File Dawn image 070911.jpg thumb A surviving protoplanet, 4 Vesta . Protoplanets are large planetary embryos that originate within protoplanetary disc s and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer sized planetesimal s that attract each other gravitationally and collide. According to solar nebula planet formation theory , protoplanets perturb each other s orbits slightly and thus collide to gradually form the dominant planet s. In the case of the Solar System it is thought that the collisions of planetesimals created a few hundred planetary embryos. Such embryos were similar to Ceres dwarf planet Ceres and Pluto with masses of about 10 sup 22 sup to 10 sup 23 sup kg and were a few thousand kilometers in diameter. Over the course of about 100 Annum SI prefix multipliers Ma they were involved in impacts with one another. The exact sequence whereby planetary embryos collided to assemble the planets is not known, but it is thought that initial collisions would have replaced the first generation of embryos with a second generation consisting of fewer but larger embryos. These in their turn would have collided ... that Earth s Moon formed from a colossal impact of a hypothetical protoplanet, named Theia planet Theia ... a protoplanet. ref cite web last first authorlink coauthors title BIG PIC 2 Pallas, the Asteroid ... protoplanet lutetia rosetta 111027.html doi accessdate 2011 10 27 ref Kuiper belt Kuiper belt ... Protoplanet frozen in time publisher MSNBC author Alan Boyle url http cosmiclog.msnbc.msn.com archive ... External links http tech.groups.yahoo.com group mpml message 25780 Thread on the definition of a protoplanet ... de Protoplanet es Protoplaneta fr Protoplan te id Protoplanet it Protopianeta lt Proplaneta ja no Protoplanet pl Protoplaneta pt Protoplaneta ru sk Protoplan ta fi Protoplaneetta sv Protoplanet zh ... more details
Thea may refer to Thea award , the annual award from the Themed Entertainment Association Thea TV series Thea TV series , a 1993 television series starring Thea Vidale and Brandy Norwood THEA Texas Higher Education Assessment , a test similar to the ACT test ACT and SAT Thea , the former name of the tea plant genus, now included in Camellia Thea, alias for singer songwriter Cynthia L. Stacey A girl s given name, short for Theodora disambiguation Theodora , Theadora, Dorothea disambiguation Dorothea , or Althea also spelled Elithea polytonic wikt , the Greek word for goddess , the feminine of Theos disambiguation theos An alternative spelling of Theia , a figure in Greek mythology Theia planet , a Mars sized protoplanet that may have been involved in the creation of the Moon THEA Tampa Hillsborough Expressway Authority , a regional expressway authority based in Hillsborough County, Florida, which includes the city of Tampa. See also Theia disambiguation Thia disambiguation disambig fr Thea homonymie it Thea disambigua ... more details
Refimprove date December 2009 In astrophysics , the term accretion is used for at least two distinct processes. The first and most common is the growth of a massive object by gravitation ally attracting more matter, typically gas eous matter in an accretion disc . ref cite web url http www.eso.org sci facilities paranal telescopes vlti science index.html title Science with the VLTI publisher European Southern Observatory date 2008 08 08 accessdate 2011 04 11 ref Accretion discs are common around smaller stars or stellar remnants in a binary star close binary , or black hole s in the centers of spiral galaxy galaxies . Some dynamics in the disc are necessary to allow orbiting gas to lose angular momentum and fall onto the central massive object. Occasionally, this can result in stellar surface fusion . See Bondi accretion The second process is somewhat analogous to the one in Accretion atmosphere atmospheric science . In the nebular theory , accretion refers to the collision and sticking of cooled microscopic dust and ice particles electrostatically, in protoplanetary disc s and Gas giant protoplanet systems, eventually leading to planetesimals which gravitationally accrete more small particles and other planetesimals. cn date December 2011 Use of the term accretion disc for the protoplanetary disc thus leads to confusion over the planetary accretion process, although in many cases it may well be that both accretion processes are happening simultaneously. T Tauri star T Tauri is an example of this phenomenon. Runaway accretion Expand section date April 2011 Example The Jovian protoplanets probably had discs of their own, in close analogy to the solar system as a whole. A Jovian protoplanet may accrete gas from its surrounding protoplanetary disc, as in the first process at the same time, dust and ice particles in the disc would accrete into moonlets and ring systems, as in the second process. See also Accretion disc Bondi accretion Nova Footnotes reflist colwidth 30e ... more details
refimprove date October 2009 The Chamberlin Moulton planetesimal hypothesis was proposed in 1905 by geologist Thomas Chrowder Chamberlin and astronomer Forest Ray Moulton to describe the formation of the solar system . It was proposed as a replacement for the Pierre Simon Laplace Laplacian version of the nebular hypothesis that had prevailed since the 19th century. The theory was based on the idea that a star passed close enough to the sun early in its life to cause tidal bulges to form on its surface, which along with the internal process that leads to solar prominence s, caused material to be ejected repeatedly from the sun. Due to the gravitational effects of the passing star, two spiral like arms would have extended from the sun, and while most of the material would have fallen back, part of it would remain in orbit. This orbiting material would cool and condense into numerous small bodies that they termed planetesimal s and a few larger protoplanet s. Their theory proposed that as these objects collided over time, the planets and their moons were built up, with comet s and asteroid s being the leftover debris. The spiral nebulae photographed at Lick Observatory were thought to possibly be views of other suns undergoing this process. These nebulae are now known to be galaxy galaxies rather than developing solar systems. In 1917, James Hopwood Jeans argued that only a very close approach of a second star was necessary to eject material, instead of requiring solar prominences. In 1939, Lyman Spitzer showed that a column of material drawn out from the sun would dissipate rather than condense. By this time the theory had mostly fallen out of favor, and in the 1940s, the work of Henry Norris Russell showed that if the solar material had been pulled away from the sun with the force necessary to account for the angular momentum of Jupiter, the material would have continued out of the solar system entirely. Though the Chamberlin Moulton hypothesis is no longer accepted, ... more details
The iron catastrophe was a major event early in the history of Earth . After accumulation of the Earth s material into a spherical mass, the material was mostly uniform in composition. While residual heat from the collision of the material that formed the Earth was significant, heating from radioactive materials in this mass further increased the temperature until a critical condition was reached, when the material was molten enough to allow movement. At this point, the denser iron and nickel evenly distributed throughout the mass, sank to the centre of the planet to form the core an important process of planetary differentiation . The gravitational potential energy released by the sinking of the dense NiFe globules increased the temperature of the protoplanet above the melting point resulting in a global silicate magma which accelerated the process. This event occurred at about 500 million years into the formation of the planet. ref Charles Frankel, 1996, Volcanoes of the Solar System, Cambridge University Press, p 7 8, ISBN 0521477700 ref This large spinning mass of super hot metal is responsible for the magnetosphere , which protects the Earth from solar wind and the most harmful components of solar radiation coming from our Sun . The magnetosphere protects both Earth s atmosphere and life to the present day and distinguishes our planet from a close celestial neighbour, Mars , which no longer has a significant magnetic field nor comparable atmosphere. Ironically, contrary to the implication of a catastrophe , this event was necessary for life to emerge and evolve on Earth for, without it, the atmosphere would have also been stripped from the Earth long before the present epoch. Another theory, however, suggests Mars did once experience its own iron catastrophe and was once shielded by a magnetosphere. By this theory Mars has simply cooled faster than the Earth and has solidified its spinning iron center into sync with its continental crust, gradually shutting dow ... more details
. There is also no exact dividing line between a planetesimal and protoplanet. Notes and references Reflist ... astronomy Protoplanet External links and data sources Discovering the Essential Universe by Neil F ... more details
galaxies. Protoplanets During the formation of planetary systems, dynamical friction between the protoplanet and the protoplanetary disk causes energy to be transferred from the protoplanet to the disk. This results in the inward migration of the protoplanet. Galaxies When galaxies interact through ... more details
Star formation A Herbig Ae Be star is a pre main sequence star a young 10Myr star of spectral type s A or B. These stars are still embedded in gas dust envelopes and may be surrounded by circumstellar disks . Hydrogen and Calcium emission lines are observed in their spectra. They are 2 8 Sun Solar mass objects, still existing in the star formation gravitational contraction stage and approaching the main sequence i.e. they are not Proton proton chain reaction burning hydrogen in their center . In the Hertzsprung Russell diagram these stars are located to the right of the main sequence . They are named after the American astronomer George Herbig , who first distinguished them from other star s in 1960. The original George Herbig Herbig criteria were Spectral type earlier than F0 in order to exclude T Tauri star s , Balmer series Balmer emission lines in the stellar spectrum in order to be similar to T Tauri star s , Projected location within the boundaries of a dark interstellar cloud in order to select really young stars near their birthplaces , Illumination of a nearby bright reflection nebula in order to guarantee physical link with star formation region . Nowadays there are known several isolated Herbig Ae Be stars i.e. not connected with dark clouds or nebulae . Thus the most reliable criteria now can be Spectral type earlier than F0, Balmer series Balmer emission lines in the stellar spectrum, Infrared radiation excess in comparison with normal stars due to circumstellar dust in order to distinguish from classical Be star s, which have infrared excess due to free free emission . Sometimes Herbig Ae Be stars show significant brightness variability. They are believed to be due to clumps protoplanet s and planetesimal s in the circumstellar disk. In the lowest brightness stage the radiation from the star becomes bluer and Linear polarization linearly polarized when the clump obscures direct star light, scattered from disk light relatively increases it is the same e ... more details
Image TTauriStarDrawing.jpg thumb right Drawing of a T Tauri star with a circumstellar accretion disc Star formation T Tauri stars TTS are a class of variable star s named after their prototype T Tauri . They are found near molecular cloud s and identified by their optical variable star variability and strong chromosphere chromospheric lines. Characteristics T Tauri stars are pre main sequence star s the youngest visible F, G, K, M spectral type stars 2 Sun Solar mass . Their surface temperatures are similar to those of main sequence stars of the same mass, but they are significantly more luminous because their radii are larger. Their central temperatures are too low for Proton proton chain reaction hydrogen fusion . Instead, they are powered by gravitational energy released as the stars contract towards the main sequence , which they reach after about 100 million years. They typically rotate with a period between one and twelve days, compared to a month for the Sun, and are very active and variable. There is evidence of large areas of starspot coverage, and they have intense and variable X ray and radio emissions approximately 1000 times that of the Sun . Many have extremely powerful stellar wind s. Another source of brightness variability are clumps protoplanet s and planetesimal s in the disk surrounding T Tauri stars. Their spectra show a higher lithium abundance than the Sun and other main sequence stars because lithium is destroyed at temperatures above 2,500,000 K. From a study of lithium abundances in 53 T Tauri stars, it has been found that lithium depletion varies strongly with size, suggesting that lithium burning by the Proton proton chain reaction P P chain , during the last highly convective and unstable stages during the later pre main sequence star pre main sequence phase of the Hayashi track Hayashi contraction may be one of the main sources of energy for T Tauri stars. Rapid rotation tends to improve mixing and increase the transport of lithium int ... more details
unreferenced date May 2008 article In planetary science , planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers the density denser materials of a planet sink to the center, while less dense materials rise to the surface. Such a process tends to create a Planetary core core and Mantle geology mantle . Sometimes a chemically distinct Crust geology crust forms on top of the mantle. The process of planetary differentiation has occurred on planets, dwarf planet s, the asteroid 4 Vesta , and natural satellite s such as the Moon . Heating When the Sun ignited in the solar nebula , hydrogen , helium and other volatile materials were evaporated in the area near the Sun. The solar wind and radiation pressure forced such low density material away from the Sun. Rocks, and the elements comprising them, were stripped of their early atmospheres, but themselves remained, to accumulate in protoplanet s. Protoplanets had higher concentrations of radioactive elements early in their history, the quantity of which has reduced over time due to radioactive decay . Heating due to radioactivity, impacts, and gravitational pressure melted parts of protoplanets as they grew toward being planet s. In melted zones, it was possible for denser materials to sink towards the center, while lighter materials rose to the surface. The compositions of some meteorite s show that differentiation took place in some asteroids. When protoplanets Accretion astrophysics accrete more material, the energy at impact causes local heating. In addition to this temporary heating, the gravitational force in a sufficiently large body create pressures and temperatures which are sufficient to melt some of the materials. This allows chemical reaction s and density differences to mix and separate materials, and soft materials to spread out over the surface. O ... more details
Image BlueMarble 2001 2002.jpg thumb 300px Water covers about 70 of the Earth s surface The question of the origin of water on Earth , or the question of why there is clearly more water on the Earth than on the other planets of the Solar System , has not been clarified. There are several acknowledged theories as to how the world s ocean s were formed over the past History of Earth 4.6 billion years . Origins Some of the most likely contributory factors to the origin of the Earth s oceans are as follows The cooling of the Hadean primordial Earth to the point where the Outgassing outgassed volatile components were held in an Earth s atmosphere atmosphere of sufficient Atmospheric pressure pressure for the stabilization and retention of liquid water. Comet s, trans Neptunian object s or water rich meteorite s protoplanet s from the outer reaches of the main asteroid belt colliding with the Earth may have brought water to the world s oceans. Measurements of the ratio of the hydrogen isotope s deuterium and hydrogen 1 protium point to asteroid s, since similar percentage impurities in carbon rich chondrites were found to oceanic water, whereas previous measurement of the isotopes concentrations in comets and trans Neptunian objects correspond only slightly to water on the earth. Biochemically through Mineralization geology mineralization and photosynthesis . Gradual leakage of water stored in Hydrate hydrous mineral s of the Earth s Rock geology rocks . Photodissociation Photolysis radiation can break down chemical bonds on the surface . Water in the development of the Earth See also Planetary differentiation A sizeable quantity of water would have been in the material which formed the Earth. ref cite web url http www.ingentaconnect.com content arizona maps 2005 00000040 00000004 art00003 jsessionid 7ibpocfkopqql.alice title IngentaConnect Origin of water in the terrestrial planets publisher Ingentaconnect.com date accessdate 2009 08 20 ref Water molecules would have esc ... more details
informs them that Tora is a lifeless protoplanet proto planet they have arrived millions of years ... of the eponymous intelligence depicted in Space Brain . The Brain protected the protoplanet proto ... more details
date August 2011 , and a remnant protoplanet . When Pallas was discovered by astronomy astronomer Heinrich ... Planetary differentiation differentiation , ref name Hubble which suggests that it was a protoplanet ... title Study of first high resolution images of Pallas confirms asteroid is actually a protoplanet ... more details
protoplanet reaches about 30 Earth masses it accelerates and proceeds in a runaway manner. The Jupiter ... of James Jeans Jeans 1917 , accretion model of Otto Schmidt 1944 , protoplanet theory of William ... of protoplanet systems and diversity of planetary systems journal The Astrophysical Journal volume ... a tenfold increase would suffice ref name Thommes2003 protoplanet migration, which allows the embryo ... gas onto the surface of each giant protoplanet , while solid bodies within that disk accreted ... more details
history Protoplanet theory In 1960, William McCrea astronomer W. H. McCrea proposed the protoplanet ... of the nebular hypothesis is widely accepted, ref e. g. cite journal title Formation of Protoplanet ... more details
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