28 March 2009
LPSC: The Masursky Lecture
Posted by Ryan Anderson
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.”
Artist's concept of an extrasolar planet and its moons.
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.
Exoplanet discoveries by year as of Early 2009.
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.
I think I mentioned this to you at DPS, but one of the problems is that dynamicists care about different details of planet-hood than geologists. Dynamically, Pluto and Eris and Ceres don’t act like planets, and moons don’t act like planets. But geologically, the Moon, Io and even icy moons like Titan or Europa have more in common with Earth than Earth does with Jupiter.
You’re right — you learn a lot about planets by trying to classify them.
Congratulations to Alan Stern for winning this award. Having heard him speak on several occasions, I can’t think of anyone more deserving of this honor.
We could address the concerns of both dynamicists and planetary scientists with one small change in the IAU’s planet definition. Currently, the IAU definition states that dwarf planets are not planets at all. This is inconsistent with the use of the term “dwarf” in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies.
Why not simply establish dwarf planets as a subcategory of planets that are planets because they are spherical but of the dwarf category because they do not gravitationally dominate their orbits? And why not establish round moons of planets as “secondary planets,” a term used in the 19th century to indicate that these objects’ composition is akin to that of the major planets but that their primary orbit is around another planet rather than the around the sun itself.
This is a great post. I never cease to be amazed that we are in the midst of a revolution in planetary science. More exoplanets discoveries are on the way!
Of course the IAU’s definition of a planet does not yet include exoplanets, only worlds that orbit the Sun.
Still, it is a very exciting time for this young field. The future is certainly looking up as more and more exoplanets are discovered.
You could use the biological model of binomial nomenclature and have the star be the Genus and the planet name the specific epithet. Our plant would be called Sol earth and Mars would be Sol mars. A moon of earth would be Sol earth ‘Luna’ or something. Then the name would carry info about its relationship to a star and a planet would be linked to it’s moons. Similarly, the family name could be the galaxy or other region. A Planet would then be a round thing going about a star or “thing in the middle”. Fun to think about.
You are right, this debate has been a wonderful teaching point.
This year I’m a visiting scientist on the Nez Perce reservation. While the sixth graders initially expressed regret that Pluto was no longer officially a planet, they were both excited to learn about Ceres and a little ruffled that no one had mentioned it before.
By teaching a rote Nine Planets we’d been glossing over the solar system’s true nature, dumbing it down then presenting it as a static fact to be memorized. We’d sold them on a charted territory with nothing new left to be explored.
After learning about the dwarf planets one student noted the pattern of planets-belt-planets-belt and concluded there “might be more planets” beyond the Kuiper belt. Decent conjecture for someone who started the morning believing Pluto was the edge of the solar system. She was excited, engaged, and had it been a simple matter of borrowing a telescope they would have started planet hunting that afternoon.