While Pluto had always been an oddball, the 2005 discovery of a body even bigger than Pluto in the same general vicinity forced astronomers to rethink what it takes to be a planet.

In 2006 the International Astronomical Union declared that Pluto would be reclassified as a dwarf planet, joined by the then newfound, larger body—dubbed Eris—and the previously known Ceres, which had been considered an asteroid.

The recently discovered dwarf planets Haumea and Makemake have since joined the list, although both are smaller than Pluto.

Late last month astronomers using telescopes in Chile watched Eris cross in front