On January 5, 2005, astronomers at NASA discovered Eris, the second-largest dwarf planet in the solar system. Eris is just slightly smaller than Pluto, and it orbits the sun about three times farther than Pluto. NASA first thought that Eris was bigger than Pluto, and they originally described Eris as the 10th planet in the solar system. But the International Astronomical Union, or IAU, wasn't too thrilled about adding Ceres to the list of planets in the solar system. Instead, they decided to come up with the first official definition of the word "planet," and that new definition excluded both Eris and Pluto.