Comets are small celestial bodies, consisting mainly of dust and gases that move in an elongated elliptical or nearly parabolic orbit around the sun. When a comet is far from the sun, it consists of a dense solid body or conglomerate of bodies called the nucleus, made up of frozen gases with particles of heavier substances interspersed throughout. This is a few miles in diameter. As it approaches the sun, a luminous "cloud" of dust and gases called the coma envelops the nucleus. The coma's luminosity is caused by the molecules absorbing and reflecting the radiation of the sun.
As the comet approaches the sun, the solar wind drives particles and gases from the nucleus and coma to form a tail which can extend as much as 160 million km in length. The solar wind always makes the tail stream out in the direction opposite the sun; i.e., it follows the head as the comet approaches the sun and precedes it as the comet passes perihelion (its closest point to the sun) and moves away.
Modern theories suggest that comets were formed during the formation of the solar system and are permanent members of it. Astronomer J. H. Oort has theorized that a spherical shell or cloud of more than 100 billion long-period comets surrounds the solar system at a distance of 75,000-150,000 AUs. While the comets move very slowly in this huge storage cloud, a passing star may change their orbits enough to force some of them into the inner part of the solar system. (Seeds, 410)
Astronomer G. P. Kuiper, noticed that Oort's cloud of comets did not adequately account for the short-period comets (those wit solar orbits less than 200 years), proposed the existence of a disk-shaped region of minor planets outside the orbit of Neptune...