28 March 2009
Every year at LPSC one of the big events is the Masursky lecture, given by that year’s winner of the Masursky prize recognizing “individuals who have rendered outstanding service to planetary science and exploration through engineering, managerial, programmatic, or public service activities”.
This year’s winner was Alan Stern, and he gave a thought-provoking talk about everyone’s favorite subject: What is a Planet? The official title was “Planet Categorization and Planetary Science: Coming of Age in the 21st Century.”
Stern made it clear that he favors a broad definition of what constitutes a planet, essentially saying that if it’s round, it’s a planet. Not a surprising view for the leader of a mission to Pluto. But at the same time, he did not try to push his views too hard. Instead he took a look at the state of planetary science and suggested that the whole debate over nomenclature is really an indicator that we’re in the midst of a revolution driven by the startling diversity of planets being discovered.
All of a sudden, we are discovering planets and planet-like objects everywhere we look. Right now there are 344 known extrasolar planets, many of which have bizarre, unexpected attributes. Some orbit pulsars, others have scorching orbits that circle their star every few days or hours, others plunge deep into their solar system, and then retreat to icy distances are they follow extremely oblong orbits. Some extrasolar planets are puffed up until they have the density of balsa wood, others are extremely dense. Even in our solar system, we’re finding a whole new population of objects out in the Kuiper Belt, and we know of many moons that are interesting worlds themselves.
Stern’s premise is that this startling diversity is what is driving the debate over what is and isn’t a planet and that, essentially, we just have to keep debating as we gather more information. How do we organize planetary objects? What properties are the most important in classification? What subtypes make the most sense? And who decides whether a new object is a planet? Stern suggested that these are the types of questions that planetary scientists need to be mulling over and chatting about in the halls at conferences because that’s where it will be figured out, bit by bit. Eventually the scientific method will prevail and a logical system will emerge.
I hope that’s how it works, but in the meantime, Stern made another point that I think was the most valuable one of the lecture, though he did not spend much time on it. He talked about the common complaint about the possibility of having more than nine planets: “But how will kids learn all those names?” His snarky response was: “I guess we’d better go back to nine states then.” But the serious response was that this is a fantastic teaching moment! This is an issue that the public is really interested in, and it’s a great example of how science works! We can use this to start conversations about planetary science and to help people start thinking critically and scientifically.
I’ll admit, I get tired of the “what is a planet” debate, but Stern is right. We are experiencing a revolution in what we consider a planet. It may be confusing and frustrating, but it’s an excellent teachable moment, and I plan to make the most of it the next time someone asks me about poor little pluto.