Another “Planet” Debate

CFA Asks “What is a Planet?”

Artist's impression of Pluto. Courtesy New Horizons mission.
Artist’s impression of Pluto. Courtesy New Horizons mission.

It’s been a few years since the great “Define a Planet” vote took place at the International Astronomical Union meeting. That 2006 event touched off what still remains a firestorm of controversy about what is or isn’t a planet, and in particular, whether Pluto fits the definition set out in the vote. I’ve blogged about this a few times, and my viewpoint has remained pretty constant: Pluto is a dwarf planet. That’s still a planet, but the term “dwarf” tells us something about its place in the size hierarchy of planets in our solar system. However, my opinion is just that: my opinion, based on what my understanding of the issues involved.

It seems to me that the planetary science community is best fitted to determine how we define planets. It’s also very clear that NOTHING in science should ever be up for a vote. That’s not the way science works. It’s not a democracy. It’s based on making decisions based on the clearest FACTS possible. In my opinion, the IAU vote didn’t do that, and it also wasted the time of a lot of scientists who are best-able to define the term “planet”.

Today the whole planet debate thing has really gotten out of hand (in my opinion). We need science to help determine how and why we sort objects in the solar system. It really should be that simple. Instead, we have Pluto fanatics demanding planetary status for Pluto (as if it’s something you can picket about) without understanding the issues. We also have Pluto demoters saying it doesn’t fit in the IAU definition and therefore it’s out — without perhaps admitting that the definition needs fixing. We also have anti-Neil deGrasse Tyson crowds and pro-Neil deGrasse Tyson supporters, each loudly disputing the other due to Neil’s outspoken pronouncement that Pluto is NOT a planet.

Over in the corner, quietly working away, we have planetary scientists who are trying to make sense of all the different worlds in the solar system that we DO have, and how to best characterize them. Of all the groups, I have the most sympathy and support for the planetary scientists. They ARE the ones who should be figuring this out. Some of them are working on the New Horizons mission, which is on its way to Pluto and beyond. If the planetary science community decides that Pluto does or doesn’t fit into the definition of comet that the IAU voted in (without the input of a number of planetary scientists), then I’m sure they’ll let us know why and what the facts are that support the decision. And that is the way it should be. Their insights are what should guide the definition of planet.

By way of helping the public understand what the issues are, and what implications they have for how we understand exoplanets (worlds around other stars), the Harvard Center for Astrophysics is having a debate about planets — what they are and how we understand them. It will feature Dr. Owen Gingrich, who chaired a definition committee appointed by the IAU, Dr. Gareth Williams, the associate director of the Minor Planet Center (who will cover for the IAU), and Dr. Dimitar Sasselov, director of the Harvard Origins of Life Initiative. All three are very eloquent speakers and should provide interesting insights into the central problem of defining “planet”.  The debate will be livestreamed at the Observatory Nights Youtube site at Harvard CFA. I urge you to watch the debate. It should be informative. It probably won’t change too many minds that are already made up, but I hope it will give people some historical and scientific background on the roots of the debate about planets.

8 thoughts on “Another “Planet” Debate”

  1. Many “Pluto huggers” do not just demand planetary status for Pluto. A lot of us support the planetary scientists who study planets, especially the New Horizons and Dawn missions. We understand that this is not so much about Pluto but about a geophysical versus a dynamical view of solar systems, and we prefer the geophysical view. And we have done a lot of public outreach through talks and writings that engage people and get them thinking for themselves and considering new ideas.

  2. Yes, I agree that not all Pluto huggers are bad. I’ve fixed that in the text. I think that that is all to the good. However, there are some “huggers” who simply do not engage the public and DO just make the demand, sometimes most petulantly. I have gotten some fairly nasty emails from some of them, as if I am the enemy.

    That’s not what you and the folks who try to help the community understand this whole issue do and you are right to make a delineation.

    Again, however, planetary status should NOT be up to a vote or a demand. It should be the science that determines it. I just wish the whole issue could be resolved without the unfortunate “bread and circuses” aspect. I don’t agree that the definition should rest solely on geophysical grounds, however. I think that dynamics and evolutionary history should be part of any definition, to certain extent.

  3. Thank you for making this change and acknowledging that some of us on the side of a geophysical planet definition do engage the public. As I am sure you are aware, the term “Pluto huggers” encompasses a wide variety of people and not a single, organized movement. There are people on both sides of this debate who just make demands and send nasty emails. I have occasionally received some of those as well.

    Your position that dwarf planets are still planets and that the adjective “dwarf” tells us about its context within the solar system is very much in line with Alan Stern’s position and that of many supporters of Pluto’s planethood. Ironically, Stern is the person who first coined the term “dwarf planet,” and your description is exactly what he intended in doing so.

    At least some of the “bread and circuses” aspect of this issue, as well as the notion that science can be determined by voting, should be attributed to the IAU, as they opened up this can of worms in 2006 and then refused to revisit the issue when it became clear their actions at that General Assembly were problematic.

  4. If there’s a clear distinction in the scientific community about what constitutes a big rock being a “planet”, and Pluto fits that criteria, it should be labeled as a Planet. If it doesn’t fit the criteria, it shouldn’t. I think it’s funny that the scientific community can’t agree on a clear definition of the term “planet”. Is it just argument for the sake of argument?

  5. It is more complex than that. Planetary science isn’t “finished” by any sense of the word just because of labels. Worlds and objects form, they are affected by evolutionary and dynamical forces, and what was a moon a few million years ago might today be ring particles. “Planet” originally was from the Greek “planetes”, meaning “wanderers”. It has only been in the past century or two that we’ve had the wherewithal to study them through telescopes and with spacecraft. With additional study comes additional knowledge, and that knowledge means that you can more accurately define something. We don’t know all there is to know about these places, and that’s the job of planetary scientists. If they don’t agree on these terms yet, that’s fine. That’s part of science. It allows for disagreement and growth, not stolid definitions that never change.

  6. I know there are many thoughtful people that are helping expand the public understanding of these issues. I’m one of them. I’m not wedded to one hard-and-fast definition — I see the definition of planet occupying more of a continuum of possibilities, and those possibilities expand as we learn more about the objects in our solar system — and in others.

  7. I have great respect for scientists on both side of the issue but we need to remember that a dynamic definition was forced on us in the past by circumstances. As physical data was unavailable to early astronomers, they had no other way to describe them. Kind of like watching birds from a great distance. Now we have the ability to define objects by their physical characteristics, we go backwards in our understanding if we make a definition that is largely location based. It’s like taking a fish out of water and then calling it a small swimming object. Defining objects based on an attempt to limit their numbers is like saying there are only 8 species of birds and the rest are just small flying objects.

  8. Absolutely spot on, Paul! No other branch of science does this kind of limitation, and in fact, limitations need to be dealt with carefully in science, or it becomes some kind of odd dogma.

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