Take a trip to the wrong side of the tracks -- and, by tracks, we mean the Kuiper Belt, the doughnut-shaped ring of icy worlds and short-period comets circling beyond Neptune's orbit.
Out there drifts the troublemaker, a gleaming dwarf planet so cold (minus 359 to minus 405 F, or minus 217 C to minus 243 C) that its thin atmosphere freezes, falls and glazes it like a moon-sized doughnut hole. Namesake of the Greek goddess of discord, Eris earned its official label from its role in Pluto's rancorous demotion from planetary status. The discovery of additional planets past Neptune -- particularly Eris, which then seemed larger than Pluto, but might be smaller -- convinced astronomers that the ninth world should be reclassified as a dwarf planet. Its sole moon is named for Eris' daughter, Dysnomia, goddess of lawlessness [source: NASA].
Eris is too distant to see well, but we know it reflects light as brightly as newly fallen snow and takes 557 years to orbit the sun. As the solar system's most distant dwarf planet, it's second only to Pluto as a Kuiper Belt staging ground, and makes a better jumping-off point for the Oort cloud -- the shell of long-period comets and icy bodies located around 5,000-100,000 astronomical units (AU) from the sun. Both merit a trip: Visiting them is like traveling back in time to the solar system's earliest days [sources: Martin; NASA].