What’s a “planet”? That common term sparked an uncommon controversy when scientists announced the discovery of what they believed to be a much-fabled Planet X out beyond the orbit of Neptune.

The icy world, which was initially nicknamed Xena and was later dubbed Eris, is more massive than Pluto. That finding prompted the International Astronomical Union to reclassify Pluto as a dwarf planet and designate all Plutolike objects beyond Neptune as plutoids.

As the planethood debate continues, astronomers also continue to probe the skies with the latest and greatest telescope technologies, learning more and more about distant worlds.