Therefore, each time a comet visits the Sun, it loses some of its volatiles. Eventually, it becomes just another rocky mass in the solar system. Many scientists believe that some asteroids are extinct comet nuclei, comet that have lost all of their volatiles. Most comets travel around the Sun in elliptical paths. The time it takes a comet to make a complete orbit is called its period.
Some comets have short periods of less than seven years.
Others travel in such huge orbits that they pass near the Sun only once in thousands or even millions of years. No comets seem to have approached the Sun from beyond the limits of the solar system. Therefore, all comets seen by astronomers are considered part of the solar system. Any theory of their origin must first and foremost explain the great predominance of very elongated ellipses among their orbits. According to some early theories, comets were of planetary origin.