Ever since Pluto was discovered, some have pondered if perhaps, like Ceres, it was just the first of a class of objects to be discovered. The first astronomer to suggest this was Frederick C. Leonard in 1930. He suggested that there could be a population of trans-Neptunian objects waiting to be discovered. In 1943, Kenneth Edge worth took this idea one step further and theorized there could be an entire region beyond Neptune containing icy debris too scattered to coalesce into a planet while the Solar System was forming. This theory was built on ever further by Gerard Kuiper in 1951, who hypothesized that a disk of icy bodies could have formed in the region of Neptune and beyond. Unfortunately, due to the mistaken belief of the time that Pluto was the size of the Earth, Kuiper assumed that Pluto would have disrupted such a disk. In the 1970's, astronomer Charles Kowal detected an icy object between Saturn and Uranus. This was just the first of what became a regular series of icy discoveries in the outer solar system. As these objects were found, and as it was realized that Pluto was actually quite tiny, researchers realized Kuiper's early models were correct in predicting an icy disk.
To date, observed Kuiper Belt Objects (KBOs) have had orbits spanning from roughly 30 AU to 55 A.U. Like is observed with the Asteroid Belt, the Kuiper Belt has gaps where objects would otherwise be in orbital resonance with Neptune. Due to these gaps, most KBOs linger between 42 and 48 A.U., where the effects from Neptune's gravity are lowest. These objects have a wide range of masses (with objects larger than Pluto get founded, and Earth-sized objects being theoretically possible although not yet observed), and varying characteristics, with a variety of densities (from rocky to fluffy) and albedos (from icy white, to dull) being observed.
Simulation showing outer planets and Kuiper belt: a) before Jupiter/Saturn 2:1 resonance, b) scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune, c) after ejection of Kuiper belt bodies by Jupiter. Click here for original source URL
Gravitational perturbations and collisions occasionally fling KBOs into the inner Solar System. As these icy bodies approach the Sun, they heat up and begin to melt, becoming comets. It is believed that short-period comets like Halley's comet, have their origins in the Kuiper Belt. One interesting thing to note: If Pluto were suddenly knocked into a comet-like orbit, it would behave like a comet and even grow a tail. As Space Telescope Science Institute astronomer Mario Livio has stated, this is no way for a planet to behave. Since what Kuiper predicted doesn't exactly match what is seen, and since he wasn't alone in making the predictions of an icy disk, many argue that it may be inappropriate to label these objects Kuiper Belt objects, and many instead call all the objects trans-Neptunian objects, even though some don't actually cross Neptune's orbit.
Recent observations of other Sun-like stars sparked renewed interest in the Kuiper belt. By carefully blocking out the light from these distant stars, astronomers revealed that a number of them have disks of dust — possibly comet debris — surrounding them. The disks reach out to several hundred A.U. from the star. If stars like the Sun have belts of cometary debris like the Kuiper Belt, it could be a sign that they have planets, too. Perhaps planets and comets are a natural consequence of star formation, and our rocky vantage point in space is not unique. This speculation spurs astronomers in their continued search for debris around other stars, as well as our own.