The Hubble Space Telescope may have detected a population of small comets dwelling in this region in space. Based upon the Hubble observations, astronomers estimate that this belt contains at least 200 million comets, which are thought to have remained essentially unchanged since the birth of the solar system 4.5 billion years ago. Long-period comets are thought to emanate from a vast, spherical cloud of frozen bodies called the Oort cloud, named for the Dutch astronomer Jan Hendrik Oort. This cloud of comets, which also orbits the Sun, resides in the farthest region of the solar system, beyond Neptune and Pluto. Occasionally, a gravitational disturbance caused by a passing star or an interstellar cloud causes one of these bodies in the Oort cloud to begin a journey toward the inner solar system, where it makes a passing rendezvous with our Sun.
Planets have nearly circular orbits, whereas comets have elongated paths around the Sun.
A comet is at aphelion when its orbit is farthest from the Sun. It is at perihelion when it is closest to the Sun. Due to gravitational effects, a comet will travel fastest at perihelion and will slow down as it approaches aphelion. Some of their orbital periods are shorter than 200 years. Comet Shoemaker-Levy 9 is an example of a comet that has been radically perturbed by Jupiter's gravitational effects.