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NebularHypothesis



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History The nebular hypothesis was first proposed in

1734 by Emanuel Swedenborg. ImmanuelKant, who was familiar with Swedenborg'swork, developed the theory further in1755. He argued that gaseous clouds—nebulae, which slowly rotate, graduallycollapse and flatten due to gravity and

eventually form stars and planets. A similarmodel was proposed in 1796 by Pierre-Simon Laplace. It featured a contracting andcooling protosolar cloud—the protosolarnebula. The birth of the modern widely accepted theory of planetary

formation—Solar Nebular Disk Model (SNDM)—can be traced to the

works of Soviet astronomer Victor Safronov. His book Evolution of the protoplanetary cloud and formation of the Earth and the planets, which was translated to English in 1972, had a longlasting effect on the way scientists think about the formation of the planets. In this book almost all major problems of theplanetary formation process were formulated and some of them

solved. The Safronov's ideas were further developed in the worksof George Wetherill, who discovered runaway accretion




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Formation of protostars

Protostars

Stars are thought to form inside giant clouds of cold molecularhydrogen—giant molecular clouds roughly 300,000 times the mass of the Sun and 20 parsecs in diameter. The initial collapse of a solar-mass protostellar nebula takes around 100,000 years. Every nebulabegins with a certain amount of angular momentum. Gas in thecentral part of the nebula, whose angular momentum is relatively low,

undergoes fast compression and forms a hot hydrostatic (notcontracting) core containing a small fraction of the mass of theoriginal nebula. This core forms the seed of what will become astar. As the collapse continues, conservation of angular momentummeans that the rotation of the infalling envelop accelerates, whichlargely prevents the gas from directly accreting onto the central core.

The gas is instead forced to spread outwards near its equatorial plane,forming a disk, which in turn accretes onto the core.




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Example of a PROTOSTAR

The visible-light (left) and infrared (right) views of theTrifid Nebula—a giant star-forming cloud of gas and dust located 5,400 light-

years away in the constellation Sagittarius




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Formation protoplanetary

disksProtoplanetary dis

Under certain circumstances the

disk, which can now be calledprotoplanetary, may give birth toa planetary system.Protoplanetary disks have beenobserved around a very highfraction of stars in young star

clusters. They exist from thebeginning of a star's formation,but at the earliest stages areunobservable due tothe opacity of the surroundingenvelope. The disk of a

Class 0 protostar is thought to bemassive and hot. It is anaccretion disk, which feeds thecentral protostar.

The main problem in the physicsof accretion disks is thegeneration of turbulence and the

mechanism responsible for thehigh effective viscosity. Theturbulent viscosity is thought tobe responsible forthe transport of the mass to thecentral protostar and momentum

to the periphery of the disk. Thisis vital for accretion, because thegas can be accreted by thecentral protostar only if it lossesmost of its angular momentum,which must be carried away by

the small part of the gas driftingoutwards



Example of aPROTOPLANETARY DISK


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The Orion Nebula, Four of the stars are surrounded by gas anddust trapped as the stars formed, but were left in orbit about thestar. These are possibly protoplanetary disks, or proplyds, thatmight evolve on to agglomerate planets. The proplyds which areclosest to the hottest stars of the parent star cluster are seen as

bright objects, while the object farthest from the hottest stars isseen as a dark object.




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FORMATION OFPLANETS




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Rocky PlanetsRocky planets form in the inner part of the

protoplanetary disk, within the snow line,where the temperature is high enough toprevent condensation of water ice and othersubstances into grains. This results incoagulation of purely rocky grains and later inthe formation of rocky planetesimals. Such

conditions are thought to exist in the inner 3–4 AU part of the disk of a sun-like star. Aftersmall planetesimals—about 1 km in diameter—have formed by one way oranother, runaway accretion begins. This leadsto the preferential growth of larger bodies at

the expense of smaller ones. The runaway accretion lasts between 10,000 and 100,000 years andends when the largest bodies exceed approximately 1,000 km indiameter. Slowing of the accretion is caused by gravitationalperturbations by large bodies on the remaining planetesimals. Inaddition, the influence of larger bodies stops further growth of smallerbodies. It is characterized by the dominance of several hundred of the

largest bodies—oligarchs, which continue to slowly accreteplanetesimals.




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GiantPlanets

The formation of giant planets is an outstandingproblem in the planetary sciences. In the frameworkof the Solar Nebular Model two theories for theirformation exist. The first one is the disk instability model, where giant planets form in the massiveprotoplanetary disks as a result of

its gravitational fragmentation (see above. The diskinstability may also lead to the formation of browndwarfs, which are usually classified as stars. Thesecond possibility is the core accretion model, whichis also known as the nucleated instability model. helatter scenario is thought to be the most promising

one, because it can explain the formation of thegiant planets in relatively low mass disks (less than0.1 solar masses).Giant planet core formation is thought to proceed roughly along the

lines of the terrestrial planet formation. It starts with planetesimals,which then undergo the runaway growth followed by the sloweroligarchic stage. Hypotheses do not predict a merger stage, due to

the low probability of collisions between planetary embryos in theouter part of planetary systems.




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nebular hypothesis presentaion

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