Space & the universe

Pluto didn't get demoted. It got a neighborhood.

Beyond Neptune lies a broad, doughnut-shaped ring of icy debris called the Kuiper belt, stretching roughly from 30 to 50 times the Earth-Sun distance and dwarfing the familiar asteroid belt in both width and mass. Unlike the rocky asteroid belt, most of its countless small bodies are frozen mixtures of methane, ammonia and water ice left over from the Solar System's formation. Pluto turns out to be just the largest well-known resident of this zone rather than a planet apart, a realization that led to its demotion to dwarf planet status in 2006 alongside other large icy worlds like Haumea, Makemake, Quaoar and Orcus. The region should not be confused with the distant, spherical Oort cloud, thought to lie a thousand times farther out and to be the source of long-period comets rather than the short-period ones once blamed on the Kuiper belt itself.

The idea of such a belt was floated by several astronomers across the twentieth century, including Kenneth Edgeworth and Gerard Kuiper, though it was Julio Fernández's 1980 calculation of how many comets would need to be flung from such a reservoir that most directly anticipated what was later found. Confirmation came only in 1992, when a small object beyond Pluto was spotted after a five-year search by David Jewitt and Jane Luu, opening the floodgates to thousands of subsequent discoveries. Later computer modeling revealed that the belt itself is too stable to be the main source of short-period comets; that job instead falls to a messier, overlapping region called the scattered disc, created when Neptune's outward migration flung material onto eccentric, unstable orbits. Neptune's gravity continues to sculpt the belt today, locking many objects into resonant orbits—Pluto and its fellow

Neptune's moon Triton and Saturn's Phoebe may not be native to their planets at all both may be captured bodies that started out in the Kuiper belt.

Read the whole of Kuiper belt on Wikipedia

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