Eris is a dwarf planet. It is part of the Kuiper Belt in the outer reaches of the Solar System. Eris is about 68 astronomical units from the Sun. It takes light from the Sun more than 9 hours to reach Eris!
Eris was first seen in 2005. Its discovery caused a debate amongst astronomers about whether this new object would be a planet. The discussion led to a new category of objects, called dwarf planets. The former planet Pluto was included as a dwarf planet.
Eris is one of the biggest dwarf planets. It's about the same size as Pluto but is 3 times further from the Sun than Pluto. Like Pluto, it's about three-quarters the size of our Moon. We think Eris is made of ice and rock. It's so cold that Eris' atmosphere often freezes and falls as snow.
Eris orbits the Sun every 558 years. Like other dwarf planets, its orbit is very elliptical and is tilted compared to the 8 planets. As Eris orbits the Sun, it rotates on its axis. It takes 25.9 hours to spin around once, similar to Earth! Eris has one small moon of its own, called Dysnomia. Dysnomia is only around 150 km across and orbits Eris every 15.8 days.