Almost a year ago, Eris – the, uh, most massive known dwarf planet -- passed directly in front of an otherwise anonymous star, momentarily causing the star to disappear, as seen from the earth. By carefully measuring the length of time that the star disappeared, astronomers made a very precise measurement of the size of Eris. I care about the size of Eris for many different reasons, but the most trivial yet emotional for me is the fact that, 5 years ago, I measured the size of Eris myself. We used a much more difficult and less accurate technique than watching a star disappear and timing it. We looked at Eris with the Hubble Space Telescope and carefully compared the tiny disk that we saw with a picture of a star (which should show no disk at all) and we claimed that we could tell that Eris had a diameter of about 1.3 pixels on the HST camera. Only 1.3 pixels! It’s hard to imagine that you could tell the difference between something 1.3 pixels across and 1.2 pixels. In fact, it had never been done before. Even we were not convinced at first that our technique was as accurate as it appeared to be. So we spent months on a careful analysis to make sure we had done nothing wrong. In the end our measurement technique passed every test we could dream up for it, and we became convinced that it was correct. We wrote the paper to announce it to the world. The diameter of Eris, we claimed, was 2400 km with an uncertainty of 100 km in either direction (I’ll be writing this as 2400±100 km).
I’ve been waiting for most of the past year for the new results from the stellar occultation, holding my breath. One of the nice things about science is that no matter what you do, eventually, someone is likely to come along and make the same measurement more precisely than you did, and you will get to learn whether you were right or whether you screwed something up. When you screw up, you do so in a very public manner. Everyone knows. You don’t look so good. It’s best to not screw up. The new measurement of the size of Eris would instantly tell us whether we screwed up or not.
This week the scientific paper describing the observations is finally out and we now know that the diameter of Eris is 2326±12 km. (Pluto is a similar size, by the way, but with a larger uncertainty. We have no way of knowing which one is actually biggest until New Horizons gets detailed images of Pluto in 2014. Gory details on the uncertainties in the size of Pluto here). It’s an excellent result from a top-notch team. Plus, I should point out, 2326 km is well within our 2400±100km that we had estimated. It shows that our very difficult measurement was, in fact, correct. I would like a little gold star, please.
I will admit: the most important implication of this result is not that we did our measurements correctly five years ago. But it’s also definitely not that Pluto and Eris are “nearly twins” although that is what you will read in the press release that accompanies the paper. In fact, the most important implication is that Pluto and Eris – which we used to think of as near twins – are much more different than anyone predicted.
How can I say that they are different when they are essentially the same size? Because – I hate to say it – [precise] size doesn’t matter. The precise size that an object has tells you almost nothing. Some objects will always be a little bigger, some a little smaller. There is nothing interesting in knowing that. What matters is approximate size. We have known for a long time that Pluto and Eris were approximately the same size. Nothing has changed there. No scientific value in knowing that any more precisely.
What we also thought, though, was that Pluto and Eris had approximately the same density. Unlike precise size, precise density matters, and it matters a lot. Density tells you the composition of the object, which tells you something about where it came from and what happened to it since. If you had asked me a year ago, I would have told you that Pluto and Eris came from similar locations and had similar histories, thus they should have the same density, which would mean that Eris, which is 27% more massive, should have 27% greater volume to have the same density. Thus I would have predicted that Eris was 9% larger than Pluto. That fit our measurement, too; we just needed to assume that Eris was at the high end of the 2300 – 2500 km range. It seemed the most reasonable expectation. There was no good reason to expect anything else.
We now know that the opposite is true. Pluto and Eris are approximately the same size, but that means they have very different densities (because we know Eris is 27% more massive it must also be 27% more dense. That’s a lot). As I wrote extensively when discussing this earlier, dwarf planets are crazy. Rather than repeat that entire post again, I just encourage you, when pondering what the size of Eris means, to go back and read that post. I am convinced that understanding why Pluto and Eris are so different is one of the keys to understanding the formation of the entire outer solar system. So I find it particularly funny when people say they are twins.
Though it is clear that precise size has little scientific value, I will admit that certain bragging rights come with whoever can claim to be the biggest dwarf planet. For now my money is still on Eris, as I know that the more trustworthy measurement techniques yield the smaller sizes. But we just don’t know, and won’t until New Horizons flies by in 2014. I think it will be quite funny if one of the legacies of New Horizons is to prove that Eris is, in fact, the larger of the two.
I wouldn’t be surprised, however, if, at that point, the question is even less interesting than it is now. There are surely even larger dwarf planets out there. It is only a matter of time before both Pluto and Eris are supplanted. Perhaps New Horizons will tell us which of the two is the second largest dwarf planet and which is the third largest. Perhaps they will be even further down the list. Scientifically, it will still not matter, which is good, because explaining why Pluto and Eris are so different is going to keep us busy for many years, I suspect.