Eris: One weird little dwarf planet

Earlier this week, we learned that the dwarf planet Eris is probably smaller, in diameter, than Pluto, but is simultaneously 27% more massive. That's pretty nifty. But it's also pretty strange. The going theory, prior to this, was that Pluto and Eris were basically twins—similar size, similar composition. Imagine you had a pair of identical twin children, and one was 100 pounds heavier than the other.

Both Pluto and Eris are Kuiper belt objects, masses of rock and ice that coagulated together on the outskirts of our solar system. The ice-to-rock ratio—and, with it, mass—varies a lot from one Kuiper belt object to another, says astronomer Mike Brown.

But, it was previously assumed that, by the time smaller objects had coalesced into something as large as Pluto or Eris, their composition would be about the same as any other Kuiper object in the same size category. Instead, this new data suggests that Eris contains a lot more rock, or Pluto contains a lot more ice, or a little bit of both. And it throws our entire understanding of Kuiper belt objects out of whack.

No answer is immediately obvious, but it is immediately obvious that one or more of the assumptions of the standard scenario are going to have to be discarded. Earlier this summer I had constructed a new hypothesis that did an adequate (though, frustratingly not great) job of explaining some of the crazy variability in the Kuiper belt as being due to a random series of giant collisions which knocked the ice off of some objects, leaving just the rocky cores. I gave a couple of talks on the hypothesis, and even wrote the first draft of a scientific paper describing the details. But I fear now that the draft is going to have to go to the recycle bin. Even in my hypothesis once things grow to a certain size they should be more or less the same. Eris and Pluto are just too big to be different.
So what happened instead? Did they form in different places? In different solar systems? Did Eris spend time close to the sun? None of these hypotheses is immediately appealing, but somewhere in there there must be a kernel of what really happened. Pluto and Eris and all of the rest of the dwarf planets must have a widely divergent set of histories of formation or evolution or interaction or all of the above.

Mike Brown's Planets: Dwarf Planets are Crazy