The solar system has eight planets. In 2006, astronomers reclassified Pluto as a dwarf planet, the same class that contains Eris, Sedna, Quaoar, Ceres and perhaps many more solar system small bodies. These are defined approximately as bodies that orbit the sun but that are not massive enough (unlike regular planets) to gravitationally dominate their environments by clearing away material. Astronomers wonder, though, whether there might not really be a ninth planet previously undiscovered but lurking in the outer reaches of the solar system, perhaps in the giant Oort cloud of objects that begins hundreds of astronomical units (au) from the sun and extends outward.