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Headshot of Emily Lakdawalla

This is the post where you can comment about the IAU planet definition

Posted by Emily Lakdawalla

30-04-2014 12:09 CDT

Topics: trans-neptunian objects, Pluto, Eris, dwarf planets beyond Neptune

It seems like every time I write an article about Pluto or any other denizen of the realm of the solar system that lies beyond Neptune, the comments on the article are overwhelmed by people who are unhappy with the IAU redefinition of the word "planet" and the consequent reclassification of Pluto. I understand that many people care a lot about this subject, but I find it to be pretty much the least interesting conversation to have about the fascinating worlds at the edge of our solar system. The inevitable comment arguments have actually made me wary of writing about Pluto and friends at all. And recently it's been particularly wearisome, as the participants in the conversation have accused me in comments here and on other websites of mindless sycophancy to one of the personalities in the public debate.

I've had enough. I want to be able to write about Pluto and Eris and all their neighbors without devolving into the planet-definition conversation every single time. I want to be able to discuss the fun problem of not knowing which of the two biggest trans-Neptunian objects is larger without people accusing me of irrational favoritism of one or the other and suggesting I should be fired for what I wrote. With New Horizons beginning its approach to Pluto and Charon, we are about to enter an extraordinarily productive period of scientific discovery. Pluto is an excitingly in-between world, occupying a place on the borders of so many different classes of objects, likely experiencing (or having experienced) a huge array of different processes. Studying it will inform the study of worlds across the rest of the solar system. I want to share all of that with you without constantly having to fight inappropriate behavior in the comments.

The Planetary Society is a community of space enthusiasts, both professional and amateur, of which I am merely a member, and I firmly believe in the importance of providing a forum for our community to talk amongst ourselves. Some terrific things have happened thanks to the comments on blog posts. It usually works pretty well. Except, it seems, whenever I mention Pluto.

I recognize that many of you care deeply about the topic of the IAU planet definition, and if that's the conversation you want to have about Pluto, then I'm happy to give you that opportunity, but only here. I believe that the overwhelming majority of you are capable of discussing it without resorting to ad hominem attacks and other bad behavior. This post is for you. Here, you may discuss the definition all you like, advocate whatever position you like, as long as you adhere to our comment policy, which is displayed prominently on the page once you log in to comment. If you don't adhere to the policy, your comment privileges will be revoked.

At the risk of being too idealistic, I'd like to suggest people read before arguing. There is a lot to read on the subject. For some background, here's the Wikipedia page on the IAU definition, which has lots of links to the history. Here is the full text of the IAU's Resolution B5 (PDF). For opinions, there are famous advocates for "it's not a planet," including our own Board member (and former President), Neil Tyson, who took Pluto off his list of planets at the Hayden Planetarium in 2000, writing about that in his book and NOVA show The Pluto Files. There's also Mike Brown, author of How I Killed Pluto and Why It Had It Coming, who blogs frequently about Pluto and planethood. The most vocal advocate for "it's still a planet" is, without a doubt, New Horizons Principal Investigator Alan Stern, who laid out his position on his PI's Perspective blog in an entry titled "Unabashedly Onward to the Ninth Planet". Alan's opinion carries weight, as he literally wrote the book on the science of Pluto and Charon, with coauthor Jacqueline Mitton, and will no doubt be updating that book with great new science in a couple of years. Joining Alan in support of Pluto's planethood is journalist Alan Boyle, who also wrote a book, The Case for Pluto. Recently, Astronomy editor David Eicher posted a video supporting "the IAU definition is stupid" camp. David Grinspoon has written eloquently about two really strange aspects of the definition (that dwarf planets are not planets, and that exoplanets aren't covered). There are lots of others; feel free to comment with links to other good, thoughtful writing on the subject.

In my own work, I do my best to emulate what the scientific community does, to the extent that there is consensus in the community. I expect that once New Horizons returns its expected beautiful images of Pluto, I'll hear more scientists referring to it as a "planet." That's based upon the assumption that it'll become a world with geology. Probably. Maybe. We actually don't know yet, of course. If it looks heavily cratered like the Moon, perhaps people will be less inclined to switch their usage. Maybe Pluto will be in the middle, and, like Titan, endure Jeff Moore's accusation of being "Callisto with weather," in which case the people who think it's more active will be more inclined to call it a planet, and the people who think it's a dead world with a veneer of weather will not! I look forward to the arguments about what Pluto is, and you'll see my language evolve as the scientific community learns more and adjusts their language accordingly. As I've said before, edge cases are fun. They test our understanding of how the physics of the solar system's formation and evolution work. And, more often than not, we find out we were wrong about at least some of what we thought we knew. That's what you'll see me talking about elsewhere on this site.

So, feel free to discuss the planet definition, its shortcomings, et cetera, here. Keep it polite. Most importantly, on planetary.org, keep it here. Elsewhere, I want to talk with readers about what we are learning about Pluto and its neighbors -- not what words a few English-speaking people choose to use to refer to it.

 
See other posts from April 2014

 

Or read more blog entries about: trans-neptunian objects, Pluto, Eris, dwarf planets beyond Neptune

Comments:

StevenJGould : 04/30/2014 12:56 CDT

Thanks for putting us in our place.

David Grinspoon: 04/30/2014 01:42 CDT

Great post, Emily! I say we settle this like civilized humans: with a wrestling match. Only to make it fair, Neil should be drugged and handcuffed. Otherwise he'll just crush us.

Jonathan Ursin: 04/30/2014 04:34 CDT

Here's a question for the thread. Is there anything that New Horizons could discover about Pluto that would change your mind? Would a complex atmosphere convert you to consider Pluto a planet? Could something convert you away from considering Pluto a planet?

Laurel Kornfeld: 04/30/2014 06:32 CDT

As an amateur astronomer and writer firmly in the "dwarf planets are planets too" camp, the first thing I want to emphasize is that in those other entries, you can discuss Pluto (and all dwarf planets) from a neutral stance that avoids clearly siding with one camp or another. I wrote an article on developments about Pluto between 2006 and 2009 for the 2009 IAU General Assembly newspaper "Estrella D'Alva" which managed to satisfy both camps by not taking any stand at all. Why not do this in your other discussions about Pluto and Eris? I do want to thank you here for including links and sources representing both sides of the debate. There is nothing wrong with acknowledging that this debate remains ongoing. Those who are interested in hearing back and forth discussion from both sides on this issue should listen to the transcripts of the Great Planet Debate, held at JHUAPL in August 2008. These can be found here: http://gpd.jhuapl.edu/ . The book "Pluto Confidential," co-written by Dr. Laurence A. Marschall, who voted for the IAU definition, and Dr. Stephen P. Maran, is also great as a source that presents both sides of this issue. Two books appropriate for kids that present both sides are "Ten Worlds" by Dr. Ken Croswell and "Thirteen Planets" by David Aguilar, written for National Geographic. On the pro-Pluto side, there is also the book "Is Pluto A Planet?" by Dr. David Weintraub, and my Pluto Blog at http://laurelsplutoblog.blogspot.com .

Keith Enevoldsen: 05/01/2014 02:39 CDT

(1) It is useful to have a named category for objects that are large enough to be a sphere but smaller than a star. These objects are easily recognized and so constitute a natural category. (2) It is useful to have a named category for objects that dominate their orbits. Nature very clearly discriminates objects that dominate their neighborhoods from those that do not (see Soter's discriminant and Stern and Levison's discriminant). Each of these two categories should have a simple name. It doesn't really matter what names are chosen. I like (1) planemo (planetary-mass object) and (2) planet.

LJ: 05/01/2014 06:48 CDT

A planet or a dwarf planet, it doesn't really matter. It's still there, and still the same, no matter how we call it. Really looking forward to learn more about it and see it up close. Only 439 days to Pluto ... :)

Anonymous: 05/01/2014 09:45 CDT

that the sphericity of an object does not necessarily have something to do with the mass or size of the object. a frozen droplet of water orbiting the Sun will be more or less spherical. if you don't add a gravitational criterion like the IAU did, you might as well say that it is a planet. the IAU definition actually makes perfect sense, while the words they used for it are poorly chosen. As someone recently explained in a letter to Sky & Telescope (Jan 2014 issue, p. 8 ): "if you calculate the ratio of a body’s mass to that of all the other matter it can influence in its orbital zone, the ratio is greater than 5,000 for the eight major planets but is less than 1 for Pluto, Eris, and Ceres."

Laurel Kornfeld: 05/01/2014 12:12 CDT

The geophysical planet definition DOES have everything to do with the mass and size of an object. It's not a matter of the object being spherical; it's that the object is shaped by its own gravity rather than by chemical bonds. The reason the IAU definition does not make perfect sense is that it is based solely on where an object is to the exclusion of what it is. The further an object is from its parent star, the larger an orbit it will have to clear. If Earth were in Pluto's orbit, it would not clear that orbit either. That means that according to the IAU definition, the same object can be a planet in one location and not a planet in another. Why not instead keep the term "planet" broad and then distinguish between different types using subcategories? Some planets dominate their orbits, and some don't, but that doesn't make the latter not planets. Also, we should note that only four percent of the IAU voted on their definition, most of whom were not planetary scientists but other types of astronomers. No absentee voting was allowed, so anyone not in a particular room on the last day of a two-week conference could not vote. The decision was immediately opposed by hundreds of professional astronomers in a formal petition led by Dr. Stern. The fact that the process by which the IAU came to its definition is itself flawed presents a compelling argument for revisiting this issue. Interestingly, in 2009, a group of astronomers asked the IAU leadership to reopen the discussion, and the leadership refused, resulting in those astronomers boycotting that year's General Assembly. We also have a lot more information now about Pluto, Eris, Vesta, exoplanets, etc.--certainly more than enough to re-open the discussion based on discoveries since 2006. No definition should be static and unchanging; if one is adopted at all, it should always be open to reconsideration when we learn new things about these objects.

David Frankis: 05/01/2014 01:47 CDT

'endure Jeff Moore's accusation of being "Callisto with weather,"' but isn't there a value judgement in there too? As if weather is somehow peripheral to the planetiness of a body, while geological activity is the *real* thing? I think all attempts at definition suffer from this. They tell us more about what the definer thinks is important (which often bears an uncanny resemblance to their own specialism) than about the science.

CJ Carter: 05/01/2014 01:49 CDT

I've written on this a few times since 2006, taking the IAU-goofed stance. As with others, my primary beef is with the "cleared the neighborhood" clause. While there is a kernel of usefulness to it, it's inability to scale renders it ridiculous (e.g. a lone farmhouse is a house, but a house of the same design in suburbia is a dwarf house). My proposition is that instead of the neighborhood-clearing clause, we substitute something that focuses on collisions capable of destroying or largely resurfacing the body (though I imagine most of this gets covered during system formation and thus renders the clause moot). There is a lot of room for nailing down a definition that still allows for some flexibility at the fringes -- and is applicable to systems outside our own. My most recent blather about it is here, where I expand on the definition.

Stephen: 05/02/2014 04:07 CDT

1) Who gave the International Astronomical Union (IAU) the right/power to define "planet" anyway? Aren't geologists (in the form of the IUGS maybe--the International Union of Geological Sciences) better qualified to a define what planets are? 2) Where is the IAU's definitions for "galaxy", "star". and "moon"? If the IAU was horrified at the thought having dozens of planets in the Solar System, they must be absolutely aghast at the dozens of moons littering the place, not to mention the uncountable billions of stars and galaxies in the heavens. Clearly there are way too many and steps should be taken ASAP to winnow them down to some more reasonable number. Like eight. Apiece, naturally. Eight moons, eight stars, eight galaxies. Such a nice round number is more civilised. All the rest of the riffraff can be termed "dwarf moon", "dwarf star" or "dwarf galaxy", as appropriate--as long as it is clearly understood that these are NOT the same as ACTUAL moons and ACTUAL stars and ACTUAL galaxies.

Adolf Schaller: 05/02/2014 11:28 CDT

Echo the remarks by Laurel Kornfeld. The definition of 'planet' is, and has always been, best left in its generic form. Nothing more to add to this tiresome controversy.

Stephen: 05/05/2014 10:08 CDT

@LJ: "A planet or a dwarf planet, it doesn't really matter. It's still there, and still the same, no matter how we call it. " Actually, it does matter to somebody or the IAU would never have bothered drawing the distinction between "planet" and "dwarf planet" in the first place. The IAU could have crafted their definition of "planet" to be as expansive as possible then set up subcategories (eg "dwarf planet", "gas giant", "ice giant", "asteroid" etc) based on various criteria of a more restricted sort. Instead they crafted the definition of planet in such a way as to keep the label "planet" confined to a small exclusive club (which naturally included our own Earth) whilst excluding the riff-raff (like Pluto, Eris,and Ceres). The very label of "dwarf planet" is a reminder of just how ludicrous the IAU's definition is. It's insistence that a "dwarf planet" is not a planet is as ridiculous as insisting that a "dwarf star" is not a star!

Todd: 05/07/2014 11:46 CDT

Reasons to criticize? Take your pick. A public relations disaster, the perception of scientific elitism, a science funding blunder, educational confusion, political maneuvers masquerading as science, angry space fans, beleaguered science journalists, etc., but for me the main point is that the naming of things is culture and history more than science. Geography is different from geology But even from the science is muddled. Will the probe rewrite what we know about Pluto? You announce the results after the experiment. Why not wait until after the probe's flyby? A scientific classification, in this case of planets, should be judged by what it gets you. Have we learned anything,is knowledge organizedm or are astronomers benefited somehow by making this classification? If a definition was needed right now, it should include the current planets, and if that wouldn't do, then introduce a new technical word. That would be appropriate if the IAU expects to change their definition.

Michael Welford: 05/08/2014 01:19 CDT

In their desperate rush to find an excuse to demote Pluto, the IAU actually bungled into an interesting idea: the Orbital Bully. Their clearing the orbital neighborhood language is an inadequate attempt to define a Bully. Hot Jupiters around distant stars show what happens in Bully confrontations, One gets ousted from the system (or perhaps gets sent too close to the primary), The other winds up in an irregular orbit. A Bully doesn't have to orbit a star. Titan sent a lesser bully to its doom near Saturn (thus creating the rings), and got an eccentric orbit. Triton ousted its rival from the Neptune system, and wound up in an unstable backwards orbit. (Who was the winner of that contest?) A Bully doesn't need to be round. After 40 billion orbits around Pluto, Hydra is a little bully.

Annraoi McShane: 05/08/2014 03:20 CDT

It just looks as if the IAU wanted to keep the number of planets down - and came up with a set of rules that achieve that. Mr Occam (and his razor) would be twirling in his grave at the complex logical contortions that these rules represent. Rule 1 is fine, an object that is massive enough to gravitationally pull itself into a spherical shape no problem - we expect planets to be round (other than for cultural/tradition reasons Ceres would be one too). Rule 2 orbiting the sun fine. Rule 3 is, frankly, a little more difficult to accept. It presupposes (I think) that orbits are, to all intents and purposes, fixed - when they are quite dynamical. It is theoretically possible that a planet that is currently in an orbit that is "clear" could (over long periods) drift into an orbit that isn't (for example due to resonance). Viewed at different epochs the object might be a planet - at other times it would NOT ;-) A small object (Pluto or smaller) at very great distance may, due to the size of the orbit, have the orbit to itself (a clear orbit) but only due to the orbit size rather than the size of the object. So Rule 3 might be broken by a small object (remote) that would not clear it's orbit had it been placed in a denser part of the solar system. In any event Neptune's orbit isn't clear (Pluto's crosses it, albeit not in the same plane). I'd much rather we have the headache of >50 planets than artificially constrain it. The universe is as it is - if this solar system has 10, 100, 1000 planets - so be it. I would just relax the rule about having everyone remember all their names ;-)

Annraoi McShane: 05/08/2014 03:40 CDT

Oh, and as an aside Jupiter hasn't cleared it's orbit either - the Trojan Asteroids that precede and follow it in it's orbit would be another clear break of the "clearing the neighbourhood" rule. Classifications should with minimal complexity allow you to group or classify things - this rule set just doesn't. In the case of the Trojan asteroids their location is at the Lagrange points of the Jupiter's orbit - so are not likely to be cleared. Unless one introduces further complexity (setting a size limit below which objects are ignored for this rule, or excluding objects at stable locations like the Lagrange points) or perhaps something more arbitrary....

Alson Wong: 05/08/2014 10:11 CDT

Stern and Levison proposed using a parameter determined by a body's mass, semi-major axis, and a function of the orbital elements of the small body being scattered and the degree to which it must be scattered to determine which planetary bodies control the region surrounding them. They defined objects with a Stern-Levison parameter greater than 1 as "uberplanets" and objects with a Stern-Levison parameter less than 1 as "unterplanets." http://en.wikipedia.org/wiki/Clearing_the_neighbourhood

Michael Paine: 05/12/2014 04:25 CDT

In 2000 I wrote a short essay on the definition of a planet - well before IAU "demoted" Pluto and the discovery of large KBOs: http://users.tpg.com.au/horsts/microplanet.html A pity that planetary scientists (instead of mostly stellar and galactic astronomers) did not take the initiative at that time. My preference is that Clyde Tombaugh was the first person to discover a "binary planet".

Anonymous: 05/16/2014 03:35 CDT

Thanks Emily for this post and for linking to relevant information on both sides of the debate. I will throw in my two pence here: I think the IAU’s planet definition is pretty awful and heartily agree with many of the comments above. Heavenly bodies such as clusters, galaxies, black holes and stars are classified by their physical characteristics only because this is straightforward and easy to understand. (can you imagine how messy things would become if we started classifying stars and galaxies dependent on their neighbors and positions?) Consider an iPad app to teach the public how the IAU’s naming system works. In this app people can move planets into other orbits, and they try the following: - Consider us moving Earth to either orbit Jupiter or follow/lead Jupiter as a Trojan at L4/L5. Why should Earth no longer be a planet? - Consider moving Earth out into the Kuiper Belt or Oort Cloud. Why should Earth no longer be a planet? - Let’s eject Earth from the solar system. Why should Earth no longer be a planet? - Wahoo! We discover planet X (and maybe planets Y and Z) out in the Oort cloud! It’s out beyond Sedna's aphelion, and it may be Earth sized, super-Earth or Neptune sized. So why should planet X not be a planet? The IAU’s naming system really obfuscates things for the public. We do not classify a dwarf galaxy as not being a galaxy, a star detached from a galaxy as not being a star, and a satellite galaxy as not bring a galaxy. Let us speak of rogue planets as being planets too, dwarf planets as planets too, and yes, massive spherical moons as being satellite planets. I suggest we speak of our solar system as having 8 classical planets, 18-ish satellite planets, and an every growing family of dwarf planets.

Lionel (with apologies for forgetting my name above!): 05/16/2014 03:35 CDT

Thanks Emily for this post and for linking to relevant information on both sides of the debate. I will throw in my two pence here: I think the IAU’s planet definition is pretty awful and heartily agree with many of the comments above. Heavenly bodies such as clusters, galaxies, black holes and stars are classified by their physical characteristics only because this is straightforward and easy to understand. (can you imagine how messy things would become if we started classifying stars and galaxies dependent on their neighbors and positions?) Consider an iPad app to teach the public how the IAU’s naming system works. In this app people can move planets into other orbits, and they try the following: - Consider us moving Earth to either orbit Jupiter or follow/lead Jupiter as a Trojan at L4/L5. Why should Earth no longer be a planet? - Consider moving Earth out into the Kuiper Belt or Oort Cloud. Why should Earth no longer be a planet? - Let’s eject Earth from the solar system. Why should Earth no longer be a planet? - Wahoo! We discover planet X (and maybe planets Y and Z) out in the Oort cloud! It’s out beyond Sedna's aphelion, and it may be Earth sized, super-Earth or Neptune sized. So why should planet X not be a planet? The IAU’s naming system really obfuscates things for the public. We do not classify a dwarf galaxy as not being a galaxy, a star detached from a galaxy as not being a star, and a satellite galaxy as not bring a galaxy. Let us speak of rogue planets as being planets too, dwarf planets as planets too, and yes, massive spherical moons as being satellite planets. I suggest we speak of our solar system as having 8 classical planets, 18-ish satellite planets, and an every growing family of dwarf planets.

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