Here’s Your Chance To Fund A Universe Today Project On The Pluto Planethood Debate

23 Replies to “Here’s Your Chance To Fund A Universe Today Project On The Pluto Planethood Debate”

1. My question: why does the definition matter at all? The various objects in the Solar System have various properties. A scientist usually studies only one topic at a time. If they want to study liquids, it’s Earth, Europa, Enceladus, Titan, and maybe a few more. Substantial atmospheres: Venus, Earth, maybe Mars, the gas giants, Titan. Large enough for interesting geology: anything near hydrostatic equilibrium, plus apparently Vesta. Ice: perhaps everything but Venus and Io. And there’s no term for any of those groups except the description “bodies that have such-and-so characteristic”.

   Any single definition is useful to at most one group, the people who are studying, working with, frequently referring to the objects that have that definition. For every other group, a definition is useless.

2. So, Elisabeth is asking for our money so that she can reopen the Pluto planethood debate? First of all, it eludes me what the relevance of the New Horizons mission is to this so called Pluto planethood debate. As Tim McDaniel already stated, why does the definition of a planet or dwarf planet matter at all?
   Second, there is no such thing as a controversial IAU vote about the status of Pluto. The resolution about Pluto demotion in 2006 was adopted by a large majority of astronomers that attended the IAU meeting in Prague. Of course, only a small minority of the international astronomical community attended the meeting in Prague, but with the exeption of a rather small number of mostly American astronomers the new definition of a planet was welcomed by most scientitst, even if it contains some shortcommings. At least here in Europe (I’m Dutch) there isn’t a Pluto planethood debate at all. That a small number of American astronomers think otherwise, doesn’t make the IAU decision controversial. Call it a planet, dwarf planet or whatever, Pluto is clearly a member of a large group of objects that populate the Kuiper belt.

   1. Personally I’m not concerned with Pluto. It’s clear that it bears as little resemblance to Earth as Earth does to Jupiter. It’s just not possible to have a single, precisely defined word that includes all three objects (or even two of them) while *still being useful*. Such a word would have to be so vague as to be of no practical use.

      I’m more concerned about “hot Jupiters” (very close to star) vs our Jupiter vs “cold Jupiters” (takes millions of years to orbit star) vs rogue Jupiters (booted from their stars) vs star-like rogue Jupiters (formed out of a stellar nursery with high metal concentrations rather than a planetary disk, but are otherwise identical to rogue Jupiters). The current definition is so ridiculous that you can use it to claim that some of those objects are planets, while others aren’t. That’s absurd.

      I laid out the only viable definition in my other post. I don’t care what word they use to describe such objects, it’s the existence of a spectrum definition that matters.

   2. Sorry, but yes, there is a debate, and the IAU definition has remained controversial, not just in the US, but all over the world. There was no “large majority” who voted in Prague. Four percent of the IAU, most of whom are not planetary scientists but other types of astronomers, voted on this definition. They did so in violation of the IAU’s own bylaws, which require a resolution be first vetted by the appropriate IAU committee before being put to the General Assembly floor. That was not done, as the resolution adopted was thrown together on the very last day of the General Assembly and was done so behind the backs of many astronomers who should have had a say in the matter. Dr. Owen Gingerich, chair of the IAU’s own Planet Definition Committee, said that had he known a small group of astronomers was going to throw together an alternate resolution at the last minute, he would not have left the General Assembly several days early, in accordance with his original plans.

      There was also no electronic or absentee voting allowed in Prague. At its last General Assembly in 2010, the IAU leadership finally agreed to allow electronic voting at future General Assemblies.

      Pluto is not just a member of a large group of bodies in the Kuiper Belt. The overwhelming majority of those objects are tiny, shapeless rocks. In contrast, Pluto is much larger, large enough and massive enough to be rounded by its own gravity. This makes it a complex world with geology, weather, and layering into core, mantle, and crust. Blurring the distinction between a complex body like Pluto and tiny iceballs or rubble piles is simply bad science.

      Then there is the fact that the IAU decision says a planet must orbit the Sun, not “a star.” That means no exoplanets can be considered planets.

      A requirement to “clear its orbit” is inherently biased against objects further from their parent star, which have larger and larger orbits to “clear.” If Earth were in Pluto’s orbit, it would not clear that orbit either. This means the IAU definition could result in the absurdity of the same object being classed as a planet in one location and not as a planet in another.

      It also precludes any binary planet system (because the two objects in a binary do not clear their orbits of one another) and rogue planets, which have no orbits to clear.

      Gopher65 presents an excellent idea when he advocates a branching spectrum, similar to the Herzsprung-Russell diagram used for stars. This merits serious consideration.

      No definition, even a good one, should be considered adopted and in place for all time. New discoveries always mandate reconsideration and incorporation of those discoveries into our knowledge base, often making us revise what we thought we knew on any subject. Dawn’s flyby of Ceres and New Horizons’ flyby of Pluto will both present us with new knowledge about these worlds that will compel us to view them in a new light.

      1. As I said, the IAU definition has its shortcommings, but it reflects our new insights after de discovery of the Kuiper belt in the early 1990ties. If Pluto was discovered in 1995 instead of 1930, it would never be regarded a planet but one of the many KBO’s. We don’t know (yet) if Pluto has a layered structure or if it’s geological actife. So you can’t proclaim Pluto to be a planet on those grounds.There isn’t such a thing as a Pluto debate. Please, show me the peer reviewed articles in the scientific literature that question the status of Pluto as a dwarf planet. Therefore we can safely assume that the large mayority of astronomers agree wit the demotion of Pluto. As I said, here in Europe the status of Pluto isn’t an issue. This seems to be an issue only on American websites. Because we have new knowledge about the Kuiper belt, knowlegde that Clide Tombaugh didn’t have, our ideas about planets changed. This happened before, in the 19th century as more and more asteroids were discovered. Should we obtain more relevant information in the future that would challengde our current understanding of (exo)planets, of course we should adapt our definition in that case. I agree with you on that.

      2. The IAU definition reflects one interpretation generated by the discovery of the Kuiper Belt in the 1990s. It does not reflect any new information learned about our solar system and others since 2006.

         Many astronomers disagree with the claim that if Pluto were discovered today, it would not be considered a planet, just another KBO. Again, Pluto is clearly much larger and much more complex than the overwhelming majority of KBOs. The latter do not have atmospheres or geology; they are mostly rubble piles and iceballs. Debate over what Pluto is and how it should be classified dates all the way back to its discovery. The same debate was raised by the discovery of Eris. If Pluto were discovered today, dynamicists would not consider it a planet but geophysicists would. In other words, there was debate in 1930 and still is debate today.

         Just repeating “there is no debate” does not make that statement fact. If there really weren’t any debate, why is the question of Pluto’s status and planet definition a regular topic of discussion at scientific conferences, such as those of the American Geophysical Union and European Geophysical Union? Why is the issue still debated in books and articles?

         Scientists on both sides of this debate continue to publish peer-reviewed articles on Pluto. Many scientists continue to refer to Pluto as a planet in these studies. Many also, including the astronomer who first coined the term “dwarf planet,” Alan Stern, have no problem with that term, only with the notion that dwarf planets are not planets. When they use the term “dwarf planet,” they use it in the context Stern originally intended, to mean a small planet.

         There is a special edition of the journal Icarus with papers presented at the five-day Pluto Science Conference last summer at JHUAPL. I recommend you read these papers and note the number of times Pluto is referred to as a planet.

         Pluto is believed to have a layered structure and be geologically active based on the findings of researchers studying it as well as well as our best observations from Hubble and Earth-based telescopes. In less than one year, we will know for sure.

         Are there any peer reviewed articles in scientific literature that argue Pluto is not a planet? We are talking about two competing interpretations. Once the data from New Horizons starts coming in, scientists on both sides will likely publish articles using that data to support and/or oppose Pluto’s planet status.

         With 333 IAU members having voted for resolution 5b in 2006 and another 300 opposing that decision in a formal petition, no, we cannot say the majority of astronomers agree with the demotion of Pluto.

         The notion that the status of Pluto is only an American issue is a straw man. Planetary scientists from around the world signed the petition rejecting the IAU decision. The US likely has more planetary scientists than do many other countries because of its active 50-year space program. Planetary scientists are the ones who care about this issue; other types of astronomers generally ignore it and concentrate on their own fields.

         Our ideas about planets have changed multiple times. Evidence in our solar system and others shows that planets can be located among belts of tiny objects. They can have eccentric orbits and be in orbital resonances with other planets. When classifying Pluto, one cannot just look at the Kuiper Belt; one also has to look at Pluto itself. Clyde Tombaugh was around when the Kuiper Belt was discovered, yet he continued to regard Pluto as a planet. Like him, many astronomers dually class Pluto as both a planet and a Kuiper Belt Object. The first tells us what it is; the second tells us where it is. They are not mutually exclusive.

         The 19th century reference actually refers to an error. Ceres, along with other objects in the belt between Mars and Jupiter, was demoted to asteroid. However, this turned out to be a mistake. Telescopes of the time could not resolve Ceres into a disk. Today, we know that unlike almost every object in the asteroid belt, Ceres is spherical and therefore, at least according to the geophysical planet definition, is a small planet. Like Pluto, it is suspected to harbor a subsurface ocean.

         The demotion of Pluto was not based on any new knowledge about Pluto itself, only about its surroundings. Ironically, New Horizons had already been launched at the time of the IAU vote. Between this mission and Dawn, we will have new relevant data that will challenge our understanding of these objects in less than one year. The IAU should have waited until the new data is in to make any attempt at definitions. Their primary motivation came from the very unscientific fear that we would end up with “too many planets” due to continual discoveries of new Kuiper Belt planets. Next year will offer a golden opportunity to re-examine this issue based on a huge amount of new information.

      3. Aliens will object that you called their planets second sized exo-planets because they have a different heritage.

   3. The IAU’s 2006 reworking of the definition of a planet is controversial because it’s absurd. Also there were a number of shenanigans that occurred in Prague, including no vetting of the final resolution, and out and out bullying. I know one member who has publicly stated he was threatened with the destruction of his academic career were he to vote to keep Pluto a planet. To say it is solely an American thing is very provincial. For the good of Science, the definition of a planet needs to be redone to include dwarf planets as a third subcategory.

3. Any single, precise, non-spectrum based definition that includes both Earth and Jupiter as members is stupid. ie, the current definition of planet by the IAU.

   A decent definition would be a branching spectrum definition, starting at the point where gravity in the most important force that holds objects together (instead of chemical bonds), and ending at the point where fusion takes over as a dominant process. Branches naturally come off such a definition each time a new type of object is discovered. For instance, hot Jupiters and cold Jupiters wouldn’t be on the same secondary branch, but they’d connect to the main spectrum at the same point due to their similar masses.

   This is the *only* type of definition that makes sense. We already use this method for other things (like stars), so why are we trying to stick to some outdated, useless single phrase definition when it comes to planets? Stupid.

4. Thumbs up to both first comments. This is utterly ridiculous and totally devoid of science.

   1. Then why did the IAU devote so much time and energy to it? They opened this can of worms and now refuse to take responsibility for the confusion it has caused.

5. Before you run off to claim Pluto a planet, check out:

   Cruithne, Earth’s “2nd Moon”
   http://www.orbitsimulator.com/gravity/articles/cruithne.html

   Pluto Orbit
   http://www.orbitsimulator.com/gravity/articles/pluto.html

   And Orcus
   http://en.wikipedia.org/wiki/90482_Orcus
   (Which has a nice simulation of its orbit halfway down the page. Which is the same as pluto, but 180 degrees shifted.)

   Downgrading Pluto to a moon has a better chance of succes, as long as you don’t require that a Moon -should- orbit its Parent in less than a year. Every orbital period [of a moon] longer than a year would look different than simply circular.

6. “We’ll have some stuff about the science as well; stay tuned for the details!”

   Huh? Science is an afterthought?! What else are these interviews going to be about if not about the science?

   What have we (humans) learnt of significance about Pluto and KBOs in genreal since the “controversy” broke out that hasn’t already been said before?

   Laurele:
   “If Earth were in Pluto’s orbit, it would not clear that orbit either. This means the IAU definition could result in the absurdity of the same object being classed as a planet in one location and not as a planet in another.”

   That’s a BIG If. And a bit of a straw man. For one, we wouldn’t be here to talk about it. Even the hypothetical Us would have to contend with a radically different model of the formation of the Solar System from the one we currently have, so our present definitions will not have been put forward. And if your hypothetical solar system had been formed as if someone had carefully placed the Earth at Pluto’s orbit, then we would have surely noticed a big Rocky blue-green object admidst a sea of tiny things. It’s a situation completely different from realising that Pluto is not the last in a list of nine, but a first in the list of many objects –potentially hundreds of thousands–which seem, from this distance, to share to some extent similar physical, chemical and orbital characteristics.

   I do agree that the current definition of planethood is not to everyone’s satisfaction, but it must be realised that Pluto is a member of a set of objects that are distinct from the major gravitating objects (which are not stars!) and probably has a formation history that is as different.

   1. When you place Earth at the distance of Pluto then it not would not have cleared its orbit and thus also not be classified as a planet.

      However the Moon did clear its orbit and that could be regarded as a planet.

      It would be interesting how big a planet needs to be in order to clear its orbit at Pluto distance. Could a mass of Jupiter have done it?

      1. Any of the inner planets could have cleared Pluto’s orbit. Mercury would juuuuuust have been able to do it. If there were only 2 major bodies in the universe, the sun and Jupiter, Jupiter would have to be about 100 lightyears away from the sun before it couldn’t clear its orbit. Jupiter is big:P.

         Eris, the largest dwarf planet, would be considered a planet if it were at Earth’s distance from the sun. Pluto would have to be in Venus’s orbit.

      2. Pluto is larger than Eris, gopher65. Scientists have known this for over a year. See the findings by Bruno Sicardy of the Paris Observatory.

      3. No it isn’t. Look it up. I just did, and Eris is 1.28 times more massive than Pluto.

      4. The inner planets could not clear Pluto’s orbit. Dr. Hal Levison, an astronomer who actually supports a dynamical planet definition, did a series of calculations that clearly determined no planet the size of Earth or smaller would “clear” its orbit if it were where Pluto is.

         Eris is not the largest dwarf planet; that was determined four years ago by a team of astronomers led by Dr. Bruno Sicardy, who measured Eris more accurately when it occulted a star.

         If your statement were true, if Eris would be considered a planet at Earth’s orbit, then you once again point out a major flaw in the IAU definition–specifically, that the same object can be a planet in one location and not a planet in another. That alone makes the definition highly problematic.

      5. Here is the relevant Wiki page. As far as I can tell, everything there is accurate. https://en.wikipedia.org/wiki/Clearing_the_neighbourhood

   2. Speaking of hundreds of thousands of hypothetical objects just clouds the issue with space junk. There are only three confirmed dwarf planets past Pluto, none of which are larger than Pluto. Makemake, Haumea, and Eris. Chances are the IAU will not have the data needed to confirm Sedna, Biden, or any other objects past Pluto as a dwarf planet in our lifetime unless we develop much faster propulsion systems.

   3. “We’ll have some stuff about the science as well; stay tuned for the details!”
      That comment refers to the new data we will learn from the New Horizons mission during the flyby.

      There is no single model of solar system formation. Every time a new exoplanet system that shouldn’t exist is discovered, astronomers go back to the drawing board to determine how such a system could have formed. It’s safe to say our knowledge of solar system formation, even of how our own solar system formed, is limited and incomplete. There may be a Mars-sized object orbiting the Sun much further than Pluto, and this would once more lead astronomers back to the drawing board in considering how our solar system formed.

      The argument that the IAU definition could result in the same object being considered a planet in one location and not a planet in another is not a straw man. If an Earth-sized planet were located in Pluto’s orbit, even if it did not host any life, and our Earth remained in its own location, the former would not clear its orbit at 40 AU. Astronomers have not ruled out the possibility that an Earth-sized planet could be lurking much further out than Pluto. That object would never clear such an enormous orbit. It’s not a matter of someone arbitrarily placing Earth at Pluto’s orbit. The reality is that when we look at exoplanet systems, we find a large number of planets in locations where, according to our current understanding, they should not have formed and should not be located.

      However, just because we don’t fully understand why we’re finding planets in unexpected locations does not automatically mean those objects are not planets! An object should be defined by its intrinsic characteristics, not by its location!

      There is no one “current definition” of planethood. There are several competing ones, all of which are scientifically legitimate. In the case of a dynamical versus a geophysical definition, scientists largely agree to disagree.

      We are still learning about Pluto’s formation history, as we are about the formation history of every solar system planet.

      But Pluto is not just one of a set of objects! Its composition and processes are much more akin to those of the larger planets than to those of tiny, shapeless iceballs and rocks. Blurring that distinction is simply bad science. In 2000, Alan Stern and Hal Levison published a paper on the issue of gravitational dominance. In that paper, they distinguished between “uber planets,” which gravitationally dominate their orbits, and “unter planets,” which do not gravitationally dominate their orbits but are in hydrostatic equilibrium. However, they never argued that the latter should not be considered planets! What is wrong with saying that some planets gravitationally dominate their orbits, and some do not, but all still fall under the umbrella of “planets” based on their being rounded by their own gravity?

7. I think it is wonderful that Ms. Howell would like to learn more about NASA’s New Horizons mission to Pluto and about the reasons why the IAU’s 2006 definition of a planet is in desperate need of an upgrade. We should support her trip to go interview members of the New Horizons team.

   Dwarf stars (and our sun is a yellow dwarf star) are still stars. It is only right that dwarf planets be a subcategory of planets. Like Olaf said, Earth would not clear its orbit if it were as far out as Pluto. The third rung of the current IAU definition is superfluous. A planet is a planet by virtue of what it is. A core, mantle, crust, and atmosphere, sometimes moons, or in the case of gas planets, other attributes. But the whole “clear one’s path” rung is arbitrary and capricious. I look forward to Ms. Howell’s path toward reaching a better understanding of Pluto and the case for its replanetization when she shares it with us. It is for the good of Science that we support her in her endeavor.

8. Hmm.. As stated numerous times here on Universe Today, even in a video making the declaration, Pluto is a dwarf planet. That does not mean that there is not a billion things to learn from mysterious Pluto, but it does mean that public money will be better spent elsewhere other than debating and/or exploring the different sides of the debate that is purported to be ongoing about Pluto’s planethood. I think Elizabeth, or “Laurele”, should find a new tack if she wishes to use public money. Just an opinion, of course.

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