How does a comet form? As the title explains, 


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*How does a comet form?

*What are the elements, what is a comet composed of? 

*Why didn't they become part of planets, moons or asteroids?

 A: *

*Comets are some of the material left over from the formation of the planets. Our entire solar system, including comets, was created by the collapse of a giant, diffuse cloud of gas and dust about 4.6 billion years ago. Much of the matter merged into planets, but some remained to form small lumps of frozen gas and dust in the outer region of the solar system, where temperatures were cold enough to produce ice.


*A comet is generally considered to consist of a small nucleus embedded in a nebulous disk called the coma. the nucleus, containing practically all the mass of the comet, is a “dirty snowball” conglomerate of ices and dust.For one, of the observed gases and meteoric particles that are ejected to provide the coma and tails of comets, most of the gases are fragmentary molecules, or radicals, of the most common elements in space: hydrogen, carbon, nitrogen, and oxygen. The radicals, for example, of CH, NH, and OH may be broken away from the stable molecules CH4 (methane), NH3 (ammonia), and H2O (water), which may exist as ices or more complex, very cold compounds in the nucleus.

3.Many astronomers believe that these small objects never became planets or other large objects because of the gravity of the large planets.  For example, the pull of Jupiter's kept 'stirring the pot' of the asteroid belt, so that the gravitational pull of the asteroids on each other was constantly being disturbed.
For the Kuiper belt and Oort cloud, there is a popular theory called 'planetary migration.'  The main idea behind this theory is that the large outer planets of our Solar System started out much closer to the Sun when the Solar System was formed.  As they migrated outward through the cloud of small objects still there, the gravity of these large planets pulled a lot of the small objectsout of their orbits.  Some were pulled into the planets, and some were flung far into the outer reaches of the Solar System.
The objects that were flung very far out by Jupiter became the Oort cloud.  The object that were not flung out quite as far by the movement of Neptune became the Kuiper belt.
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