In 1992, Dave Jewitt and Jane Luu at the University of Hawaii discovered a small object, designated 1992QB1, orbiting the Sun beyond Neptune at a distance of about 40 AU. Since then, more than 1,000 similar objects have been discovered beyond Neptune's orbit, and scientists estimate there are several hundred thousand objects bigger than 20 miles across waiting to be discovered in that vast region.
We call this swarm of bodies the Kuiper Belt, in honor of Dutch-American astronomer Gerard Kuiper, who speculated about the existence of small bodies beyond Neptune in the 1950s. There is controversy about who deserves credit for the idea; some people call this the Edgeworth/Kuiper Belt, sharing the honor with Irish scientist Kenneth Edgeworth, who published a similar idea in the 1940s. The inhabitants of this realm are called Kuiper Belt Objects (KBOs), Edgeworth/Kuiper Belt Objects, or simply Trans-Neptunian Objects (TNOs). It is very likely that most of the short-period comets found in the inner solar system come from the Kuiper Belt, being held there in cold storage until random perturbations nudge them inward.
The Minor Planets Center of the IAU at Harvard/Smithsonian web page shows the latest information about KBOs, including plots of their current locations and properties of various distant minor bodies.
We have learned a lot about the Kuiper Belt since its discovery — enough to classify its numerous inhabitants based on their orbits. The main categories are:
Because they are small and far away, KBOs look like faint stars even through the world's largest telescopes. They are so hard to see that the first classical Kuiper Belt Object was just discovered in 1992. Astronomers can find Kuiper Belt Objects among the myriad of faint stars because KBOs move slowly over time. Because of the changing orbital positions of Earth and the Kuiper Belt Objects, very detailed photographs of the sky taken many hours or days apart will show faint "stars" that have slightly changed positions. Those that change slowly must be very far from the Sun — these are the Kuiper Belt Objects. (More easily discovered are faster moving asteroids much closer to the Sun, residing in the asteroid belt between Mars and Jupiter.)
Modern astronomers don't exactly use "photographs" to discover Kuiper Belt Objects. They use extremely sensitive digital cameras — highly specialized versions of the digital cameras now in wide use by shutterbugs around the world. Digital cameras used by astronomers are so sensitive that they must be operated at extremely cold temperatures, around minus-58 to minus-148° Fahrenheit (minus-50 to minus-100° Celsius). Also they are very large compared to the cameras in home use, allowing them to take images of relatively large areas of the sky in a single exposure.
The largest two Kuiper Belt Objects are Pluto and Eris, each with a diameter of about 1,400 miles (2,380 kilometers). There are six other known KBOs with diameters that are approximately 600-900 miles (1,000-1,500 kilometers), including Charon. Scientists believe additional KBOs in the 600-1,200 mile (1,000-2,000 kilometer) size range will be found, but most KBOs are much smaller.
Kuiper Belt Objects exhibit different reflectivity and colors. Pluto is very bright, with a reflectivity of 60%. For comparison, Earth's Moon is only 10%. The high reflectivity of Pluto implies the existence of relatively fresh ice or snow, which might be expected from recent condensation of volatiles from the atmosphere or even geologic activity.
Other KBOs have darker surfaces with reflectivity in the 4%-20% range. The darkest bodies may be covered by chemically complex carbon-rich polymers. The wide range of KBO colors - from grey to red - suggests a wide diversity in the composition and evolution of these bodies. Due to the faintness of KBOs, it has been difficult to obtain infrared spectra that indicate the existence of specific minerals and ices. The faint signature of water ice has been detected on the surface of several KBOs, including Pluto and Charon.
Roughly 80 KBOs have companions, and more are being discovered all the time. They are often called "binary KBOs" because the two objects have similar size, so it's not clear which is the "KBO" and which is the "moon!" The best-known pair is Pluto and Charon; Charon orbits Pluto every six days at a distance of about 10,000 miles (17,000 kilometers). Some pairs have much slower orbits (up to 17 years) and greater separations (up to 60,000 miles or 100,000 kilometers), while others are very close. We don't yet know how these binary KBOs form, but collisions or close encounters with other objects are likely involved. Only the largest KBOs are expected to have atmospheres, and a tenuous atmosphere has already been detected on Pluto. Pluto's atmosphere - with a density about 2,000 times smaller than Earth's - could potentially freeze out on the surface as Pluto continues to recede from the Sun over the next several decades.
Pluto is in some ways a typical Kuiper Belt Object, but in other ways quite exceptional. The Kuiper Belt consists of myriad worlds with average orbital distances of about 30 to 50 AU from the Sun - that is, beyond the orbit of Neptune. Pluto's orbit is within the Kuiper Belt and has a special relationship to Neptune's orbit; namely, Pluto makes two revolutions around the Sun for every three Neptune orbits. This relationship is called a "mean motion resonance," specifically a 3:2 resonance. Many KBOs (but not the majority) are in the same mean motion resonance as Pluto.
Pluto is unlike the other planets in that it's not much larger than its moon, so Pluto and Charon together are known as a binary system. However, the Pluto-Charon system is not all that exceptional among KBOs - telescope observations show that a few percent of all KBOs are in binary systems.
The combination of Pluto's large size, its high reflectivity, and its current proximity to the Sun (compared to many other KBOs) makes it the brightest known KBO. That is why Pluto is the easiest KBO to see from Earth and why it was discovered in 1930, some 48 years before the next KBO was discovered (Charon, in 1978). And because it was discovered so many decades before any other member of the Kuiper Belt, Pluto is the only KBO to ever have been given the status of "planet."
Comets are usually classified into two main families depending on their orbits around the Sun. Oort Cloud comets come from a roughly spherical-shaped region between 10,000 and 100,000 AU from the Sun and typically have orbital periods of about a million years. Jupiter family comets have orbits strongly influenced by Jupiter's gravity and usually need less than 20 years to travel once around the Sun.
When the solar system formed nearly 4.6 billion years ago, about 10% of the comets that formed near the giant planets (perhaps a trillion total) were ejected into the Oort Cloud. Subsequent gravitational perturbations by the galactic tide and nearby passing stars send some of the Oort Cloud comets toward the Sun, creating the "new comets" we see today. In contrast, Jupiter family comets are thought to be byproducts of collisions between KBOs. They're literally chips off the old blocks, pieces of KBOs, pulled into the inner solar system by Jupiter's and Neptune's gravity. Nearly 150 short-period comets are now classified as "JFCs."