary disk, as collisions and mergers of blobs of gas average out their motions in favor of the direction of the net angular momentum. In physics, density is mass m per unit volume V. For the common case of a homogeneous substance, it is expressed as: where, in SI units: ρ (rho) is the density of the substance, measured in kg·m-3 m is the mass of the substance, measured in kg V is... Potential energy is the energy that is by virtue of the relative positions (configurations) of the objects within a physical system. ... The kinetic energy of an object is the extra energy which it possesses due to its motion. ... 1. ... This gyroscope remains upright while spinning due to its angular momentum. ... This gyroscope remains upright while spinning due to its angular momentum. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ...

The protostar

Main article: protostar

At the center of the solar nebula's gravity accumulates an increasingly dense protostar. During the process of planet formation in the disk, the protostar gradually compacts further, until after about 10-50 million years, it finally reaches the conditions of temperature and pressure needed to initiate hydrogen nuclear fusion, and a star is born. A young star of this kind (a T Tauri star) produces a stellar wind, much stronger than that of a fully formed star, which eventually blows the remaining gases out of the disk, and largely ending the accretion process (particularly for any gas giants). Like most processes in a star's life, the time spent in the protostar phase depends on mass: massive stars collapse more quickly. A Protostar is an object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. ... A Protostar is an object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ... STAR is an acronym for: Organizations Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astronomy club. ... 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 plasma in the solar wind meeting the heliopause For the British comic, see Solar Wind (comic). ... This article or section does not cite any references or sources. ...

The gas in the protoplanetary disk, meanwhile, gradually cools from the gravitational heating of its collapse, and as it cools, dust (metals and silicates) and ice (hydrogen compounds such as water, methane, and ammonia) grains condense out of the gas (solidify). These grains gently bump into neighboring grains (collide) and stick together electrostatically, beginning the accretion process. Gas atoms and molecules are present in great abundance, but cannot be accreted, because they are moving too quickly to be held electrostatically. Hydrogen and helium, 98% of the mass of the disk, remain gaseous throughout the solar nebula, never condensing. Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... Methane is a chemical compound with the molecular formula CH4. ... Ammonia is a compound with the formula NH3. ... Water vapor condensing over a cup of hot tea Condensation is the change in matter of a substance to a denser phase, such as a gas (or vapor) to a liquid. ...

Planetesimals

Main article: planetesimal

The solid component of the disk is initially in the form of microscopic dust grains that seeded the precursor cloud; such grains in the interstellar medium are typically less than a micron in diameter, but through collisions in the protoplanetary disk they stick together and grow in size to become planetesimals (literally meaning an infinitely small planet). This dust is initially spread throughout the disk, but is expected to rain out into the disk midplane: just as the initial molecular cloud collapses under gravity into a disk, so the grains sink to the midplane but cannot move radially towards the protostar without losing angular momentum. Dust grains of different sizes fall down at different speeds, gathering more dust along the way.^[2] Larger grains may grow faster by clumping together randomly to produce fractal structures;^[3] such arrangements have more surface area for other grains to bump against and stick to. A population of large, fluffy grains may also inhibit the effects of gas drag,^[4] which could otherwise cause the solids to migrate towards the new-formed star before planets can form. Fast collisions may shatter forming planetesimals, meaning the transition from dust to planetesimal is reversible. Turbulence in the disk may play a role in these collisions: if the turbulence is too violent, rainout into the midplane may be hindered, and destructive collisions between particles may be more common. Once planetesimals become sufficiently massive, their gravity helps bring more grains into contact,^[5] yet strong turbulence may also prevent this gravitational clumping, leading to growth through binary collision only. Nontheless, if gas giants are to form then planetesimals of about 1 km across must form within around 10,000 years.^[6] Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. ... The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. ... A micrometre (American spelling: micrometer, symbol µm) is an SI unit of length equal to one millionth of a metre, or about a tenth of the diameter of a droplet of mist or fog. ... Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. ... The boundary of the Mandelbrot set is a famous example of a fractal. ... In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. ... This article or section does not cite any references or sources. ... KM, Km, or km may stand for: Khmer language (ISO 639 alpha-2, km) Kilometre Kinemantra Meditation Knowledge management KM programming language KM Culture, Korean Movie Maker. ...

Because planetesimals are so numerous, and spread throughout the protoplanetary disk, many 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 gravitational 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. It has been suggested that minor planet be merged into this article or section. ... Comet Hale-Bopp Comet McNaught as seen from Swifts Creek, Victoria, Australia on 23 January 2007 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. ...

Oligarchic growth

As the planetesimals grow, they decrease in number and collisions become less frequent. Due to the stochastic nature of growth, not all planetesimals grow at the same rate and some will become more massive than others. As the planetesimals orbit the new star, dynamical friction keeps the energies (momentum) of the planetesimal population evenly distributed, so that the largest bodies possess the slowest velocities, typically orbiting in near-circular fashion, while the smaller planetesimals move much faster, on more eccentric orbits. Significantly, slower-moving bodies possess a larger collisional cross-section, the radius over which gravity can enhance a planetesimal's ability to capture another planetesimal. Consequently, the slower, more massive bodies are more effectively able to accrete the surrounding planetesimals, while the faster-moving, smaller bodies hardly grow at all. This quickly leads to a runaway process, where the largest bodies in each region of the disk come to dominate, growing much larger than the surrounding "planetesimal sea".^[7][8] These massive bodies now completely dominate the solid material in the disk and are called oligarchs, meaning the few that rule; the process is known as oligarchic growth.^[9] These few planetesimals rapidly increase in size, increasing from a few tens of kilometres across before oligarchic growth begins, to several hundred and eventually several thousand kilometres in diameter. Stochastic, from the Greek stochos or goal, means of, relating to, or characterized by conjecture; conjectural; random. ... Dynamical friction is a term in astrophysics related to loss of momentum and kinetic energy of moving bodies through a gravitational interaction with surrounding matter in space. ... In classical mechanics, momentum (pl. ... m. ... (This page refers to eccitricity in astrodynamics. ... There are very few or no other articles that link to this one. ... Forms of government Part of the Politics series Politics Portal This box:      Oligarchy (Greek , Oligarkhía) is a form of government where political power effectively rests with a small, elite segment of society (whether distinguished by wealth, family or military prowess). ...

The process of oligarchic growth is self-limiting: each oligarch has a limited feeding zone (determined by its collisional cross-section), and can grow no further once all the planetesimals within it have been accreted. It is doubtful whether these zones contain enough solids for the oligarchs to grow to terrestrial masses, suggesting the planetesimals' growth should stall at a few hundred kilometres in size.^[10] However, it is possible that turbulence once again plays a part, as it can deposit or take away angular momentum from the planetesimals, providing a random component of radial motion. This may provide a steady influx of new material to the feeding zones, allowing the oligarchs to continue with their growth.^[11] The inner planets, Mercury, Venus, Earth, and Mars, their sizes to scale. ... This gyroscope remains upright while spinning due to its angular momentum. ...

One way or another the oligarchs continue to grow, until (interior to the frost line) they have reached masses of typically 0.05-1 Earth mass within a million years or so,^[12] large enough to be considered protoplanets; bodies in the outer disk will grow larger still, due to the higher density of solids available for growth. The terrestrial planetary region now likely consists of a few dozen widely spaced oligarchs,^[9] dynamically isolated and unlikely to collide for hundreds of thousands or even millions of years. 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. ... In cosmogony, a protoplanet is a quasi-planetoid which is slightly larger than a planetesimal and orbits within a solar nebulas protoplanetary discs. ...

Non-uniform temperatures

The temperature in a protoplanetary disk is not uniform, and this is the key to understanding the differentiation between terrestrial and jovian planet formation. Inside the frost line, the temperature is too high (above 150 K) for hydrogen compounds to condense: they remain gaseous. The only grains available for accretion, then, are the heavier metal and silicate dust grains. Thus the planetesimals in this region are composed entirely of rock and metal, such as the asteroids, and make up the terrestrial planets. 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. ... The kelvin (symbol: K) is a unit increment of temperature and is one of the seven SI base units. ...

Outside the frost line, hydrogen compounds such as water, methane and ammonia are able to solidify into 'ice' grains, and accrete. Rock and metal grains are also available, but are vastly outnumbered (and outweighed) by the hydrogen compounds, which are much more abundant everywhere. Thus the planetesimals in this region are icy bodies with small amounts of rock and metal mixed in. The Kuiper Belt and Oort Cloud objects, comets, Neptune's huge moon Triton, and probably Pluto and its moon Charon, are all examples of these 'dirty snowball' planetesimals. Due to the greater amount of solid materials available, as well as less frequent collisions and lower velocities (being in much larger orbits), the largest of these planetesimals grow so massive (about 10 times the mass of the Earth) their gravity begins to collect and retain helium and even hydrogen gases. Once that starts, they grow rapidly, as hydrogen and helium are 98% of the disk, and collecting these gases increases their mass and consequently the size of their gravitational net. Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... This image is an artists rendering of the Oort cloud and the Kuiper Belt. ... Adjectives: Neptunian Atmosphere Surface pressure: ≫ 100 kPa (cloud level) Composition: 80% ± 3. ... Triton (trye-tÉ™n, IPA: , Greek Τρίτων), or Neptune I, is the planet Neptunes largest moon. ... Adjectives: Plutonian Atmosphere Surface pressure: 0. ... Charon (shair-É™n or kair-É™n (key), IPA , Greek Χάρων), discovered in 1978, is, depending on the definition employed, either the largest moon of Pluto or one member of a double dwarf planet with Pluto being the other member. ...

Jovian planetesimals

Soon the jovian planetesimals are nothing like the icy bodies they came from, but are more or less dominated by the hydrogen and helium gas they have captured, huge gaseous clouds with dense cores. These jovian gas balls then, in close analogy to the solar system itself, gradually collapse gravitationally, heating up, rotating more quickly, and flattening. Some moons of the jovian planets may be formed in an analogous process to the planets themselves, coalescing from condensed grains in the disks which formed as the gas giant protoplanet collapsed. This can explain why, in our own Solar System, the jovian planets all have many moons and rings in the same plane, and why jovian planets rotate quickly. The growth of the jovians ends when the young star's strong stellar wind blows the remaining gas and dust out of the disk and into interstellar space. A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. ... This article or section does not cite any references or sources. ... A solar wind is a stream of particles (mostly high-energy protons ~ 500 keV) which are ejected from the upper atmosphere of a star (in the case of a star other than the Earths Sun, it may be called a stellar wind instead). ...

In the simplest possible terms, the innermost giant protoplanetary core forms where the disk density is highest and dynamical times (the typical timescale for collisions) are shortest; hence this body reaches the critical mass for gas capture earliest, and in the densest regions of the disk, and so has longest to accrete the surrounding gas. In our own Solar System Jupiter was the largest protoplanetary core beyond the frost line, and so fulfilled this role, becoming the largest planet in that system. In reality, the process may be more complicated, with planetary migration and turbulence muddying this picture; compared to the extrasolar planetary systems observed to date, the distribution of the planets in our own system may even be considered somewhat unusual. 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. ... Planetary migration is the act of a stellar satellite altering its orbital parameters, especially semi-major axis, through various means during its lifetime. ... In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. ...

Giant Impacts

Finally, after the stellar wind has cleared the gas out of the disk, a large population of protoplanets and planetesimals may be left over. Over a period of 10-100 million years, these protoplanets - typically with a mass between that of the Moon and several Earths - will perturb each other until orbit-crossing occurs, leading to collisions. The bodies resulting from these collisions will be the final planets of that system. Such a collision, between the proto-Earth and a Mars-sized protoplanet, is believed to have formed the Earth's moon. The process is highly random; a forming terrestrial system near-identical to that which produced the inner planets may easily end with fewer or more planets than we observe in our Solar System. In the solar system the inner planets are the solid planets nearest the Sun: Mercury, Venus, Earth and Mars. ...

The smaller planetesimals, being vastly more numerous, remain within the planetary system for much longer. These bodies may be swept up by the planets that have formed in a process known as "clearing the neighbourhood", either by slinging them in the distant outer reaches of the system (the Oort Cloud in our Solar System), or continually nudging their orbits into collisions or stable orbits with other planets. This period of bombardment lasts several hundred million years, and may leave evidence of cratering which is still visible on geologically dead bodies. In some respects, as long as there are small rocky or icy bodies available to the system, it may be argued that this stage of formation never really 'finishes', as the threat of asteroid collisions with Earth or the recent impact of comet Shoemaker-Levy 9 upon Jupiter ably demonstrates. This article or section may be confusing or unclear for some readers, and should be edited to rectify this. ... This image is an artists rendering of the Oort cloud and the Kuiper Belt. ... Hubble Space Telescope image of Comet Shoemaker-Levy 9, taken on May 17, 1994. ...

In our own Solar System it is believed this episode of formation was exceptionally strong due to a 2:1 resonance crossing of Jupiter and Saturn, catastrophically disturbing a large outer planetesimal disk, and the process is known as the Late Heavy Bombardment. The Late Heavy Bombardment (LHB) was a period approximately 3. ...

Solar system features explained by theory

The nebular hypothesis effectively explains all the major features of our solar system:

1. regular motions of the planets and moons (all revolve in the nearly same plane, in nearly circular orbits, in same direction the Sun rotates, and nearly all rotate in the nearly same direction too)
2. all major differences between terrestrial and jovian planets (mass, distance from Sun, composition, moon and ring systems)
3. small bodies (asteroids and comets, both short- and long-period)
4. exceptions to the trends (terrestrial moons, axial tilts, non-coplanar jovian moons, Triton)

Challenges to the hypothesis

The current challenges for the nebular hypothesis include explaining:

1. missing mass in Kuiper Belt
2. capture process for Triton
3. sideways rotation of Uranus
4. discovered "hot Jupiter" exoplanets
5. discovered exoplanets in binary and trinary stellar systems

Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... Triton (trye-tÉ™n, IPA: , Greek Τρίτων), or Neptune I, is the planet Neptunes largest moon. ... Adjectives: Uranian Atmosphere Surface pressure: 120 kPa (at the cloud level) Composition: 83% Hydrogen 15% Helium 1. ... Artists impression of roaster extrasolar planet HD 209458b (Osiris). ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ...

The meaning of accretion

Use of the term accretion disk for the protoplanetary disk leads to confusion over the planetary accretion process. 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. ...

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. 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 protosun in the protoplanetary disk, colliding and sticking together and gradually growing, up to and including the high energy collisions between sizable planetesimals. A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ...

In addition, the jovians probably had accretion disks of their own, in the first meaning of the word. The clouds of captured hydrogen and helium gas contract, spin up, flatten, and deposit gas onto the surface of each jovian protoplanet, while solid grains within that disk accrete into planetesimals and eventually forming the jovian moons. An accretion disc (or accretion disk) is a structure formed by material falling into a gravitational source. ...

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. ... The planet Earth, photographed in the year 1972. ... For details on the physical properties of bodies in the asteroid belt see Asteroid and Main-belt comet. ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... This image is an 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

1. ^ Swedenborg, Emanuel. 1734, (Principia) Latin: Opera Philosophica et Mineralia (English: Philosophical and Mineralogical Works), (Principia, Volume I)
2. ^ Weidenschilling S.J., (1980). "Dust to planetesimals - Settling and coagulation in the solar nebula". Icarus 44: 172-189. 
3. ^ Meakin P.; Donn B., (1988). "Aerodynamic properties of fractal grains - Implications for the primordial solar nebula". Astrophysical Journal, Part 2 - Letters 329: L39-L41. 
4. ^ Takeuchi T.; Clarke C. J.; Lin D. N. C., (2005). "The Differential Lifetimes of Protostellar Gas and Dust Disks". The Astrophysical Journal 627: 286-292. 
5. ^ Goldreich P.; Ward W.R., (1973). "The Formation of Planetesimals". Astrophysical Journal 183: 1051-1062. 
6. ^ Lissauer J. J., (1993). "Planet formation". Annual review of astronomy and astrophysics 31: 129-174. 
7. ^ Wetherill G. W.; Stewart G. R., (1989). "Accumulation of a swarm of small planetesimals". Icarus 77: 330. 
8. ^ Ohtsuki K.; Ida S., (1990). "Runaway planetary growth with collision rate in the solar gravitational field". Icarus 85: 499-511. 
9. ^ ^{a b} Kokubo E.; Ida S., (2000). "Formation of Protoplanets from Planetesimals in the Solar Nebula". Icarus 143: 15-27. 
10. ^ Lissauer J. J., (1987). "Timescales for planetary accretion and the structure of the protoplanetary disk". Icarus 69: 249-265. 
11. ^ Fogg M. J.; Nelson R. P., (2005). "Oligarchic and giant impact growth of terrestrial planets in the presence of gas giant planet migration". Astronomy & Astrophysics 441: 791-806. 
12. ^ Weidenschilling S. J.; Spaute D.; Davis D. R.; Marzari F.; Ohtsuki K., (1997). "Accretional Evolution of a Planetesimal Swarm". Icarus 128: 429-455. 

External links

• SPACE.com: Discovery Hints at a Quadrillion Space Rocks Beyond Neptune (Sara Goudarzi) 15 August 2006 06:13 am ET