Comets can be divided into two basic groups depending on their orbital periods. There are long period comets with orbital periods that can be thousands to millions of years long, and short period comets with orbital periods less than about 200 years. Their alignments with the plane of the planet orbits is also different. The long period comet orbits are oriented in all different random angles while the short period comets orbits are within about 30 degrees of the solar system plane. These orbital characteristics point to two regions beyond the realm of the planets: the Oort Cloud and the Kuiper Belt.
At the great distances of the Oort Cloud, comets can be affected by the gentle gravitational tugs of nearby passing stars. The passing stars tug on the comets, ``perturbing'' their orbits, sending some of them into the inner solar system. The comets passing close to a jovian planet are deflected by the planet's gravity into an orbit with a shorter period, only decades long. Jupiter and Saturn tend to deflect long period comets completely out of the solar system (or gobble them up as Jupiter did with Shoemaker Levy-9). Uranus and Neptune tend to deflect the long period comets into orbits that stay within the solar system. Halley's Comet may be an example of a deflected comet. Unlike other short period comets, Halley's Comet's orbit is retrograde.
The Oort cloud comets probably formed at the about the same distance as Uranus and Neptune from the Sun 4.6 billion years ago and were then deflected outward when they passed to close to the two planets. Comets forming at the distance of Jupiter and Saturn were either ejected from the solar system by these massive planets in a ``gravitationally slingshot'' or gobbled up. Comets forming further out than Neptune never coalesced to form a planet and now make up the Kuiper Belt.
The comets observed in the Kuiper Belt have more circular orbits and do not stray close to Uranus or Neptune. Many of the Kuiper belt comets observed from the ground are 100 to 300 kilometers in size (but some are Pluto-size) and orbit between 30 and 60 A.U. from the Sun. Another group of objects mostly between Saturn (9.5 A.U.) and Uranus (19.2 A.U.), called ``Centaurs'', may be an extension of the Kuiper Belt. These objects include Chiron (170 km in diameter) and Chariklo (about 240 km in diameter) and many others.
Because of its small size and low density, some astronomers view the planet Pluto (2300 kilometers in diameter and just 1/6th our Moon's mass; on the left in the image above) as just a large comet. Pluto and its moon, Charon (1200 kilometers in diameter; on the right in the image above), are members of the Kuiper Belt. (Pluto is now known to have at least two other smaller moons orbiting it.) In July 2005 the discovery of a Kuiper Belt object larger than Pluto was announced, called Eris (formerly UB 313). Is Eris the tenth planet? If Pluto is a planet, should not Eris be considered a planet too? How about Ceres in the asteroid belt? Although the discovery of a Kuiper Belt object the size of Pluto or larger was considered likely, Eris' discovery finally forced astronomers to decide what is to be called a "planet", what is a "minor planet", what is an "asteroid", "large comet", etc.
On August 23, 2006, the International Astronomical Union (IAU, the official authority responsible for naming stars, planets, celestial bodies and phenomena, etc.---the official body of astronomy) re-classified Pluto as a "dwarf planet". A "planet" in our solar system is a celestial body that "(a) orbits the Sun; (b) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and (c) has cleared the neighborhood around its orbit." Pluto fits (a) and (b), but not (c). Pluto, Eris, Ceres, and others will be called "dwarf planets" because although they fit (a) and (b), they have not cleared the neighborhood around their orbits. Also, a "dwarf planet" is not a satellite (which may leave out Charon, but its large mass compared to Pluto may make Charon to be a "dwarf planet").
The third criteria (c) of a planet from the IAU has caused a considerable amount of debate---what does "cleared the neighborhood around its orbit" mean? One interpretation is to say that the object gravitationally dominates its orbital zone where an orbital zone includes all objects whose orbits cross each other, their orbital periods differ by less than a factor of 10, and they are not in a stable resonance. Within that orbital zone, if a round object is much more massive (say, by at least 100 times) than the other objects combined mass, it will gravitationally dominate its zone. With this interpretation there is a clear separation between the eight "planets" (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and the "dwarf planets". All eight planets are at least 5000 times more massive than the other objects in their orbital zones while Pluto is only 0.07 times the mass of the rest of the objects in its orbital zone (Ceres is just 0.33 times the mass of the rest in its orbital zone). By the time all of the bodies in the Kuiper Belt are found, the number of dwarf planets will probably number well over 200. See Mike Brown's "The Dwarf Planets" page for more about dwarf planets and their number (he is the discoverer of Eris).
Even smaller objects (comets, most asteroids, etc.) will be called "Small Solar System Bodies". This does leave open the question of how this applies to planets outside the solar system, especially the truly planet-sized objects that are not bound to any star. Another controversial issue behind the IAU 2006 decision was the small proportion of members who voted on the decision. There will undoubtedly be a much larger proportion of IAU members actively participating in the planet definition discussion and voting at the next meeting in 2009.
The image below compares Pluto, Charon to Earth and the Moon. Eris is probably about 20% larger than Pluto, so it is still smaller than the Moon. In mid-January 2006, the New Horizons spacecraft was launched on a 9.5 year trek to Pluto-Charon. After flying by Pluto-Charon in July 2015, it will be directed to another Kuiper Belt object.
Earth, Moon, Pluto and Charon to the same scale (using images from NASA and
USGS)
The currect list of objects of the Kuiper Belt is at the Minor Planets Center (the following links will display in another window). They keep a list of the tran-Neptunian objects and a list of the Centaurs. A plot of the positions of the observed Kuiper Belt objects is also available from the Minor Planets Center.
Regardless of where it is in the solar system, the Sun's gravity is always pulling on the comets. When a comet is close to the Sun, it moves quickly because of the great force of gravity it feels from the Sun. It has enough angular momentum to avoid crashing into the Sun. Angular momentum is a measure of the amount of spin or orbital motion an object has---see appendix A for more on angular momentum. As a comet moves away from the Sun, the Sun's gravity continually slows it down. Eventually, the comet slows down to the aphelion point and the Sun's gravity pulls it back.
A comet's motion around the Sun is sort of like a swing on the Earth. When the swing is closest to the ground, it moves quickly. As the swing moves up, the Earth's gravity is continually pulling on it, slowing it down. Eventually, the swing is slowed down so much that it stops and the Earth's gravity pulls it back down. The swing has enough angular momentum to avoid crashing to the ground.
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last updated: March 20, 2008