Eris

A new study shows that the Kuiper Belt object Eris, which orbits the Sun about three times farther out than Pluto, has almost exactly the same diameter as Pluto. The discovery of Eris in 2005, when astronomers thought it was actually larger than Pluto, led to the demotion of Pluto to dwarf planet status.

The new study also finds Eris is roughly 27 percent more massive than Pluto, which makes it much denser-- implying a rocky core of some size under the ice field that seems to cover the entire world.