News Planetary Science

Eris dwarfs Pluto

By Ron Cowen

- More than 2 years ago

Ex-planet Pluto just got a further demotion. Observations in 2005 had already revealed that it wasn’t the kingpin of the outer solar system: A more remote denizen called Eris has a diameter 5 percent bigger than Pluto’s.

KINGPIN. The largest and heaviest known outer–solar system object, Eris, is pictured with its tiny moon, Dysmonia. Brown, NASA, ESA

That finding triggered last year’s decree by the International Astronomical Union that Pluto should no longer be considered a planet.