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The Minor Planet Eris On 'Utah Skies'

NASA, ESA, and M. Brown

Eris (AIR-ees) is the second-largest minor planet in our solar system and is only slightly smaller than Pluto by a mere 31 miles. Eris has an orbital period of 551 years, compared to Pluto’s 248 years.

Its distance varies from 37 to 97 times the distance of the earth from the sun. Because it moves so slowly against the background of the stars it was not recognized immediately on the first images that recorded this minor planet. Eris was discovered at the Palomar Observatory by a team of astronomers led by Mike Brown on January 5, 2005.

At first, it was thought that Eris was somewhat larger than Pluto and so it was considered to be the 10th planet. Later after Eris was occulted by a star it was determined to be just slightly smaller than Pluto. Later both Pluto and Eris were demoted to a new class of solar system objects called minor planets.

Dysnomia, (Dis-nom-ia) a small moon, was found to be orbiting Eris. The presence of a moon made it possible to calculate the mass of Eris. It turns out that Eris is more massive than Pluto by 28 percent. Eris is also about 2/3 the diameter of our moon and is large enough that if you took all of the objects in the asteroid belt they would fit inside it