Naptune, the 8th planet outward from the sun, is now considered to be the solar system's farthest planet – now that Pluto has been demoted from planetary status by the International Astronomers Union.

Many astronomers were uncomfortable with this prospect, according to Stern, and that discomfort was a big factor in the decision to demote Pluto.

The discovery of an icy object slightly larger than Pluto in the Kuiper Belt last year reinvigorated the argument over whether to demote Pluto or add other planets.