Final answer:
The Oort cloud is believed to be the main source of comets in our solar system, with their distinct appearance arising from the sublimation of icy compounds and dust when they approach the Sun.
Step-by-step explanation:
The Oort cloud is suspected to be the reason for the existence of Comets, or at least most of them. This vast reservoir of ancient icy objects resides near the edge of the Sun's sphere of influence. Astronomer Jan Oort originally suggested that the presence of new comets in our solar system could be attributed to the gravitational perturbation of these objects by nearby stars, which occasionally sends them into the inner solar system where they become visible due to the sublimation of their ices. The content loaded within these cometary nuclei includes water ice, silicate grains, and dust, commonly referred to as the "dirty snowball" model first proposed by Fred Whipple. When a comet from the Oort cloud approaches the Sun, the increase in temperature causes the icy core to evaporate, forming a coma or head, and often resulting in a tail that can be clearly seen from Earth. This process, driven by evaporation of cometary ices such as water (H₂O), is what creates the spectacular activity that makes comets such intriguing celestial objects. The comet's head and tail are formed respectively from escaping gas and dust, illuminated by sunlight, while the nucleus remains the source of the material feeding the atmosphere.