Pluto really is unlike the other planets, and if it were found today would not likely be categorized as a planet. In fact, there was debate when it was found whether it should be classified as a planet or asteroid. Pluto is an icy ball, that may be more closely related to comets than to the other planets. Pluto has a sattelite called Charon, which is almost exactly half of Pluto's diameter. Pluto's orbit is very eccentric - extending from within Neptune's orbit to more than 1.5 times neptune's orbit. Pluto's orbit is also tilted about 17 degrees from the plane of the rest of the planets. All of these things point to pluto being a large kuiper belt object (see below) rather than a proper planet. Pluto's "demotion" to dwarf-planet status in 2006 had many people upset, but it made perfect sense.
Asteroids are small, irregular shaped rocky bodies, sometimes called planetessimals. These may be the remains of collisions early in the solar system, or debris that never quite formed a planet. The main asteroid belt lies between the orbits of Mars and Jupiter, held in place in part by Jupiter's tremendous gravity. Some asteroids swing within the earth's orbit, and are called Near Earth Asteroids. The largest main belt asteroid is Ceres, which is about 960 km long.
Comets are "dirty snowballs" formed far out in the outer solar system. They have an elliptical orbit that carries them close to the sun and then far away again. Some comets complete one full orbit in only a few years or decades, while others, called long period comets, may take thousands of years to complete an orbit. As a comet gets close to the sun, it warms up and spews gas and dust, creating a cloud that follows it in its orbit. This cloud is called the "coma". The solar wind pushes on this cloud, stretching it out and making a tail. Oddly, this means the tail always points away from the sun, so the comet sometimes looks as though it is backing up!
If this cloud is bright enough we can see it from earth as a comet. Most comets are visible only in binoculars or telescopes, though occasionally one is bright enough to be seen with the naked eye. The last such "Great Comet" was Hale-Bopp in 1997.
Image Copyright Paul Mortfield, 1997
This image of comet Hale-Bopp shows a bright Coma, and a double tail. The nucleus of Hale-Bopp is only about 40 km long, but the visible coma is several million kilometres in diameter, and the tail is 10's of millions of kilometres long. The double tail is the result of the solar wind affecting dust and ions in the tail differently. This image was taken by amateur astronomer Paul Mortfield.
See NASA's Asteroid and Comet page for more information, or search Google for images of comets and asteroids.
The diagram below illustrates the elliptical orbit and position of the tail of a comet. In a sense, the tail acts as a flag blowing in the solar wind!
The Kuiper belt is a region outside Neptune like the asteroid belt -- though most kuiper belt objects are icy comets. Pluto is the largest Kuiper belt object. Every once and a while one of these objects is knocked out of its orbit by the gravity of one of the gas giants. This can then become an active comet.
There are several Kuiper Belt Objects that share similar orbits to Pluto. These have been dubbed "Plutinos".
Sedna is a strange, reddish planetoid about 1800 km in diameter, discovered just last year. It is the most distant object yet imaged in the solar system. Sedna's orbit is elliptical, ranging from 90 -- 800 AU from the sun. At this distance, Sedna's orbital period is 11, 487 years. Sedna is too far out to be a kuiper belt object, but too close to be an Oort cloud object, and may have been captured from a wandering star that passed near our solar system about 4 billion years ago. More information can be fount at http://www.gps.caltech.edu/~mbrown/sedna/
The discovery of object 2003 UB313 really generated the discussion of the definition of "planet". This object, at a distance of about 97 AU, is larger than Pluto - so if Pluto is a planet, this object must also be a planet. If this object is not a planet, then Pluto must also not be a planet. This led to the reclassification of the definition of "planet", and the subsequent demotion of Pluto to dwarf planet status. Named Eris (the goddess of discord - suitable given the debate it generated), this dwarf planet also has a moon, named Dysnomia. Although Eris is currently the most distant object seen in our solar system, it's orbit is closer in than Sedna's. But Sedna is near the inner part of it's orbit, while Eris is near the outer part (aphelion), and the two orbits cross.
The oort cloud is a theoretical, spherical cloud of comets 1000 times farther out than Pluto that may contain billions of comet-like bodies. This cloud has not yet been detected, so we do not know for certain that it exists, but there is enough evidence to keep looking. The main piece of evidence is the regular appearance of long period comets. They have to come from somewhere!
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