The Kuiper belt (pronounced /ˈkaɪpər/, rhyming with "viper"), sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. While the asteroid belt is composed primarily of rock and metal, the Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water.
(The picture on the left shows known objects in the Kuiper belt, derived from the Minor Planet Center data. Objects in the main belt are coloured green, while scattered objects are coloured orange. The four outer planets are blue. Neptune's few known Trojan asteroids are yellow, while Jupiter's are pink. The scattered objects between the Sun and the Kuiper belt are known as centaurs. The scale is in astronomical units. The pronounced gap at the bottom is due to obscuration by the band of the Milky Way.)
If you take a close look at the picture of the Kuiper Belt, you will notice that the Kuiper belt terminates suddenly at a distance of 48 astronomical units (AU) from the Sun. Normally, the objects in the Kuiper Belt should have fade away slowly with distance from the Sun, like our air gets gradually thinner with altitude.
There is some speculation that this sudden drop-off, known as the "Kuiper cliff", may be attributed to the presence of an object with a mass between that of Mars and Earth located beyond 48 AU. This object's gravity sweeps up the material in the Kuiper Belt and over time cleans up everything farther out. In one disk, this sharp boundary indicates a planet 200 AU out. Neptune orbits at 30 AU from the Sun, so 200 AU is a long way out.
Astronomers have not excluded the possibility of a more massive Earth-like planetoid located further than 200 AU with an eccentric and inclined orbit. Computer simulations have suggested that a body roughly the size of Earth, ejected outward by Neptune early in the Solar System's formation and currently in an elongated orbit between 80 and 200 AU from the Sun, could explain the Kuiper cliff.
"THERE'S something funny going on out there." Marc Buie of the Lowell Observatory in Arizona is talking about the Kuiper cliff. Buie won't be drawn too far but, when pressed, he speaks of the possibility that some "massive object" has swept the zone clean of debris.
And his is not a lone voice. Alan Stern, a planetary astronomer at the Southwestern Research Institute in Boulder, Colorado, has been saying for years that there could be another large planet out there. Just how large became clear when Mario Melita from Queen Mary, University of London, and Adrian Brunini from the University of La Plata, Argentina, published a paper on the very same anomaly (
Sources:
http://www.newscientist.com/article/mg19726381.600-the-mystery-of-planet-x.html?page=1
http://www.dailygalaxy.com/my_weblog/2008/02/scientists-at-k.html
http://www.newscientist.com/article/mg17623735.300-the-hunt-for-planet-x.html