Mike Brown's Planets

In part 1 of this story I told about the discovery of Sedna, the first – and still only – body found far beyond the edge of the Kuiper belt. Part 2 described some of our early theories on how Sedna had gotten there and what it was telling us about the early history of the solar system. Here I’ll begin talking about the most recent searches for more things like Sedna and how we’re doing so far.

Seven years ago, I knew with certainty that the discovery of Sedna in a strange orbit that never brought it close to any planet was telling us something profound about our solar system. I also knew that Sedna would never divulge her secrets alone. To learn more, we’d have to find more things like Sedna.

These days, a question like that is easy to answer: type it in to Google, click on the Wikipeadia entry, and read the answer: 2306 +/- 20 km. The  +/- (to be read “plus or minus”) is important here: every measurement has limitations and an often critical  part of science is correctly quantifying those limit. The correct interpretation of 2306 +/- 20 km is that 2306 km is the most likely value, but, within a certain range of probability, the value could be as low as 2286 or as high as 2326 km. The value could still be higher or lower, but the probability is small.

So when the occultation of a start by Eris a few weeks ago lasted for such a short period of time that it was clear that Eris could be no larger 2360 km, Pluto was declared, throughout the land, the once-again largest dwarf planet and king of the Kuiper belt. Because, um, 2306 km is greater than 2360 km? [see updated size on Eris, below]

Brian Marsden, long time director of the International Astronomical Union Minor Planet Center died today. While it is easy to say “he was the nicest guy…” in this case it was simply true. Everyone who came across him has stories about Brian. My book, coming out in out a few more weeks has a few too. Just last week I autographed a copy for Brian and bookmarked the spots where he appeared. I say nice things about him, which made me nervous to send him the copy. I was always a little worried about exactly how he would react to praise.But I really wanted him to see it to make sure he knew how much I appreciated everything he had done for me, for astronomy, and for the solar system.

The news last week that Eris might actually be a tiny bit smaller than Pluto led to the inevitable question: doesn’t this mean that Pluto should be a planet, after all? [update: the final analysis suggests that to the best of our ability to measure Pluto and Eris are the same size. No way to know which is bigger or smaller.]

The simple obvious answer to this question is no. Pluto was not demoted in 2006 simply because it was no longer the largest known object beyond Neptune, but because it was one of many many such small objects beyond Neptune. The fact that it might still be the largest gives it some bragging rights at the  next dwarf planet convention, but – just like we never considered Eris a planet when we thought it larger than Pluto – being the largest known thing beyond Neptune doesn’t get you an invitation to the planet ball.

[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.

[sorry: a brief interruption in the ongoing Sedna story for some late breaking news]

Eris, the goddess of discord and strife and the most massive dwarf planet, is up to her usual tricks.

On Friday night Eris was predicted to pass directly in front of a relatively faint star in the constellation of Cetus. You might think that this sort of thing happens all of the time, but you’d be wrong. Eris is so small in the sky and stars are such tiny points of light that, though they get close frequently, their actually intersections are rare. When they do intersect, though, something amazing happens: the star disappears. And since we know how fast Eris is moving across the sky, seeing how long the star disappears gives us a very precise measure of the size of Eris. Or, to be more exact, a very precise measure of a single chord passing through the body.