Reflecting on Pluto

The past few days I, along with much of the rest of the world, have been  thinking about Pluto a little more than usual, although I am forced to admit that my usual thinking about Pluto is fairly minimal. I have taken two images of Pluto in my entire life. I paste them in here for your pleasure. If you have trouble seeing which dot is Pluto you can go here to blink the images. Look about a quarter of the way down from the top of the image and centered left to right to see how much Pluto moved one night to the next. The field of view is about half a degree on a side for those of you who care. Actually, it is half a degree on a side even if you don’t care.


I learned a long time ago that when I am tired and not in the mood to interact with strangers when I get on a plane, I answer “I’m a physicist” when asked what I do. If, however, I am feeling like chatting I answer “I’m an astronomer.” In my mind astronomy is simply the application of physical principles to the universe at large and non-terrestrial objects so that there really isn’t much difference in these answers. By the way, this strategy failed me once when I was working on some practice problems for a graduate school qualifying exam. The person next to me leaned over and said, “I certainly hope you don’t plan to do Lagrangian mechanics all the back to California.” No system is foolproof.

When I do identify myself as an astronomer the most common question I get is whether or not I think Pluto is a planet. I never quite know how to answer this question and do so vaguely, at best. In part, this evasion is based on the principle that at least once in a while one should avoid weighing in on things one knows essentially nothing about. But beyond that, my participation in this debate would, I fear, send the wrong message to my students. One of the handful of broader ideas that I work hard to help students think about is that it might be beneficial to avoid thinking in binaries unless it is absolutely necessary. To me planet/not a planet is just such a binary. Pluto is an object in the outer solar system, similar in ways to other objects and different from other objects. I’ve just never been able to develop much passion for thinking about whether it is a planet, a dwarf planet, a comet or something else, although the argument does help bring the physical nature of solar system objects into better focus. Just for fun here is one  recent “It’s a planet” story from the web and one  recent “Still not a planet” story.

Next time in this space we will return to the topic of the prior post when I talked about recalibrating our data. The research students have just over a week of work left this summer and the recalibration has gone quite well. Of course, we need to avoid that good/bad binary when describing it.