Back in something like seventh grade, I learned how science works. Scientists formulate a hypothesis and then they do experiments, and if enough experiments support the hypothesis, eventually the hypothesis becomes a theory. Right? Only, in my twenty-something years of actually doing science, things have rarely worked that way. Usually the process is more like this: do observations; discover something; develop explanation; repeat. This new paper, though, is a real live old fashioned hypothesis. It even has the word in the title: A hypothesis for the color diversity of the Kuiper belt.
While most of the scientific papers I’ve written have been an attempt to make a scientific point and marshal evidence that this scientific point is almost certainly true, a hypothesis paper gets to be delightfully different. My goal is not to prove the hypothesis true, but merely to suggest the hypothesis and show that it is plausible. Even coming up with a plausible explanation for colors of Kuiper belt objects is a good step; fifteen years after we first started realizing how diverse Kuiper belt colors are we still didn’t even have a plausible hypothesis on the table for explaining these colors until now. The object is to get the community of researchers studying the Kuiper belt to think about their data through the lens of this particular hypothesis and help determine whether it is right or it is wrong. I’m hoping that the answer turns out to be that the hypothesis is right, but more than that, I’m hoping that other scientists will feel challenged to try to prove it wrong.
Understanding the hypothesis requires understanding much of the stuff that we’ve figured out about the Kuiper belt over the past decades. The details are like pieces of a puzzle that all have to fit together somehow. The difficulty is that you don’t know ahead of time how many puzzle pieces there are or whether or not you have them all or perhaps too many. And you don’t even know what picture the puzzle is trying to make. But you take the pieces, put them together as best you can, and try to figure out what the right outcome really is.
In very very abbreviated form, the hypothesis we developed attempts to answer one simple question: why are Kuiper belt objects the colors that they are? You would think that question would have been answered sometime in the past twenty years, but every attempt that has made has been proven to be wrong. Perhaps this one will suffer the same fate. But perhaps not.
My answer to the question of why Kuiper belt objects are the color that they are all hinges on the molecule methanol. All Kuiper belt objects likely start out with some amount of methanol (we infer this because all comets – many of which used to be in the Kuiper belt – are seen to have some amount of methanol in them). Kuiper belt objects which formed closer to the sun became so hot that all of the surface methanol evaporated into space (though methanol in the cold interior happily stayed). The evaporation of methanol is so sensitive to temperature that there is essentially a line in the solar system: everything outside can keep surface methanol, everything inside must lose it. Over millions of years, the surfaces of these Kuiper belt objects get bombarded from the solar wind and cosmic rays, and whatever molecules there are on the surface slowly get broken apart and remade. In general, dark complex organic molecules start to form (generally I describe these as “goo” which I think we should turn into their technical term). Laboratory experiments have shown, however, that if methanol is in the mix, the goo is unusually red. Thus, objects which formed closer to the sun and have no surface methanol have blueish non-methanol goo, while objects which formed further away have reddish methanol goo.
|This is the main figure from the paper. It shows the evaporation lines for major molecules (methanol is CH3OH). The evaporation lines are mostly just a function of distance from the sun, but a little bit also a function of the object's. Inside of one of the lines, all of that molecule evaporates from the surface. Outside the line, the surface still contains the molecule. The shading is an attempt to show the two major regions important for determining surface colors. It is possible that some objects which formed much further out (the "cold classical Kuiper belt objects") are also influence by the presence of ammonia (NH3) on their surfaces, but that's a story for a different day.|
Simple enough, right?
There are a few hitches.
First, no one has ever done laboratory studies of precisely the right mixes of goo to make sure that what I said above is actual true. Enough laboratory studies have been done for me to say that it is plausible. But that is not good enough. Have some nice lab equipment and looking for a project? The paper tells you precisely what we need to know.
Second, most people who study the Kuiper belt believe that there are more than just two kinds of objects (the red and blue, in our hypothesis). Most people would tell you that there is a fairly uniform mix of colors from red to blue. We show in the paper, however, that this belief is actually not supported by the data. The data show that the colors of Kuiper belt objects could well be either red or blue with nothing in the middle. But the important word here is could. All we could do in our paper is show that common interpretation of the data – colors are uniform – has no statistical support. But our interpretation cannot be supported statistically, either. The data are simply not good enough. We predict, therefore, that when the right measurements are made, astronomers will find that the objects really divide into these two families. If they don’t, we’re just plain wrong.
A couple of things have happened even in the last few months since we wrote the hypothesis paper that we find encouraging . First, our very large survey of colors with the Hubble Space Telescope (we call it the H/WTSOSS survey, for Hubble/WFC3 Test of Surfaces in the Outer Solar System, but mostly so that we can pronounce it hot sauce) shows quite nicely the red vs. blue distinction of the Kuiper belt. And a paper we’re working on at this very moment shows that methanol, when detected on the surfaces of Kuiper belt objects, is only detected on the reddest of objects. Neither of these proves our hypothesis right, but both give me warm feelings inside.
How long will it take? I would think that the right laboratory experiments could take a few years. Acquiring more precise data for the Kuiper belt could be even longer. It’s OK. We can wait. We’ve know about the Kuiper belt for 20 years and haven’t understood the colors so far. Perhaps we won’t have to wait all that much longer.