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

by Neil DeGrasse Tyson

  Long before Mark Sykes had come to New York City to get in my face and argue about Pluto, he sent an e-mail that was entirely consistent with his long-standing posture on the subject, outraged and full of energy:

  I recall the “issue” starting out as a joke by Brian Marsden at a party in the late 80s. It seems the joke is on your institution. Unfortunately, it is the public who loses out. Minority opinions can be a wonderful starting point for an illuminating discussion, but to the extent your exhibit is meant to be educational, it should be identified as such and the argument engaged. Otherwise, you make a misrepresentation, in your silence, of the view commonly held by planetary scientists.

  My Pluto in-box would not be complete without a quip from Alan Stern, sent years after the exhibit dustup. In a postscript to a letter on another subject, Stern was characteristically terse and firm, punctuated with a winking smiley face at the end:

  It’s a planet, man. You gotta get over this.;-)

  Bill Nye the Science Guy®, (seen in Figure 3.11), a consummate educator and hobbyist of linguistics, opined with a tutorial on much-needed nomenclature in the field of planetary science:

  The great thing about this debate is that it has gotten people every-where thinking about planets and our place among them. It’s remarkable. The whole world it seems is full [of] people puzzling Pluto.

  Words include more than they leave out. Words never say all there is to say about anything. So, I advocate some adjectives. I favor Pluto, Xena (or whatever its ultimate sobriquet), Sedna, and others being called generally “planets.” Then we’d have teaching opportunities with the adjectives or descriptors:

  “Main Plane planets” (those in the plane of the ecliptic)

  “Ice dwarf planets” or even “Plutonian” planets (spherical icy worlds akin to their namesake Pluto)

  The expression Main Plane has assonance and is thereby easy to say and remember.

  If all that was not enough, a brief lesson in Latin followed:

  There are apparently untold “Plutonian,” or “Ice Dwarf,” planets beyond Neptune. These would or should be described as “ultra-Neptunian” planets. Nota bene, I feel strongly that my Latin teachers would suffer greatly to know that certain of my astronomical colleagues use the Latin “trans” to mean “beyond.” Sigh. “Trans” means “across.” There are occasional usages of traneo for “go past.” But to my ear, it’s not the same. For “beyond,” Romans use “ultra.” I hope the nomenclature committee(s) can come around on this one.

  There aren’t many of them in the world, but Geoff Marcy, of the University of California at Berkeley, is one: a hunter of exoplanets.32 In reference to the views of Mark Sykes and the need to not rock the boat, Marcy had this to say:

  He feels that mere science museums should communicate only the IAU party line. That’s not how I read the constitution, and it’s not how I read productive scientific discourse. Discussion of the issues should prevail, especially when the observables lean clearly one way. That the IAU has political agendas and recalcitrant members is no reason for us to shade the truth about Pluto.

  Don Brownlee, planetary scientist at the University of Washington, was simple and direct with an opposing view:

  Demoting Pluto to just another KBO is revisionist science and a cheap shot at history.

  Of course, one might alternatively view revisions in science as a good thing—a sign of progress and discovery.

  Wesley Huntress, former associate administrator of space science at NASA, wrote from the Carnegie Institute of Washington, where he served as director of its geophysical laboratory. After a brief admonishment, questioning whether the Rose Center has fallen wayward of scientific consensus, he reasons himself back to a position not fundamentally different from our own:

  The Science Citadel of the Capital of the World should not confuse the public. Given the continuing discovery of Kuiper Belt Objects, and in particular, objects larger than Pluto and also having their own satellites, we need a new map of the “world” of the solar system as we explore its wider seas…. Our solar system has two belts of multiple small objects, the first between Mars and Jupiter containing rocky bodies, and the second beyond Neptune stretching far into interstellar space to the Oort Cloud that contains icy objects…. So we have an asteroid belt and a comet belt. The objects within them can be called minor planets if they are self-gravitating and therefore round; rock dwarfs in one case and ice dwarfs in the other. Otherwise they are asteroids or comets. Ceres is a rock-dwarf, Pluto is an ice-dwarf, and there are eight planets in the solar system.

  Huntress ends by reflecting, as only a wise, empty-nest parent would—

  Sometimes you have to let your children go.

  Of course scientists were not the only ones whose opinions came to my in-box. Before there was the New York Times, before there was CCNet, before there was Mark Sykes, before the New Horizons mission was sent to Pluto, there was Will Galmot, the first person to notice that Pluto was missing from our displays and to write us about it. This astute visitor to the museum did so ten months before the Rose Center’s Pluto story broke in the press. Mr. Galmot was apparently paying closer attention than everyone else during our first month of operation and had carefully researched the problem. Reproduced in Figure 5.1, Mr. Galmot’s terse correspondence was clear and to the point. And in case we were unsure of what Pluto looked like, Will used the artistic tools available to him and supplied a detailed image for our exhibit professionals to use.

  By mid-2001, I was receiving organized packs of student letters every several weeks, sent by zealous teachers eager to tell me how their class voted on the matter. In June 2001, Miss Fedi’s fourth graders from Dean La Mar Allen Elementary School in Las Vegas, Nevada, voted 90 percent to 10 percent in favor of retaining Pluto’s status. You didn’t have to be a kid to feel this way. Craig Manister, a full-grown acquaintance, muttered to me at a cocktail party, “It’s like knowing when you get out of bed that the floor is solid.”

  Over the years, I noticed a trend in the mail to my office from elementary schools. Slowly, legions of angry students move on, making room for new crops of students who may have never known the certainty of nine planets in the first place.

  In a stack of letters sent in March 2005 from Mrs. Chemai Gray’s middle school class in Marysville, Washington, the split of votes for Pluto’s planet status had reached 50 percent in favor, 50 percent against, with students tending to offer arguments about size and tradition. A year later, other stacks of letters, including a second stack from the same teacher, showed that the students had become fluent in the Kuiper belt and the difference between icy and rocky objects in the solar system. They further gleaned a basic understanding of circular and elliptical orbits, and their correspondence was mostly devoid of emotion and sentimentality. By the end of 2006, letter tallies approached 90 percent against planet status and 10 percent in favor.

  Meanwhile, others with opinions could not resist e-mailing me their commentary.

  From the downright grumpy:

  Figure 5.1. Letter from Will Galmot.

  Date: Feb 13, 2005, at 10:50 PM

  I do not appreciate your attempting to demote Pluto to a non-planet.

  I am 59 years old. I grew up on Tom Corbett, Space Cadet. Although I have seen many changes in my life, one thing I am sure of is that there are nine planets in the Solar System, and that the smallest and most distant is Pluto.

  Leave it alone.

  Dan E. Burns

  To “the child shall lead them”:

  Date: November 18, 2004 7:09:13 PM EST

  My name is John Glidden. I am six years old and my favorite planet is Pluto. I disagree with you that Pluto is a Kuiper Belt Object. I think Pluto is a real planet and I took a poll of 11 people. The question was, What do you think Pluto is?

  A Planet

  A Double Planet

  A Kuiper Belt Object

  A planet and A Kuiper Belt Object

  I think it is a double planet and everyo
ne else thought it is a regular real planet that is very cold.

  I had a half day at school yesterday so my mom brought me to the Museum of Natural History and the Hayden Planetarium. I wanted to see you so I could tell you this in person.

  John Glidden

  To pleading with tongue-in-cheek arguments:

  Date: Thu, 28 Jan 1999 07:45:58 -0500

  C’mon Neil. Let’s not go messing around with Pluto’s planethood.

  Grandfather the little guy and get on with it. (If Ceres starts to make a fuss, toss him an honorary something). Why, stripping Pluto of his planethood is like stripping George Washington of his citizenship because the US wasn’t really a country when he was born.

  In any case, the cost to make the change will be huge. There must be thousands of sets of twenty year old encylopedias that will have to be replaced. The number of encyclopedia salesmen required to do it will surely tilt the social fabric of the world.

  Steve Leece

  To mild accusations of cultural insensitivity:

  Date: December 6, 2004 9:06:50 PM EST

  Would you say a small child or midget wasn’t a person? Of course you wouldn’t, although they are a different versions of the normal standard that is set as what a person would like, but they are still classified as people. By saying that Pluto is not a planet, is like saying a midget or a small child is not a person.

  Brooke Abrams

  And to blunt obstinacy:

  Date: November 13, 2003 9:01:07 AM EST

  Pluto is a planet because I say so. I don’t think that something that I have been told all my life (namely, that Pluto is in fact a planet) should be doubted to the extreme that we must write a letter to some organization that does not in fact care what we think.

  Lindsey Greene

  Letters such as these provided months of entertaining reading, but little did I know, the assault had only just begun.

  6

  Pluto’s Judgment Day

  AFTER TWO YEARS OF COMMITTEE DELIBERATION, THE International Astronomical Union (IAU) could not converge on what exactly a planet should be. And so the IAU formed an ad hoc Planet Definition Committee to see if they could succeed where others had failed. This fresh group of seven—five scientists, a journalist, and a science historian—met for two days before the Prague meeting to cogitate and then decided on what, in their judgment, would be the best solution for all concerned parties, Pluto included. On August 16, 2006, they recommended to IAU membership that a planet be officially defined as an object that (1) is in orbit around a star, but not in orbit around another planet, and (2) is large enough for its own force of gravity to shape it into a sphere, but not so large that it would trigger fusion in its core becoming a star. This definition would have kept Pluto as a planet and added, on the spot, three more objects to the planet list: Ceres, Charon, and Eris, with many more surely to come.

  Figure 6.1. The seven-member Planet Definition Committee of the International Astronomical Union (IAU). Top row, left to right: André Brahic, University of Denis Diderot (Paris) planetary scientist and popularizer; Iwan Williams, Queen Mary University (London) planetary theorist; Junichi Watanabe, director of the Outreach Division of the National Astronomical Observatories of Japan; and Richard Binzel, MIT planetary scientist. Bottom row, left to right: Catherine Cesarsky, director general of the European Southern Observatories and president-elect of the IAU; Dava Sobel, best-selling science writer and journalist; and Owen Gingerich, Harvard astronomer, historian of science, and chair of the committee.

  In spite of the undeniable cosmic expertise represented among the committee members, they were nonetheless absent researchers who specialize in the discovery and analysis of Kuiper belt objects or in the discovery and analysis of exosolar planets, two frontiers of planetary science that bring daily insight to what kind of solar system we live in. Based on the comments and reactions already expressed by Kuiper belt codiscoverers David Jewitt and Jane Luu, for example, had either of them been on the committee, it surely would have led to yet another hung jury.

  During the week that passed between the IAU proposal going public and the formal vote on the recommendations, the roundness criterion received substantial media attention. In an appearance on Comedy Central’s The Colbert Report, I shared this information with faux ultraconservative host Stephen Colbert, who had been supportive of Pluto all along, but was deeply concerned that if being round was what made you a planet, then “that means anything could be a planet,” and “if everything is a planet, then nothing is a planet.” His concern, shared by many, was simply that the decision was “taking away the specialness of Earth’s planetness.” He then proceeded to trash-talk the other three planet candidates in the solar system, beginning with Pluto’s very round moon Charon:

  Hey Charon, you’re orbit is so big, you only get Christmas once every 248 years, even then all you get is earmuffs because it’s so cold!

  Next came Ceres, the largest and only round asteroid:

  Hey Ceres, guess what? They call you a planet, but we both know you’re just a big fat ass-teroid. Yeah. You’re so ugly, God tried to hide you in an asteroid belt!

  Last was the yet-to-be-named icy Kuiper belt object 2003 UB313 that would later become Eris:

  Hey 2003 UB313, if that is your real name, you’re not a planet, you’re just a lazy comet. Your mama’s so ugly, she named you 2003 UB313.

  Back in the real world, the conference attendees hotly debated the roundness criterion for planethood, leading to two additional criteria: (1) that the round object not be in orbit around another, larger world—precluding Charon from being called Pluto’s companion planet; and (2) that the round object has cleared its orbit of wayward debris—the death knell for Pluto, whose orbital regime remains rich with countless thousands of craggy chunks of icy Kuiper belt objects. This leaves the Sun with an eight-planet family instead of either twelve or nine. Coincidentally, on August 16, 2006, my friend and museum colleague Steven Soter (the fellow who first called my attention to the Pluto problem back in 1998) submitted for publication a research paper titled “What Is a Planet?” in which he quantifies what it would mean for an object to clean its orbit.33 This criterion is subtle because without a quantitative account of a clean orbit, the requirement can be arbitrarily invoked. For example, as noted earlier, Earth continues to plow through hundreds of tons of wayward meteoroids per day in its annual journey around the Sun. So have we cleared our orbit? Clearly not. The objective is to assess the total mass of cleanable debris and compare it with the mass of the planet in question. If the debris does not amount to much, then you can claim to have cleaned or dominated your orbit. Otherwise, you’re just one of the crowd.

  For example, Earth far outweighs the sum of all matter it will ever collide with. Earth can plow through its daily dose of debris for a quadrillion (1,000,000,000,000,000) years and become a mere 2 percent heavier than when it started. A quadrillion years is 10,000 times longer than the current age of the universe. Meanwhile, the countless Kuiper belt comets outweigh Pluto by a factor of at least 15.

  Steve Soter’s paper provided perspective on what was, at the time, a hastily added criterion to the original roundness definition of the IAU resolution. Soter and I had collaborated on this paper during its early stages, but by the end, he had done 95 percent of the work while I was (regrettably) distracted by administrative matters. So I withdrew as coauthor but was delighted to be recognized in the paper’s acknowledgments.

  Figure 6.2. The 26th (triennial) General Assembly of the International Astronomical Union, held in Prague. Of the 2,500 attending members, 424 remained for the last day of the conference (August 24, 2006) to vote overwhelmingly (90 percent in favor) on a revised definition of the word planet that excluded Pluto, formally “demoting” it to dwarf planet status.

  Back at the IAU conference in Prague, anxious reporters waited outside the assembly hall, which was off-limits to the press. They lurked with the kind of silent anticipation one finds only during
the election of a new pope by the college of cardinals, for which eager onlookers in St. Peter’s Square search for the smoke to rise from the Vatican Palace chimney—black smoke, a failed ballot; white smoke, a new pope is elected. All week long my e-mail in-box logged more than a hundred inquiries a day on Pluto alone, from concerned citizens and from the press wanting me to comment. When the final vote was cast on August 24, 2006, a revised definition of a planet emerged—and a revised status for Pluto:

  Pluto is officially demoted to the status of “dwarf planet.”

  More than 90 percent of the 424 voters voted for demotion. (See the full amended resolution 5A in Appendix F.) The same criteria that downgraded Pluto elevated Ceres to the class of dwarf planet from the ranks of asteroids. Eris, the freshly discovered spherical Kuiper belt object, joined Pluto among the ranks of dwarf planets as well.

  By now, the Pluto e-mails to my in-box were arriving at about two hundred a day, with assorted subject headers that betrayed the sentiments contained within: “Now Look What You’ve Done!” “Congratulations on Winning the Pluto Is Not a Planet Debate,” and “Honk if you think Pluto is still a planet.” And a dozen major media outlets had called or e-mailed for my reaction to the decision. I happened to be at the beach that entire week, on vacation with my family, and could take no interviews. So this feeding frenzy would have to happen without me.

  In the weeks that followed, there were one or two supporters amid the e-mail barrage:

  Date: October 27, 2006 2:56:24 PM EDT

  I was compelled to write…after hearing what I perceived as a breath of fresh analytic air, after spending so much time reading what amounted to philosophical flatulence.