Occultation Data Show Pluto and Eris Are "Twins"

http://www.space.com/13403-dwarf-planet-eris-pluto-twin.html

In fact, Eris having a higher density than Pluto means Eris likely has a higher percentage of rock than does Pluto, which is estimated at 70 percent rock. This rocky nature is part of what makes these objects planets. We now know definitively that the 2006 IAU decision was based on erroneous information--that Eris is larger than Pluto, which turns out to be false--which only serves to reinforce the notion that that decision was premature.