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.
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.