Scholars tried to explain the existence of comets through both scientific and religious reasoning. Today, science has succeeded in answering many of the questions posed by the ancients, but it has also produced many more of its own. Comets are thought to be the remnants of the material which gave birth to the solar system. As a large cloud of primordial gas contracted it eventually formed into the Sun and the planets around it. However, some of this material did not get incorporated into these objects and it is this leftover material which is thought to be what formed the comets.
Comets are small, irregularly shaped bodies composed mainly of ice and dust.
Most comets have highly elliptical orbits which take them very close to the Sun and back out into deep space, far beyond the orbit of Pluto. These comets will take hundreds of thousands or even millions of years to complete one orbit. Other comets will never pass far beyond Pluto's orbit and have orbital periods less than 200 years. These comets with short orbital periods are called periodic comets. The main body of the comet is called the nucleus and is composed of dust particles trapped in a mixture of ices of water, carbon monoxide, carbon dioxide, methane, and ammonia.