A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

Discovery 2008

A friend asked me a few days before Christmas, “What big discoveries are you going to make in 2008?” to which my immediate reply was “I have no idea.” In thinking a little more about this question for the past week, I realized that the real answer is a little more complicated than that. I think that discovery comes in three different flavors. I think I should have said “I have a very specific idea about some, I have vague hopes about some others, and some might just happen.”
The first type of discovery is the classical type of science that we all learn about in eighth grade: a scientist has a hypothesis and sets out to prove it. Some of the most famous science stories of the past involve this sort of hypothesis testing: Einstein theory predicts that the sun’s gravitational field would deflect the positions of nearby stars in the sky; Eddington mounts an expedition to view stars during an eclipse and proves Einstein right; and relativity is instantly enshrined as the newest part of physics. In many ways this classical type of discovery is also the most respected. It is certainly the most dramatic. In thinking back over interesting discoveries I have made over the years I can think of none whatsoever that fit this category.
Why not? Partially it is because not a lot of actual science works in this way. In looking over the list, say, of Nobel prizes in physics, most of them are not for this type of eighth-grade-textbook hypothesis-testing science. Most science proceeds in one of the other two flavors.
Interestingly, though, for 2008, I am indeed working on one such type of hypothesis-testing discovery. It would not be nearly as dramatic as Einstein and Eddington or as universally encompassing as relativity. But, still, it’s a hypothesis I have that I am attempting to test. The hypothesis suggests that collisions between large objects like Pluto and Eris and others out in the Kuiper belt played a much larger role in sculpting these objects than anyone previously thought, and, in particular, the collisions removed much of the ice that these cold objects would have had in their interiors. The key to testing a hypothesis like this is to figure out a test, like Eddington realizing that an eclipse would provide the key measurements. For my hypothesis there are two tests. The first is to show that a giant collision indeed removes ice. The second is to show that an object that has a lot of ice didn’t have a giant collision.
The giant Kuiper belt object and dwarf planet Quaoar (for details on these large Kuiper belt objects see my dwarf planet website) looks like it indeed had a giant collision. The key piece of evidence is that it has tiny moon circling it which appears to be a remnant of the collision, much as our Moon is a remnant of a collision between a Mars-sized object and the Earth 4.5 billion years ago. In my hypothesis, this giant collision that happened on Quaoar long ago should have removed most of the ice inside of Quaoar, meaning that the interior should be much more rock than ice. I can test this prediction by figuring out how much Quaoar weighs and seeing if its weight is more rock-like than ice-like. And I can figure out the weight by using a method first used by Newton more than 300 years ago. If I track the path of the moon around Quaoar and calculate how much time each orbit takes I can use that information to get the weight. If Quaoar is heavy and rock-like, the moon will be pulled along quickly, while if Quaoar has more ice and is lighter, the moon will take longer to go around.
The moon of Quaoar is, unfortunately, so tiny and so close to Quaoar itself that it is impossible to see it with anything other than the Hubble Space Telescope. Last year I wrote a proposal asking to use the Space Telescope to track the moon, the proposal was accepted, and now the observations are scheduled for sometime in the spring (turnaround time is not fast!).
One of the nice things about hypothesis testing is that the answer can sometimes be instantly clear. As soon as the pictures from the Hubble Space Telescope are taken I will be able to quickly calculate the amount of rock and ice inside Quaoar and see if I’m right or wrong. If I’m wrong the whole hypothesis is dead and thrown out the window. If I’m right it will be time to go on to the second test.
The second test is to show that an object that has a lot of ice didn’t have a giant collision. Another giant Kuiper belt objet and dwarf planet, Orcus (which is about the same size as Quaoar, that is to say about half the size of Pluto), also has a moon, and we had observations from the Hubble Space Telescope about a year ago which allowed us to track the orbit of that moon and determine that Orcus is much icier than anyone had anticipated. The moon of Orcus might have formed in a giant collision, but the moon is a bit larger than the other moons of dwarf planets that look like collisional remnants. Perhaps, then, Orcus never experienced a giant impact and the moon is simply another Kuiper belt object captured from space. For my hypothesis to be correct, this moon must be captured, because otherwise my hypothesis would predict that Orcus should be rocky, not icy.
Happily, I think we can test whether or not the satellite of Orcus came from a collision or was captured from space by looking in detail at the composition of what is on its surface. Typical Kuiper belt objects (like one that Orcus would capture) appear to look very different in surface composition from collisionally formed satellites. The best way to look at the surface composition is to examine the spectrum of sunlight after it has reflected off the surface of the satellite. Like the satellite of Quaoar, the satellite of Orcus is small, faint, and close in, so looking at the spectrum is particularly difficult. We are thus going out to the Keck telescope – the largest telescope in the world – on the summit of Mauna Kea in Hawaii at the end of March to spend three nights trying to get a clear glimpse of the satellite. We’ll be using the fancy new Laser Guide Star Adaptive Optics system at the observatory which shoots a laser beam into the sky to help correct the smearing caused by the earth’s atmosphere. I’ll write in detail about that at the end of March.
Like the first test, the results of this second test will be apparent almost instantly. In only a few hours I should be able to figure out what the moon of Orcus has on its surface. If the moon of Orcus looks like it was made in a collision I will again have to toss the hypothesis out. But if the moon appears captured, the hypothesis will have survived two important tests, and it will suddenly have to be considered seriously. That doesn’t mean it will have been proved correct; other astronomers will look for other ways to test and challenge the hypothesis. But if it is correct, it will pass all of these tests and challenges and become part of our understanding of the outer solar system.
So while I said “I have no idea” to my friend who asked about discoveries for 2008, it is clear that, at least in this case, I have a very specific idea and definite observations to confirm or refute the idea. I presented these ideas at a scientific conference in December and suggested that I thought there is approximately a 15% chance that I am correct. I think that’s about right. My hypothesis would require a fairly major revision of some of our fundamental ideas about how the outer solar system formed, and such major revisions don’t come along too frequently, so if you felt like betting I would recommend betting against me. But, still, at least I have one idea of what might be a discovery in 2008.
Next week I’ll write about the other two types of discoveries and what else might happen in 2008.

Happy New Year

If you had walked out into my backyard around 4:40 the last few afternoons you would have been greeted with the orange ball of the sun setting with a final low glare over the tops of the buildings that I can see low on the horizon out across the Los Angeles basin. At this time each late afternoon I like to get out the binoculars that I keep next to the back door, and I step outside to watch the last seconds of the sun setting and to find the spot where the last glimmer of light for the day appears. Every night that glimmer has moved a little further to the south. Just a few weeks ago the last glint vanished just behind the cupola of the Pasadena city hall. By just the next day, the cupola was clear, but the sun disappeared behind the building to the left of city hall. Last night it set 4 or 5 office buildings further to the left, still, behind an anonymous office tower that I can't recognize, but through the binoculars appears impressive with the sun directly framing it and the occasional stray bit of light going through a window on the far side, rattling around on the inside, and emerging as the last bit of bit of light before a long winter night.
Tonight I watched again, and the sun set behind exactly the same anonymous tower. It hadn't moved at all. Today, therefore, must be the solstice. The solstice is many things: the first day of winter, the earliest sunset, the longest night of the year, the latest sunrise. Most people notice the sunset more than anything else. But solstice comes from the latin "solstitium": sol for sun, and stitium for a stoppage ("armistice" comes from the same root: a stoppage of arms). The stoppage of the southern progression of the sun -- the turnaround to come back to the north -- was considered a big enough phenomenon to give the event its name. The sun stoppage. As the darkness tries to ascend (quickly; these winter twilights don't last) the other part of the season becomes clear. While the nearby glare of Los Angeles means that we never truly have darkness in these parts, this time of year everyone is doing their best to cut the darkness even more. I can see Christmas lights on the houses throughout Pasadena, and, with the binoculars, I can see to downtown Los Angeles where the buildings have been strung with lights. And who can blame them? With the nights so long and the sun moving further and further south, who would not want to try to do their part to make up for the absence of the light and the heat? Who would not be at least a little afraid at this time every year that the sun would somehow not decide to stop and then come back?

At our house we celebrate the solstice with our best attempt to coax back the sun. When the night is as dark as it will get, we gather with friends around our Christmas tree, turn out all of the lights in the house, and slowly refill the house with the yellowy-orange glow as we one by one light the dozens of candles hanging in the branches of the tree. Lighting candles on Christmas trees is a well known Bad Thing to Do, but we find that with a tree cut down the day before (and a fire extinguisher on hand just in case), all goes smoothly. Like the sun, the candles slowly go out. Some catch a few warm drafts and burn more quickly, some get less air and burn more slowly, but one by one they all eventually go until, with just two or three left, the house is dark again and the shadows of branches shimmer sinisterly on the ceiling. Finally the last candle sputters and dies, sometimes with a long glow and sometimes with a sudden final pop, and the longest night of the year totally envelopes us.

The night sky gets in on the act this time of year, too. Many people who claim to know no constellations in the sky can look up and identify Orion in the winter sky. With the three bright stars making the belt, the scabbard of stars hanging below, and the quartet making the shoulders and knees, Orion is truly simple to identify. But Orion is also composed of some of the brighter of the stars in the sky. In fact, look outside, and look around Orion. Bright stars are all around. The constellation of Taurus, Sirius, the brightest star around. The seasons of the sky are not created equally. Winter is a spectacular display of stars and constellations unlike any other, as if the stars, too, are trying to help us out on the longest winter nights by saving the best show for the very end of the year. None of this is true, of course. The spectacular winter skies are caused by the fact that we are looking straight in to the Milky Way galaxy, instead of out of it as we do in the spring and fall. But still, it is hard not to see the similarity between the lights strung in the town below trying to dispel the night and call back the sun, and the lights above, also seemingly strung for the same reason.

Tomorrow, if the weather holds, I'm going to go outside with my binoculars and see exactly where the sun sets again. Because I do this every year, and because I can look up the precise date and time of the solstice, and because I know that the earth will continue to go around the sun with the same tilt for my entire lifetime, I know what will happen: the sun will have moved away from the anonymous office building and finally started moving right again. The day will get imperceptibly longer. Really, there is not much suspense in what will happen, just a certain reassuring inevitability. But if I didn't know these things and didn't have confidence in the inevitable, I can imagine myself holding my breath as the last rays of the sun were shooting out and I was trying to see just where it was setting. I stopped yesterday, but is it really turning around today? Will the days really get longer again? Will my crops (well, ok, my vegetable garden) come back to life? And I'll then see the spot and it will be clearly north and I'll know. And at that point, I will say to anyone within sight: happy new year. For while the calendar claims I have another week to go, the Christmas lights and the candles and Orion and Taurus and Sirius will have done their jobs, and the sun will have started its new year already today and we should all be glad for the solstice.