Astronomy: What is the "Kuiper Cliff? Planet X?""
"The Kuiper belt (pronounced /ˈkaɪpɚ/, rhymes with "viper") is a zone of the solar system extending from the orbit of Neptune (at 30 astronomical units (AU) to 50 AU from the Sun. The objects within the Kuiper belt, together with the members of the scattered disk extending beyond, are collectively referred to as trans-Neptunian, along with any hypothetical Hills cloud and Oort cloud objects. Together with the asteroid belt which is the source of meteorites, Kuiper belt objects give important clues to the distribution of materials and evolution history of the Solar System through their orbital characteristics and compositions. Kuiper belt objects coming as comets and dusts are gift for humanity to go back in time and study the early stage of the Solar System."
"If you travel out to the far edge of the solar system, into the frigid wastes beyond Pluto, you'll see something strange. Suddenly, after passing through the Kuiper belt, a region of space teeming with icy rocks, there's nothing. Astronomers call this boundary the Kuiper cliff, because the density of space rocks drops off so steeply. What caused it? The only answer seems to be a 10th planet. We're not talking about Quaoar or Sedna: this is a massive object, as big as Earth or Mars, that has swept the area clean of debris. The evidence for the existence of "Planet X" is compelling, says Alan Stern, an astronomer at the Southwest Research Institute in Boulder, Colorado. But although calculations show that such a body could account for the Kuiper cliff (Icarus, vol 160, p 32), no one has ever seen this fabled 10th planet.There's a good reason for that. The Kuiper belt is just too far away for us to get a decent view. We need to get out there and have a look before we can say anything about the region. And that won't be possible for another decade, at least. NASA's New Horizons probe will head out to Pluto and the Kuiper belt, but won't reach Pluto until 2015, so if you are looking for an explanation of the vast, empty gulf of the Kuiper cliff, watch this space."
"Earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50 AU; however, observation has revealed that in fact, at 50 AU, the number of observed objects in the Kuiper belt falls precipitously. This falloff is known as the "Kuiper cliff," and its cause is unknown, though Alan Stern of the Southwest Research Institute has claimed that a large planetary object might be responsible. Bernstein and Trilling et al. have found evidence that the observed rapid decline in objects of 100 km or more in radius beyond 50 AU is a real decline in the number of objects, and not just an observational effect."
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