What In The Planet?

The Pluto Files: The Rise and Fall of America's Favorite Planet by Professor Neil DeGrasse Tyson Professor Neil DeGrasse Tyson found himself on receiving end of one of the most controversial non-issues of our time. The demotion of Pluto from planet to “dwarf planet” made the front page of the New York Times and prompted legislation in the state of New Mexico to rebuff the consensus of the International Astronomical Union recognizing Pluto’s planethood a matter of law. It’s a bizarre story well told by the unassuming astronomer who unintentionally found himself at the center of a firestorm of controversy.

In the Pluto Files
Tyson chronicles the events of “the rise and fall of America’s favorite planet.” This subtitle at first seems odd in that there does not seem to be any American consensus on what its favorite planet might be, but judging by the energetic reaction of the press, various disagreeable astronomers, and children in grade school who wrote letters of protest at the behest of their teachers, it is not far off the mark that the tiny ball of ice five billion kilometers away really does have a special place in the hearts of Americans.

It all began when Tyson, a leading astronomer at the American Museum of Natural History in New York City, helped design a scale model of the solar system that did not include the ninth planet. The decision to not include Pluto was not made solely by him or anyone else, but by a consensus agreement of several astronomers who devised their classification system on the basis of shared properties. This was not without its controversy as some astronomers thought it would be best to leave the matter alone and not risk confusing the public with technical scientific details. The curators of the Museum disagreed, however, believing it was their duty to inform and educate the public of the finer points of astronomy. Yet, with that in mind, they would soon find out that there really wasn’t a clear set of criteria that defined the necessary and sufficient conditions of planethood.

The problems with Pluto as a planet are legion. Its very small size takes up a width that would cover the coast of California to Kansas. It’s orbit floats in and out of the Neptune’s and is set 17 degrees off the orbital plane which all the other planets share making for an angular revolution around the sun. True, Pluto does have a small moon named Charon, but it does not revolve around a gravitational center found within Pluto itself. Instead, it revolves around a center that is in between the two bodies along with several other small chunks of ice. Furthermore, astronomers have found other bodies similar to Pluto named Sedna, Orcus, Quaoar, Varuna–all smaller than Pluto–as well as one that is larger named Eris (see here).

The “shared properties” thesis states that if certain bodies have the same or similar features, they can be classified accordingly. Thus, the so-called “terrestrial planets” are those that share a gravity strong enough to make an object spherical, though not strong enough to produce star-like nuclear fusion at the core. They have a rocky surface and crust that is smooth (a note on Earth, while we as tiny humans see a huge level of difference between the ocean deep and the mountain highs, if the Earth was the size of a cue-ball in our hands it would feel just as smooth). The Jovian planets, or the gas giants, share a huge size, a gaseous atmosphere, and rings. Asteroids like Ceres located in between Mars and Earth share properties with several smaller bodies that float around the sun in a similar manner. Having a moon certainly doesn’t matter since asteroids can have moons too.

Pluto shares properties with bodies that have been recently discovered called “trans Neptunian objects” that were hypothesized by Gerald Kuiper. So many of the objects have been discovered recently that they now bear his name as being part of the “Kuiper belt.” This renaming precisely follows the pattern of discovery that uncovered the asteroid belt. A large object named Ceres was floating in between Mars and Earth that was once thought to be planet, but hundreds of other smaller, yet similar objects were found along side of it undermining the idea that it could be called a planet. What was this key idea? According to the International Astronomical Union a planet must be large enough to dominate its orbit. Earth is hit by several meteors a day, but it is large enough to clear them out of the way with its orbital path being uninterrupted. Pluto has too many objects in and around its path to be recognized as such.

With these shared properties in mind the Museum thought it best to classify Pluto accordingly, but people didn’t like it. While there really wasn’t much of a scientific argument to be made in rebuttal, the cultural sentimentality ran strong among museum-goers, reporters, educators, and some astronomers.

Tyson spins several theories to explain why downgrading Pluto was so controversial. He lays blame at the feet of Disney and Micky Mouse’s lovable dog, he faults educators and their pneumatic devices for memorizing the planet names (My Very Educated Mother Just Served Us Nine Pizzas), and even goes so far to criticize scientists who write textbook that emphasize the importance of counting planets. Yet each of these explanations seems a bit ad hoc and fails to account fully for the protracted and sometimes severe psychological reaction to Pluto’s demise.

I have my own theories, of course. Part of it is somewhat tongue and cheek as evidenced by the humorous songs and cartoons people made in the wake of Pluto’s demotion. This can be chalked up to America’s affection of the little guy. We love to root for the underdog. But part of it is more seriously bound up in the cultural perception of science. Science is something that is always subject to change, and this can be quite disconcerting to our sense of knowledge. If we are taught something is true, we assume that it will be true tomorrow. In fact, if something really is true we should never expect that one day it will become false. I was born in 1978, and I should not expect that fact to change. There is an immutability to truth that we intuitively grasp.

When we are taught that something is scientifically true, we are inclined to feel certain about its truth. When science says it, we believe it, and that settles it. Science’s authority in our culture is rightly respected, and we would never question the idea that the Earth goes round the sun or that water is made from one part oxygen and two parts hydrogen. But when our scientific knowledge is challenged by further evidence or a new way of reframing the evidence we are naturally resistant. We don’t like being told one thing and then being told another, especially when all of the authority of science, at least at one time, was invested in each. It is like if the Pope one day said we could not eat fish on Fridays and then on another said we could.

Dr. Neil DeGrasse Tyson’s book is a treasure in that it captures the cognitive dissonance between professional science and popular science well. The academic hob-knobing, the Museum-building, and the popular educating are all wonderfully set on a collision course with a culture that is more than willing to trust its scientists to get it right… and not change their minds.


6 thoughts on “What In The Planet?

  1. You are operating under the assumption that the definition adopted by four percent of the IAU, most of whom are not planetary scientists, is both useful and accurate, when that is not true. This is why hundreds of professional astronomers led by Dr. Alan Stern, Prinicipal Investigator of NASA’s New Horizons mission to Pluto, immediately signed a formal petition rejecting the IAU definition.

    Ceres should be classified as a planet because it is spherical, meaning it is large enough for its own gravity to pull it into a round shape, a state known as hydrostatic equilibrium. This is a condition of planets, not of shapeless asteroids, which gain their shape solely by chemical bonds.

    Having an elliptical orbit and being inclined 17 degrees to the plane of the other planets do not disqualify Pluto from being a planet. Mercury has a 7 degree inclination to the ecliptic, and many exoplanet systems discovered have huge gas giants within the same star system orbiting all on different planes. Many have highly elliptical orbits. Also, the fact that Pluto and Charon orbit around a common barycenter does not mean Pluto is not a planet; it means this is a binary system with two planets orbiting one another.

    The IAU definition makes no sense for two reasons. One, it states that dwarf planets are not planets at all, a statement that makes no sense and is inconsistent with the use of the term dwarf in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies. Two, the IAU definition classifies objects solely by where they are while ignoring what they are. If Earth were in Pluto’s orbit, according to the IAU definition, it would not be considered a planet either. A definition that takes the same object and makes it a planet in one location and not a planet in another makes absolutely no sense.

    Many planetary scientists prefer a broader planet definition that encompasses any non-self-luminous spheroidal body orbiting a star. That gives our solar system 13 planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris. The last four are both planets and Kuiper Belt Objects.

    It is important to note that Tyson has distanced himself from the controversial 2006 IAU decision, which he himself admits is flawed. At this point, he even admits that the debate is not over, that it might be too early in the study of planetary scientists for anyone to be defining what a planet is in the first place. This was pretty much his message at the Isaac Asimov Memorial Debate, which he moderated at the American Museum of Natural History on March 10, 2009.

    For another perspective, anyone interested in this topic should read “Is Pluto A Planet” by Dr. David Weintraub and/or “The Case for Pluto” by Alan Boyle, which will be out in October 2009.

  2. If you assume Ceres should be a planet then of course you will think Pluto is one. I applaud the consistency, but I don’t think astronomers will grant it. It doesn’t dominate its own orbit just as the thousands of other asteroids in the belt do not.

  3. Pluto will always be my favorite, ever since I played Pluto in our 5th grade rendition of “You are My Sunshine: A Musical About the Solar System” (it only had one song, you can guess what it is). Anyway! One minor error to point out (which was actually kind of humorous). You write: “he faults educators and their pneumatic devices for memorizing the planet names (My Very Educated Mother Just Served Us Nine Pizzas)”. I think you meant that educators utilize mnemonic devices (a way to help people memorize something) as opposed to pneumatic devices (things that are operated by air pressure, although one could argue that a lot of mnemonics are just a lot of hot air…)

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