Solar System lie within a nearly flat disc called the ecliptic. The Solar System also contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets.
Planet object that is neither a true planet nor a natural satellite; it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit. As of July 2008 the IAU has recognized five dwarf planets: Ceres in the asteroid belt, and Pluto, Haumea, Makemake, and Eris in the outer Solar System. Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space. Some