The Pluto Files: The Rise and Fall of America's Favorite Planet

by Neil DeGrasse Tyson

  Jane Luu, professor at Leiden University in the Netherlands and codiscoverer not only of the first actual Kuiper belt object but of many others that followed

  Brian Marsden, a comet and asteroid specialist at the Harvard-Smithsonian Center for Astrophysics and director of IAU’s Minor Planet Center and the Central Bureau for Astronomical Telegrams, a clearinghouse for discoveries and transitory astronomical phenomena

  Alan Stern (see Figure 3.9), of the Southwest Research Institute in Boulder (later to become associate administrator for science at NASA), specialist in everything small in our solar system, author of the wonderfully titled Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System, and the person who would become principal investigator of NASA’s New Horizons mission to Pluto and the Kuiper belt.

  If any set of people was going to give us insight into the Pluto problem, it would be them. The right people at the right time and at the right place.

  After I briefly reviewed the scientific and pedagogical challenges we all faced, each panelist opened with introductory remarks before the roundtable discussion began. During the 90-minute panel, A’Hearn came off as the strictly-business Pluto rationalist, willing to let you call Pluto a planet if you were investigating the inner working of sizable round bodies or to call it a Kuiper belt object (KBO) if you were investigating its origins. Levy was the unabashed sentimentalist—about Pluto, about astronomy, about science in general. Luu, supremely articulate and confident and by far the youngest person on the stage, was the dispassionate Pluto denigrator and rejectionist, terminally bored by what she considered an old and irrelevant question. Marsden, with his British accent, was the witty, affable multicategorizer. Stern took the Pluto-is-a-planet posture, simultaneously committing to the verdicts of both the laws of physics and the feelings of fifth graders.

  For that evening, I was the dispassionate facilitator, inwardly open to arguments on all sides even though I’d already declared a preference in print.

  Jane Luu kicked off the proceedings—and wasted no time getting her point across:

  When Jewitt and I discovered the Kuiper belt and showed that Pluto was just another member of [it], we were quite pleased…. But I found out that our discovery raises questions that have proved to be painful for many people: Is Pluto truly a planet? Some scientists assert that Pluto’s tiny size and its membership in a swarm of similar objects mean that it should be classified a minor planet, like asteroids and comets. Others are outraged by the idea, insisting that regardless of how its identity has changed, demoting Pluto would dishonor astronomical history and confuse the public.

  I personally don’t care one way or the other. Pluto just goes on the way it is, regardless of what you call it.

  If that was not enough, Luu next drew the battle line between science and sentiment:

  If Pluto continues to be referred to as the ninth planet, it would only be due to tradition and sentimental reasons. People are fond of planets, because the idea of a planet conjures up notions of home, life, happy things, and astronomers are always looking to find more planets, not to lose them. So in the end, the question goes back to this: Should science be a democratic process, or should logic have something to do with it?

  She also reminded the audience, as I do earlier in this book, that when Ceres was first discovered, it (like Pluto) was thought to be a missing planet, but when other asteroid discoveries quickly followed, astronomers realized that Ceres was just the largest member of a new class of objects in a new place in the solar system—the asteroid belt—and so “its planethood was promptly revoked.” Therefore, she contended, if other members of the Kuiper belt had been discovered right after Pluto was, the planethood of Pluto would have been revoked with equal speed.

  Luu ended by raising a blindingly obvious question:

  We are continuing to try to find more Kuiper belt objects, and the search is going pretty well. What if we find other objects fairly close in size to Pluto—maybe even bigger, or maybe just a bit smaller—will these objects also be called planets, or what?

  Her delivery was uncompromising, her tone icy. The audience loved her.

  Next up was Alan Stern, on the opposite side of the fence and only slightly more willing than Luu to compromise, but a bit friendlier in tone. Like Luu, he declared that Pluto’s planethood should not be a matter of democracy, and like Luu (as well as the rest of the panelists), he chose a clever analogy. Referring to the problem of misconceived terminology, Luu had pointed out that Native Americans had been initially referred to as Indians “simply because Columbus screwed up” and thought he’d arrived in India, but that now we know Native Americans are not the native inhabitants of the Indian subcontinent. Using his own analogy, Stern referred to the alleged problem of Pluto’s small size by pointing out that nobody thinks a Chihuahua isn’t a dog just because it’s small—that there’s “something innate” about a Chihuahua, “something doggy” that automatically puts it in the class of dog for any observer. By analogy, Pluto’s roundness puts it in the class of planet.

  In an effort to frame the ultimate definition of a planet—an effort that the IAU had been studiously avoiding—Stern presented the audience with a “rational sieve,” a physical test that anyone in the audience would be able to apply to any celestial object, whether already known or yet to be discovered. Its ease of application, he said, derived from its having an upper size limit and a lower size limit. At the upper end, any object big enough to fuse hydrogen in its interior is acting like a star and must be called a star. At the lower end, any object that doesn’t have enough mass to “become round of its own volition” by virtue of the physical process of hydrostatic equilibrium, which happens in objects with a minimum of about half the mass of Pluto, doesn’t deserve to be called a planet. Anything in between would be some sort of planet—although if it was orbiting another planet rather than a star, it could be called a planetary body.

  Clear. Even persuasive. Yet, despite announcing his opposition to subjecting Pluto’s planethood to the democratic process, Stern appealed to a democratic invocation of sorts—the authority of public perception:

  I guess the best sort of a test is the test that my favorite fifth grader, my daughter Sarah, suggested. It’s the “duh” test…. Like the Supreme Court justice on [the definition of] pornography, when it comes to a planet I’m not sure I can give you an exact definition, but I know it when I see it. By the same token, give a fifth grader a picture of Pluto and ask him if it’s a planet, and you get back: “Duh.”

  Marsden, bearing encyclopedic knowledge of the cosmic catalog and willing to agree with everyone onstage, grouped Pluto with its neighbors in the trans-Neptunian Kuiper belt but was perfectly content to embrace dual status—major planet (one of the nine) and minor planet (asteroid)—in the same way as certain small bodies are classed as both asteroids and comets. Once upon a time his hope had been that Pluto would become officially known as number 10,000 in the list of minor planets, in which case, rather than being “a piddly little thing between Mars and Jupiter,” it could have been accorded the “nice, unargumentative name” Myriostos, which is Greek for 10,000. Having lost that fight, if a fight it was, he was now amenable to all points of view, provided that the large-enough-to-be-round asteroid Ceres would be treated the same way as Pluto.

  Next came A’Hearn, who, like Marsden, was willing to be a dual classifier. He stated his case with utmost and characteristic precision:

  Why do we care about classifying Pluto as a planet or as a minor planet, or as anything else for that matter? Why do we do classifications at all in astronomy, or in any other science for that matter? Why do we bother separating humans from chimpanzees?

  The reasons we do the classifications is to try to find patterns that will help us to understand how things work or how they came to be. So the way we classify Pluto should be something which helps us to understand how it works or how it came to be, and if what you want to understand is how the interiors of solid bodies wor
k, then you should probably be thinking of Pluto as a planet. If, on the other hand, you want to know how things got to where they are in the solar system, there is no question that Pluto got to where it is in exactly the same way as a large fraction of the other trans-Neptunian objects…. So if that’s the question you’re interested in, you absolutely have to classify Pluto as a trans-Neptunian object. Now, this basically means that you should have dual classification.

  But A’Hearn had a surprise up his sleeve. Classifying Pluto as a trans-Neptunian object would place it in the region that produces comets and would subject it to the IAU’s distinctions between comets and asteroids and between comets and minor planets. In other words, if you can see fuzz around an object, it must have an atmosphere, and like icy comets, icy Pluto would have an atmosphere only during perihelion, the few years when its orbit takes it closest to (though still very far from) the Sun’s warmth. So, he concluded, “I think it’s clear we can come out in favor of comet Tombaugh.” The audience loved it.

  Last in line was Levy. Invoking the heroism and devotion of Clyde Tombaugh and the emotional attachment to Pluto felt by children and the cosmic discovery stories recounted to him by his father at the dinner table and the meanness of expert taxonomists who decided that the galumphing brontosaurus of his childhood was actually an apatosaurus and that the beautiful Baltimore oriole was actually a northern oriole, he came down squarely on the side of Pluto’s planethood:

  Science, to me, is not just for scientists. Science, to me, is for everyone; it’s for us. It’s for the children at the Clyde Tombaugh Elementary School. It is for the young people in this audience, who have a better way than we do to look at an object and say, “That’s a planet.” “That’s a brontosaurus.”

  But most important, when we go out under the night sky, and we look up at the stars, we don’t see them as being something incredibly complicated, but instead we see them as something beautiful and simple…. Let’s send a [spacecraft] out to Pluto. If it gets there, and if it clearly takes a picture that shows that there is a dog instead of a planet, then we can have this debate again, and then we can decide that Pluto is not a planet, it is a dog or a brontosaurus. But until then, please, let’s all enjoy the night sky and leave Pluto alone.

  That evening was the first time that we at the Hayden Planetarium—and certainly the people in the audience, or perhaps anybody anywhere—listened to a sustained encounter on the status of Pluto based primarily on the science, but also on culture. And the panelists were divided: one for uncompromising iceballhood, two for dual status, two for planethood. In retrospect, what started as a homework exercise to assist our design of the planet exhibits in the Hall of the Universe was actually a watershed event.

  After the speakers’ opening remarks, I ran a mental applause meter to take the temperature of the room: Who would be perfectly happy to kick Pluto out of the planet club? Weak applause. Who favors planet status? Modest applause plus a scattering of loud whoops. Yet by the end of the evening, everyone from the Hayden Planetarium connected with the Pluto exhibit design had come to believe that Pluto needn’t retain any kind of status at all, except for reasons of nostalgia. And judging by the crowd’s laughter and applause as the debate progressed, a majority of them became convinced as well.

  Monday, May 24, 1999. The night Pluto fell from grace.

  The time quickly arrived for us to design the planet exhibits. We didn’t have the power or the authority (or the interest) to declare that the solar system has only eight planets, but that didn’t mean we couldn’t invent innovative ways to treat the subject. That’s when we decided to present the contents of the solar system as families of objects with similar properties, rather than as an enumeration of orbs to be memorized—a trend that was already being seen in textbooks of the day.

  One of the “families” is simply our star, the Sun, because it’s so much more massive than everything else combined. We then have the terrestrial planets: Mercury, Venus, Earth, and Mars. All of them have more in common with one another than any one of them has with anything else in the solar system. They’re small, they’re rocky, they’re dense, they’re near the Sun. Beyond the terrestrial planets, we have the asteroid belt, made up of hundreds of thousands of craggy chunks of rock and metal—debris that never became part of a planet, as well as the fragmented remains of planetesimals that formed but were subsequently shattered. Then come the so-called Jovian planets, the gas giants: Jupiter, Saturn, Uranus, and Neptune. As with the terrestrials, the Jovians have more in common with each other than any one of them has in common with anything else in the solar system. They’re big, they’re bulbous, they’re low density, they’re ringed, they’re moon-rich, and they’re in sequence. Beyond them we have the Kuiper belt of comets, whose orbits all lie more or less in a plane, and then far beyond the Kuiper belt we have a swarm of comets whose orbits go every which way, called the Oort cloud.

  Where does Pluto fit? The Kuiper belt. End of story.

  We saw no value in counting planets—or counting anything. That exercise to us seemed pedagogically and scientifically vacuous. In an equally unenlightening exercise, consider the answer to “How many countries are there in the world?” There are 192, but that depends on how you define country.22 The number is as high as 245 if you include places that see themselves as countries, such as Palestine or the Turkish Republic of Northern Cyprus, but are not internationally recognized as such. Want instead to use the official United Nations list? Sounds like a good idea, but that would mean Switzerland was not a country until 2002, when it was finally admitted as a member state. Odd, because the Swiss city of Geneva is where you could always find one of the four world offices of the United Nations itself, as well as the original location for the League of Nations.

  One could also list all the countries alphabetically from somebody’s compilation and check off their individual characteristics. But why not simply group countries by “family” resemblances, using data and demographics that tell you something useful, such as region, population size, per capita income, temperature range, life expectancy, or proportion of arable land. Divisions such as these, taken in turn or together, enable you to compare and contrast countries in meaningful ways.

  The Rose Center’s Cullman Hall of the Universe, named for New York philanthropists Dorothy and Lewis Cullman, is split in four principal sectors: the Planets Zone, the Stars Zone, the Galaxies Zone, and the Universe Zone. In a day gone by, each wall panel in the Planets Zone might have been devoted to a single planet: Mercury and its properties, then Venus and its properties, continuing out to Pluto and its properties. Nine panels. And that would be that.

  We did something different.

  We looked across the solar system and asked ourselves what physical features about planets and other objects could be taken together and discussed as common properties or phenomena, allowing us to compare and contrast families of objects in whatever way those families would naturally delineate. One such feature is storms; wherever you have a thick, rich atmosphere on a rotating object, you have storms. Another feature is rings. Yet another is magnetic fields. So we took nontraditional cuts through the data of our solar system and presented them among these panels. Pluto was displayed among other Kuiper belt objects, but we neither counted these objects nor made a list of who is or is not a planet.

  We knew that no matter how the Pluto debate would ultimately resolve, our familial treatment of the solar system was pedagogically and scientifically sensible—a kind of intellectual high road that sidesteps nomenclature altogether.

  The Rose Center for Earth and Space also contains a 400-foot, square walkway that we call the Scales of the Universe. Not to be confused with the famous exhibit on how much you would weigh while standing on various cosmic objects, the Scales of the Universe consists of multiple vistas along a path that surrounds the giant Hayden Sphere. The sphere houses not only the Space Theater but also, in its belly, a walk-through Big Bang experience—a separate venue where we re-cre
ate the first few moments of the universe. We further exploit the outside of the sphere as an exhibit element, allowing us to compare things of greatly varied size. With models mounted on the railing of the walkway and suspended from the ceiling, the exhibit shifts by a power of ten in scale for every few yards you walk. At the beginning, the sphere represents the entire universe, and the model on the railing represents the Local Supercluster of galaxies. Take a few steps, and the sphere now represents the extent of the Local Supercluster, while the next model you encounter represents the Milky Way galaxy. Take a few more steps, and the sphere represents the Milky Way, and the model on the railing represents a star cluster. Keep walking, and the scale of comparison keeps dropping and dropping and dropping until you enter the nucleus of the hydrogen atom.

  About halfway along this walkway, you reach a vista where the sphere represents the Sun, while models on the railing represent the terrestrial planets, ranging from the size of your fist for Mercury to the size of a cantaloupe for Venus and Earth—all in correct relative size to each other. Suspended near the Sun are scale models of the Jovian, gas giant planets, which are much too large and glorious to mount on the railing.

  It’s an exhibit on the relative sizes of things. In this picture-perfect spot the visitor is comparing the size of the Sun with that of the terrestrials and the Jovians in one glance. We had no compulsion to include Pluto. Why? We don’t show comets. We don’t show asteroids. We don’t show the seven moons in the solar system that are bigger than Pluto. We were making a simple comparison of how two families of objects in the solar system contrast with the Sun in size.

  Figure 4.6. Models representing the terrestrial planets, seen from the Scales of the Universe walkway in the Rose Center for Earth and Space. With the Sun represented by the Hayden Sphere (see Figure 4.5), models of the terrestrial planets, in correct relative size, are mounted on the railing. On this scale, Mercury (left) is a little larger than a baseball; Earth and Venus, soccer balls; and Mars (right), a bocci ball. Pluto, not being a terrestrial planet, does not appear among them. (Earth serves as the principal size referent to the Sun and so remains unpainted, along with other fiducial models of the exhibit.)