To Eris, human!

To Eris, human! In 2006, Pluto got downgraded to "dwarf planet", partially because researchers found several celestial bodies of about the same size, and one that was actually larger. That husky cousin, Eris, is in the news again this week as new measurements suggest it might be smaller, in diameter, than Pluto, after all. Everybody seems to be turning this into a, "Will Pluto be a real planet, again?" story. But Scientific American gets into the really interesting part. The new research shows Eris having a smaller diameter than Pluto, but much more mass. The next questions: Is this right? And, if so, what's Eris made of?