Mike Brown's Planets

I grew up in a universe teeming with life. Alien forms lived on and traveled throughout the planets. Sometimes they could even be found on neutron stars or giant rings constructed around a sun, or, shockingly, returned from the earth’s own upper atmosphere. When people meet me for the first time and realize I am an astronomer, the second thing they ask is often: What do you think about the possibility of life on other planets?  (The first thing they ask, of course, is: What happened to Pluto? To which I have a book-long answer to hand them).

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.

Is it real, or is it cat hair?

Seven years ago this week I was preparing one of my favorite lectures for The Formation and Evolution of Planetary Systems, a class I frequently teach at Caltech. “Preparing” is probably the wrong word here, because this lecture, called The Edge of the Solar System, was one I could give even if instantly wakened from a cold deep sleep and immediately put on stage with bright lights in my eyes and an audience of thousands and no coffee anywhere in sight. The lecture explored what was known about the edge of our main planetary system and the ragged belt of debris called the Kuiper belt that quickly faded to empty space not that much beyond Neptune. Conveniently, one of my most active areas of research at that time was trying to figure out precisely why this ragged belt of debris had such an edge to it and why there appeared to be nothing at all beyond that edge. I could wing it. So instead of preparing the lecture, I really spent that morning doing what I did whenever I had a few spare moments: staring at dozens of little postage-stamp cutouts of pictures of the sky that my telescope had taken the night before and my computer had flagged as potentially interesting. Interesting, to my computer, and to me, meant that in the middle of the postage stamp was something that was moving across the sky at the right rate to mark it as part of the Kuiper belt. I was not just lecturing about this debris at the edge of the solar system, I was looking for more of it, too.

For most of the past decade the last thing I would do before going to bed was to step out on to my back patio and stare up at the sky for a few minutes, checking for clouds. If the skies were clear I always slept better. In the morning, I would hop out of bed and do the same thing, to see if any unexpected weather front had passed or cirrus had snuck in while I had been sleeping. If all was well with the skies, I knew that my robotic telescope 95 miles southeast of me, likely had a successful night scanning the skies, and I was excited to get up and get to my office to see the results. I knew that any clear night we might (and eventually did!) discover something larger than anything else ever seen past Neptune. It was just a matter of time and of keeping those pesky clouds away.

After writing last week about a pretty major 5 year anniversary – the discovery on Dec 28^th 2004 of what is now called Haumea – it seems funny to be writing once again about a 5 year anniversary. But that’s just the way that reality worked. Eight days after discovering Haumea, and just a few days into the new year of 2005, I was back in my office again. I wanted to be studying Haumea – or Santa, as we called it then – since I was certain that it had to be bigger than Pluto, but, sadly for me, we still didn’t have any new data on it. We only had those first three pictures and there was nothing new to learn. We were scheduled to get more data soon, but not soon enough for sooth my anxiousness. My fingernails were nubs.