Comets - the formation(nutshell)

Comets are believed to be coagulations of material left over during the formation of the solar system through gravitational contraction and spinning up of the solar nebula. The spinning up is a consequence of conservation of angular momentum. In this process , a time came when the spinning material was concentrated into an ever-thinking disc. The shape was ordained by the fact that the epuitorial regions of the spinning cloud could overcome the gravitational contraction while the polar regions could  not. Finally when the central core of  the cloud became dense and hot inside, thermonuclear reactions started, thereby converting hydrogen into helium and producing a lot of energy.

Outflow of this energy produced the outward pressure to stabilise the sun against the contracting force of gravity.

Density fluctuations in the disc led to coalescence of mass into planets going around the sun. But a lot of mass in the outer fringes of the thin disc escaped congregation into large aggregates and was retained as dispersed material in the far reaches of the solar system. The additional pressure of the solar wind probably aided dispersal . This cloud is known as the Oort cloud .

The mass in the thinning edge of the pre -solar disc was too sparse to form large bodies through coagulation and was left alone to form it's own small colonies. Comets could be considered as colonies  of this kind and are the transformed components of Oort cloud .  They also go around the sun in long period elliptical orbits that are often perturbed when they travel into the inner solar system. In the early history  of the solar system, they occurred more frequently  and there is a feeling that much of the water on earth and other planets was brought in by comet collisions.