Characteristics of the Dwarf Planet Eris

Plutoid

• The name plutoid was put forth by the IAU's Committee on Small Body Nomenclature. Plutoids are heavenly bodies in orbit round the sun at a distance greater than that of Neptune and that haven't cleared the neighborhood around their orbit. They have enough gravity to become somewhat round and have a magnitude brighter than + 1, which is a magnitude a bit more than halfway between the sun and the full moon. So far Eris and Pluto are the only plutoids. They are also considered dwarf planets along with Ceres, Makemake and Haumea. Ceres isn't a plutoid because it's in the asteroid belt between Mars and Jupiter. Makemake, in the Kuiper belt beyond Neptune, is being considered for plutoid status. Scientists don't know enough about Haumea -- they do know it's shaped like a cigar.

Eris vs. Pluto

• Eris is the largest dwarf planet yet found. It is believed to be bigger than Pluto, which dwells with it in the Kuiper belt, a ring of icy bodies beyond Neptune that orbit the sun. Eris is 10 billion miles from the sun and takes 560 years to orbit around it. Eris has a diameter of 1,491.29 miles, plus or minus 62 miles, though this might change with more accurate measurements. Pluto has a diameter of 1,375 miles, is 4,572,500,000 miles from the sun and takes 248 years to orbit it.

Why So Bright?

• Eris has a surface temperature of 405 degrees below zero and reflects more light than any other body in the solar system besides Saturn's satellite Enceladus, and Enceladus has frost volcanoes that coat the surface with highly reflective material. It's postulated that Eris is as bright as it is because its methane atmosphere has fallen to its surface and frozen solid, which gives it a bright shining reflective coat.

What's it Made Of?

• Astronomers use Pluto as a guideline to what Eris is made of. Pluto's density is somewhere between that of rock and ice, and so scientists think its interior may be half rock and half ice. They believe the same about Eris. Pluto has a somewhat reddish surface and is mottled and Eris' surface is smooth and white, probably because of the frozen atmosphere on its surface.