l> Formation of the Earth

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In the inner part of the solar system, only things which existas a solid at high temperature are available to be used as raw materials for planetary assembly (so how come thereis so much water on the earth? -- answer later)So in the inner part of the solar system you can only make a rocky planet via acretion of grains.In the outer part of the solar system, ices can exist so you canmake larger planets (e.g. Uranus and Neptune) out of the much more abundant icesVery large planets like Jupiter and Saturn, whose composition isprimarily Hydrogen and Helum, formed in a manner similar to the Sun,that is not by accretion of building blocks. Note that Jupiter can never becomea star. A star is a ball of gas sufficiently hot to excite nuclearreaction . The minimum mass require for this is about 8% themass of the Sun. Jupiter"s mass is an order of magnitude belowthis limit. Jupiter will never be a star.Jupiter has a very large mass and perturbs orbits of objects near them.There were lots of these objects scattered between Jupiter and PlutoJupiter redirected some of this cometary material into the innersolar system and most of the earth"s water was delivered throughcomet bombardment (therefore would we be here without Jupiter?)Steps in the accretion process from small grains to large planetare represented in the following sequences:
Step 1: accretion of cm sizedparticles. countless numbers of these small particles collide at low relatively velocity and coalesce into objects which are a few km in diameter
Step 2: Physical Collision on km scale. once these km size objects are formed, they start to smack into one another and stick thus creating fewer new objects of size 10-100 km
Step 3: Gravitational accretion on10-100 km scale. Once an object gets to a radius of 10-100 km, its cross section is larger than its physical cross section because it exerts a gravitational influence on surrounding smaller bodies and, in essence, sweeps them up. This process will create a planet withsignificant gaps between each swept up zone.
Step 4: Molten protoplanet fromthe heat of accretitop top . For each protoplanet that forms, there are thousands of left over planetisimal objects. These planetisimals are then accelerated by the relatively large mass of the formed planets and they start to move at high velocity throughout theinner solar system - many of them ultimately impact the surfaces of thecooling planets.
Final step is differentiation of the molten proto-planet (like earth:) Light objectsfloat; heavy objects sink. The result is a planet with aIron-Nickel Core (which can generate a magnetic field) and an oxygen-silicon crust. In the outer part of the solar system, the same 4 step processof accretion occurred but it was accretion of ices (cometisemals)instead of grains.Things to note about the formation of planets via accretion There is a lot of heat dissipated in the final accretionprocess resulting in initially molten objects Any molten object of size greater than about 500 km has sufficientgravity to cause gravitational separation of light and heavy elementsthus producing a differentiated body The accretion process is inefficient, there is lots of leftover debris. In the inner part of the solar system, the leftover rocky debriscratered the surfaces of the newly formed planets. In the outer part of the solar system, much of the leftoverrocky debris was ejected from the solar system due to the largemasses of the planets which formed there. Some of this materialwas ejected into a large "Comet Cloud" which has a distance ofabout 100,000 AU from the Sun and some of the leftover debris (beyond Pluto) could not be ejected (as it was far away from Uranusand Neptune) and hence remained there. This material is known asthe Kuiper Belt and it was newly discovered by the Hubble Space Telescope More information on the Kuiper belt and the kinds of objects thatare located there can be found here The asteroid belt represents a relic of the accretion process.A planet tried to form in that location but the gravitational influenceof the large mass planet Jupiter was sufficient to accelerate thematerial there to high velocity.


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High velocity collisions betweenchunks of rocks cause them to be shattered and indeed, over thehistory of the solar system, the sizes of the largest asteroidsare decreasing. The asteroid belt is therefore not the remains of a planetthat was blown up by the Death Star.