Most of the rock and ice from the Solar Nebula that didn’t fall into the young Sun eventually went into building Jupiter and, to a lesser extent, the other big planets of our solar system. But there were still some leftover building blocks that never got incorporated into planets, like the small (0.6–6 miles [1- 10 kilometers]) rocky planetesimals in the main asteroid belt that Jupiter’s gravity prevented from growing into full fledged planets, and similar icy ones beyond Neptune that were simply too far apart and that collided too rarely to grow into large planets.
This latter class of so-called trans-Neptunian objects is of special interest, partly because the first one ever discovered is also the most famous: Pluto.
Pluto is a small, icy, rocky world in an elliptical orbit between about 30 to 50 astronomical units (AU). It is only about 20 percent the mass and 35 percent the volume of our Moon, and yet it has a large icy moon of its own, Charon, along with at least four other smaller icy moons, and a thin comet-like atmosphere of nitrogen, methane, and carbon monoxide.
Since the early 1990s, astronomers have discovered many more “Plutos” out beyond Neptune, orbiting within a doughnut-shaped disk called the Kuiper belt, named after the Dutch-American astronomer Gerard P. Kuiper. Beyond the Kuiper belt, which contains small icy bodies that formed in the zone between about 30 and 55 AU, another “scattered disk” consists of icy bodies that formed closer to the Sun but were flung out there by gravitational encounters with Jupiter to between 30 and 100 AU. More than 1,100 trans-Neptunian objects are now known.
As it became clear that there are huge numbers of Pluto-like objects in the Kuiper belt and scattered disk, the International Astronomical Union demoted Pluto and other similar objects in 2006 to dwarf-planet status.
Pluto is the last well-known body in our solar system not yet visited by a space mission. That will change in 2015, when the New Horizons mission flies past Pluto and its moons and reveals them as new worlds rather than just fuzzy points of light.