[yet another Eridian digression in lieu of continuing the Sedna story. Sorry! Plus this one is written while staying up all night and trying to run a telescope at the same time. Not the best for coherent writing or fixing typos, but I am losing sleep pondering the strange results from last week’s Eris occultation. And I have not been getting enough sleep to be able to afford the loss.]
I’m in Hawaii for a few precious nights to point the Keck telescope – one of the largest in the world – at dwarf planet Eris – one of the largest in the solar system. A week ago I would have just said “the largest in the solar system,” but as of last weekend I’m less sure.
The news that Pluto might indeed remain the largest dwarf planet is big, and, even though it will have no impact on the well-settled question of whether Pluto is a planet or not (still: no), it should make fans of the former planet smile a little bit. At least until they remember that Eris is substantially more massive, and in a head-on collision Pluto would suffer the brunt of the damage.
To me, though, Eris has suddenly become substantially more interesting than it was a week ago.
I will admit to you a somewhat embarrassing secret of mine: when I go around the world giving scientific talks about all the exciting scientific insights coming from the new discoveries of the dwarf planets, I often spend almost no time on Eris itself. I talk about the strange orbit of Sedna, the rapid rotation and collisional family of Haumea, and the irradiated surface of Makemake. But when it comes to Eris there has not been much more to say than “Yeah, it has a surface composition like Pluto and is a little bigger and more massive than Pluto, but, basically, it is a ever-so-slightly-larger Pluto twin.” Scientifically, finding a twin of Pluto doesn’t teach you much new, except that whatever made Pluto did it twice. Good to know, but not a huge scientific insight.
Now it seems that Eris is far from being a Pluto twin. The only plausible way for Pluto and Eris to be essentially the same size but for Eris to be 27% more massive is if Eris contains substantially more rock in its interior than Pluto. In fact, the amount of extra rock that Eris contains is about equal to the mass of the entire asteroid belt put together. That counts as a pretty big difference.
Now, crazy dwarf planet, what are you trying to tell us?
First, why is this difference in composition so surprising? It’s because Pluto and Eris should be identical. Here’s the standard scenario: About 4.5 billion years ago, the outer solar system was a swirling cloud of ice and rocky dust. Over time the ice and rock began sticking together to make small bodies, which in turn grew to bigger and bigger bodies until things the size of Pluto and Eris finally formed. Making Pluto or Eris requires sweeping up so much material from around the ice and dust cloud that any small local variations should be averaged out. Even if some of the smaller objects that initially coagulated differed from each other in composition, by the time objects got to the size of Pluto or Eris they would all be more or less the same.
This scenario even had good evidence to support it. For years, the only two icy objects known in the outer solar system were Pluto and the much larger Triton, the moon of Neptune which is thought to be a captured Kuiper belt object. When the masses and sizes of these two objects were finally measured and their densities and interior compositions computed it was realized that, as predicted, they are indeed nearly identical. Entire theories were constructed out of the fact that Pluto and Triton gave us the most pristine measurement of the composition of the entire early cloud of ice and gas and dust that led to the whole solar system and planets and life. The only thing left to do was to book the flight to Stockholm.
Things started getting a little more confusing about 5 years ago as we finally were able to start to measure the interior compositions of some of the newly discovered Kuiper belt objects. Eris, one of the early measurements, was, at least, consistent with having the same composition as Pluto and Triton (and thus we assumed that it probably did, because, well, that just made sense), but then there were objects like Haumea which is mostly rock with a thin layer of ice, and small Kuiper belt objects made almost entirely of ice, and a new measurement of Charon showing it has more ice than Pluto, and the word this summer that Quaoar has essentially no ice in it whatsoever. Orcus, just to keep things interesting, is somewhere in the middle.
So Eris is not the only crazy dwarf planet. They all are. I have absolutely no faith that any dwarf planet out there gives you a confident measure of what the early solar system was like. They are all thoroughly different. How could this possibly be?
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.
And, so, in just a week, Eris has gone from not really teaching us much new about the solar system to potentially demanding that we throw out some of our most cherished assumptions. Eris, goddess of discord and strife, indeed.
These two nights studying Eris at the Keck telescope are not turning out to be terribly effective. Clouds, instrument problems, and, now, fog. But we’ll be back. We’ve now got an entire solar system to figure out, and Eris might just be that puzzle piece that someday makes all of it finally make sense.
update: So is Pluto a planet after all?
update: So is Pluto a planet after all?