Demoting Pluto

The artist's rendition shows the newly discovered planet-like object, dubbed "Sedna," in relation to other bodies in the solar system, including Earth and its Moon; Pluto; and Quaoar, a planetoid beyond Pluto that was until now the largest known object beyond Pluto. The diameter of Sedna is slightly smaller than Pluto's but likely somewhat larger than Quaoar.

The artist’s rendition shows the newly discovered planet-like object, dubbed “Sedna,” in relation to other bodies in the solar system, including Earth and its Moon; Pluto; and Quaoar, a planetoid beyond Pluto that was until now the largest known object beyond Pluto. The diameter of Sedna is slightly smaller than Pluto’s but likely somewhat larger than Quaoar.

I would like to revisit in this blog post the 2006 decision of the International Astronomical Union (IAU) demoting Pluto as the Solar System’s ninth planet to “minor planetary” status. It caused a furor both inside the astronomical community and among many in the general public. This really wasn’t surprising, however, Pluto’s status as a planet had been a point of debate among astronomers for at least a decade before this action.

Neil deGrasse Tyson famously excluded Pluto from the Solar System display at the Hayden Planetarium in New York City and Brian Marsden and others campaigned to demote it. Caltech astronomer Mike Brown wrote a wry book in 2012 infamously titled, How I Killed Pluto and Why It Had It Coming, to sum up the rationale for the decision. This decision, of course, was predicated on advancing knowledge about the outer Solar System and the Kuiper Belt of small and icy bodies that reigned there. Marsden, Tyson, Brown, and others succeeded at the IAU meeting in 2006, having Pluto and other bodies of similar size reclassified as “dwarf planets.”

The rationale for this decision rested on the advances in scientific understanding made in the first half of the decade; especially the discovery of three additional bodies of similar size in the Kuiper Belt—Eris, Makemake, and Hauemea—as well as the asteroid belt body named Ceres. At its simplest the conundrum revolved around whether to designate these other bodies as planets or to exclude Pluto from the list as well. After a tortured process, not a little controversy, and numerous false starts the IAU announced the creation of the “dwarf planet” category and assigned these newly discovered bodies along with Pluto to it.

The idea of a region of the outer solar system with large orbiting bodies, no called the Kuiper Belt, first gained currency in 1992 with the detection of a 150-mile wide body, called 1992QB1. Several similar-sized objects were discovered thereafter, confirming that a belt of icy objects once theorized by Gerard Kuiper did indeed exist. The planet Pluto, discovered in 1930 by Clyde Tombaugh, is only the largest member of the Kuiper Belt. Other named objects soon joined Pluto, including 1992QB1, Orcus, Quaoar, Ixion, 90377 Sedna, and Varuna.

The discovery of these many objects, nearly as large as Pluto and occupying the same range in the outer solar system, led to the IAU decision in 2006 to redesignate Pluto a “dwarf planet.” The first members of the “dwarf planet” category were Ceres, Pluto, and 2003 UB313. Astronomers agreed that the following criteria defined a planet: (a) it is in orbit around the Sun, (b) it has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) it has cleared the neighborhood around its orbit.

If the IAU meeting where the astronomers decided this was contentious it was nothing compared to the uproar coming thereafter from the general public. Some refused to accept it—protests took place at several locations, state legislatures passed resolutions refusing to accept it, and memorials appeared at the Pluto displays at museums and science centers around the U.S.—at the National Air and Space Museum in Washington, D.C., someone hung black crepe paper around the display on Pluto in the “Exploring the Planets” exhibition and others left cards and flowers on the floor before the panel. At the same time, some put a decidedly political slant on the 2006 decision, asserting that it could be blamed on resentment by many other nations of the world for America’s unpopular foreign policy in Iraq.

But the IAU decision made a lot of sense once one pondered the situation. It pointed up well the manner in which the scientific method worked to further understanding as new discoveries and analyses required modification of what we thought we knew about the natural universe.

From the point that Percival Lowell was searching for a fabled Planet X in the early twentieth century, a mythical ninth planet whose gravity affected the rest of the solar system, astronomers knew something was out there as yet undiscovered. They thought in terms of a single body, a planet, although scientists have learned since that time that the mass of the Kuiper Belt accounts for the perturbations measured  by Lowell and others.  Clyde Tombaugh’s 1930 discovery of Pluto, as well as its embrace by the American public, seemed to settled the matter for a while. Pluto was Planet X, or so many thought, but as refinements of the planet’s size and mass proceeded it appeared more and more that there had to be something bigger than Pluto in the outer solar system. Over time the identification of what became known as the Kuiper Belt containing thousands of bodies, some even larger than Pluto, demonstrated that Pluto’s status as a planet needed to be reassessed.

After years of debate among the scientists advancing knowledge of the solar system, the case of Pluto finally emerged as a major agenda items at the IAU in 2006. There some 2,500 astronomers gathered to reconsider how to define planets and their place in the Solar System. There were rivalries, priorities, disciplinary loyalties, and the like that  played into the decision to reclassify Pluto as a “Dwarf Planet.” Much of it had to do with how scientists defined the problem and what type of science they conducted, and whether or not they were liberal or conservative not in their politics but into their perspective on scientific nomenclature.

People gather to protest Pluto's loss of status as a planet, September 1, 2006..

People gather to protest Pluto’s loss of status as a planet, September 1, 2006.

Not a little about the deliberations were humorous. As reported by science writer Alan Boyle about the tenor of the IAU meeting: “Mercury may be a burnout case, and Mars isn’t what it used to be. Venus is a hottie, but she’ll make your life hell. With Saturn, it’s all about the rings and the bling. Jupiter takes himself waaaay to seriously. Uranus won’t stop with the off-color puns, while Neptune’s jokes will leave you cold. But Pluto? Now that’s one funny planet!”

In the end, at the IAU meeting scientists did what scientists do best, fought like cats and dogs over important issues and came to a consensus that makes sense of the scientific data as then understood. That’s what happened in Prague in 2006 as the IAU struggled, perhaps a bit too publicly, in deliberations over the definition of a planet.

The decision-making process concerning Pluto at the IAU in 2006 was an impressive object lesson in how the scientific method works, as ideas are brought forward and shot down until something that best explains what is known about the natural universe is crafted. No other method of dealing with knowledge works so well. At some level I was sorry to see Pluto demoted, but this process has led to better understandings of planetary science and how and why decisions are made.

After I got used to the idea of only eight planets in the solar system, along with thousands of other smaller bodies, the story of the Prague meeting is actually comforting. It speaks to the manner in which we can live in a fact-based universe and make changes to beliefs long-held.

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4 Responses to Demoting Pluto

  1. mike shupp says:

    And some of us are still dissatisfied. Let’s consider that Pluto orbits the sun by itself; it’s not an object in the middle of the Kuiper Belt. It’s round. It has satellites. It is reasonably large. It meets what most people consider as requirements for a planet.

    More to the point, it was the last major planet-like thingy to be discovered before Oct 4, 1957. Yes there are other big round things, we know, but they can be excluded from the set of objects which the general public as planets. Ceres and Juno are clearly part of the Assteroid Belt. Sedna has a far-from-circular orbit and wasn’t found until 2003. Other large objects in the Kuiper Belt again were not observed prior to the Space Age. In other words, Pluto might be considered a planet for “social” reasons even if it fails to meet a definition created long after its discovery. As an analogue, I’d suggest that a good practical definition of “US citizen” should be equally “social” – – general and encompassing (“any child of a US citizen”) rather than narrowly legalistic (“except for immigrants from Serbia and Albania born before 1 January 1975 and arriving after 3 February 1979”).

    I will also make the philosophical point that the IAU’s naming scheme seems clearly aimed at exactly one solar system — our own, with a size-based distribution of small and large planets which have moved over time into what looks like a stable configuration, quite different from the planets our instruments have revealed at literally thousands of other stars. Granted, few members of the IAU could have guessed how unusual our social system might appear when Pluto was being expelled from it, but this just helps make the point that our definitions are more a product of society (and psychology) than rigorous thought or clear physical distinctions.

    But perhaps I’m just being romantic myself. I still think Xena and Gabrielle were perfectly fine names for what professional astronomers prefere to call Eris and Dysnomia. (Dammit, beings from television serials are PERFECTLY suited for providing names to objects never viewed before the space age!) Oh well.


  2. The IAU process in 2006 was a disaster, and the planet definition adopted was terrible, which is why seven years later, that definition is still rejected by a large number of astronomers and planetary scientists. Only four percent of the IAU voted on this, and most are not planetary scientists. Their decision was opposed by hundreds of professional astronomers in a formal petition led by New Horizons Principal Investigator Dr. Alan Stern.

    Only 424 IAU members actually voted, as the rest of the original 2,500 attendees had already left by the time the vote was held on the last day of the General Assembly. The vote was actually held in violation of the IAU’s own bylaws, which state that a resolution must first be approved by the appropriate IAU committee before being voted on during a General Assembly. The IAU’s committee had recommended a resolution including Pluto, Eris, and Ceres, but the General Assembly voted it down. At that point, the issue should have gone back to the committee; instead, a small group of astronomers with an agenda of excluding Pluto threw together a hastily crafted alternate resolution at the last minute, after most participants had already gone home, assuming the issue would go back to committee and be brought up again in three years. Dr. Owen Gingerich, who headed the IAU Planet Definition Committee, admitted publicly that had he known this would happen, he would not have left the conference early.

    In 2009, a group of astronomers petitioned the IAU to reopen the discussion, but the IAU leadership refused, leading the petitioning astronomers to boycott that year’s General Assembly. Many have permanently distanced themselves from the IAU.

    Based on the assumption that Eris was larger than Pluto, later proven incorrect, the decision was premature. The “need” for a reclassification was based on the completely non-scientific notion that our solar system cannot have too many planets because kids wouldn’t be able to memorize them. Memorization is not even important for learning, and besides, since when does convenience drive science? Nobody says we cannot have billions of stars or billions of galaxies. Our solar system has whatever number of planets, and artificially keeping the number small makes no sense.

    Dwarf planets are not “minor planets.” Minor planet is a synonym for asteroids and comets, objects the IAU classifies as “Small Solar System Bodies.” Unlike those, Pluto is large enough to be rounded by its own gravity and is a complex world with geology, layering, and weather. It may even harbor a subsurface ocean. In fact, Dr. Stern is the person who first coined the term “dwarf planet” back in 1991, but his intention was to designate a third class of planets in addition to terrestrials and jovians, not to designate non-planets. The 424 IAU members who voted misused his term.

    Significantly, in astronomy, dwarf stars are still stars, and dwarf galaxies are still galaxies; thus, it makes sense for dwarf planets to be a subclass of planets. Dwarf planets have all the features that larger planets have; they just are smaller versions of them.

    The idea that an object has to “clear its orbit” to be a planet was contrived specifically to exclude Pluto. It is a nebulous idea, as no planet fully clears its orbit of asteroids, and Neptune does not clear its orbit of Pluto. Many exoplanets have extremely elliptical orbits that cross the orbits of another planet in their system or take them through asteroid belts during their orbit.

    Several points you make in your article are incorrect. First, neither Neil de Grasse Tyson nor Mike Brown was at the General Assembly at all. Brown is not even an IAU member. He did not discover Eris alone but with two partners, Dr. Chad Trujillo, and Dr. David Rabinowitz. Interestingly, Rabinowitz is one of the 300 professional astronomers who signed the petition rejecting the IAU decision.

    Tyson has since moderated his stand on Pluto, apparently with the help of the late Patsy Tombaugh, widow of Pluto discoverer Clyde Tombaugh. He met Patsy when he visited the family while making his TV program “The Pluto Files.” Acknowledging that the debate remains ongoing, he put a plaque in the Hayden Planetarium stating that many astronomers disagree with the IAU and do still consider Pluto a planet.

    So far, no objects larger than Pluto have been found in the Kuiper Belt. Objects like Haumea, Makemake, and Eris are not tiny iceballs–they are worlds rounded by their own gravity as opposed to shaped only by their chemical bonds. That is what makes them small planets. And Pluto, estimated to be 70 percent rock, is hardly an iceball.

    The IAU definition is flawed because it defines an object solely by where it is to the exclusion of what it is. If Earth were in Pluto’s orbit, it would not “clear” that orbit either. This means that according to the IAU definition, the same object can be a planet in one location and not a planet in another, which makes absolutely no sense.

    Our solar system does NOT have eight planets. It has 14 and counting, if one counts Pluto-Charon as a binary system. There is no consensus among the astronomy community on the question of a dynamical versus a geophysical planet definition. To pretend there is and that all is settled once and for all is to mislead the public.

    Pluto’s status has actually been in question not for 20 years but since its discovery more than 80 years ago, given that it was too small to be resolved into a disk with the telescopes of the 1930s. Today, we know that even Ceres, smaller than Pluto, is spherical and therefore, a small planet rather than an asteroid.

    You are also selective in the books you choose to mention. Alan Boyle wrote an excellent book, “The Case for Pluto” while Dr. David Weintraub wrote “Is Pluto A Planet?” Why do you not mention those? I am working on a book as well, “The Little Planet That Would Not Die: Pluto’s Story.”

    Far from being “comforting,” the story of the Prague meeting is one of a tiny group imposing their interpretation on the whole world, obscuring the fact that debate is ongoing, and then refusing again and again to reconsider the issue. Science is not decreed by an authority on high. Nobody ever voted on gravity or relativity. Not all change is good. This one was accurately described by Dr. Stern as “an embarrassment to astronomy,” as it caused and continued to cause major confusion among the general public. It also violates the Copernican principle by only dealing with our solar system and ignoring exoplanets at a time when they are being rapidly discovered, and we are finding out they are stranger than anyone had previously thought.

    All these reasons are why I along with other amateur astronomers, professional astronomers, and members of the public are actively urging the overturning and/or ignoring of the ignominious 2006 debacle.


    • launiusr says:

      I very much appreciate your perspective on this episode in the history of astronomy. And I especially thank you for corrections to the blog post.


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