The Tenth Planet
In 2005, a distant rock was discovered orbiting the sun in the icy, debris-filled Kuiper Belt. It’s currently about 96.6 AU from the sun—three times as far away as Pluto—and it was christened Eris, for the Greek goddess of chaos of strife, while its moon was named for Eris’s daughter, Dysnomia, the demon goddess of lawlessness. In Greek mythology, Eris stirred up jealousy and anger among the goddesses and basically started the Trojan War, so it’s a beautifully fitting name, because Eris’s discovery shook the international astronomical community and outraged the world. Some thought Eris should be classified as the tenth planet since it’s thought to be bigger than Pluto, but others disagreed because it’s smaller than our own moon, and this sparked debates over what constitutes a planet. The International Astronomical Union met in 2006, and their discussions led to Pluto’s planetary demise—they stripped it of its planethood status, reclassifying it as a dwarf planet (otherwise known as ‘plutoids’). Eris was put into that same category, never becoming a planet at all. It’s an interesting little world, though: it takes 557 years to complete a single orbit, and it’s so far away from the sun that its surface temperatures are between −243 and −217 degrees Celsius. The icy surface is dominated by nitrogen methane, similar to the surface of Pluto, and its atmosphere is most likely frozen, so Eris is extremely reflective and gleams brightly. Since it’s in an elliptical orbit, it will get closer to the sun in years to come and warm up, so hopefully we’ll learn new things about it.
(Image Credit: ESO/NASA)