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

by Neil DeGrasse Tyson

  In spite of its later-to-be-determined diminutive size, Pluto, the god of death, is forever enshrined on our periodic table of elements and associated, by name, with the atomic bomb, one of the greatest weapons of destruction ever devised.

  The Periodic Table has memorialized other cosmic objects as well. The first two asteroids discovered, Ceres and Pallas, led to cerium and palladium. Earth and Moon are there, too, in the guise of the rare elements tellurium and selenium (from the Latin Tellus for Earth and the Greek Selene for Moon), found naturally together in ores.

  Meanwhile, back in Los Angeles on September 5, 1930, the fledgling Disney Brothers Studio releases a cartoon titled “The Chain Gang,” featuring two bloodhounds hot on the trail of Mickey Mouse, an escaped convict. These unnamed canines would serve as the model for the character who would become Pluto, Mickey’s pet dog, but not before some further experiments with the concept.

  On October 23, 1930, Disney releases “The Picnic,” featuring a bloodhound character, but with the name Rover, who in this cartoon belongs to Minnie Mouse. Both Rover and Minnie join ex-convict Mickey for a picnic. Minnie wants to eat. Rover wants to play. And Mickey, having spent so much time in jail, is horny. But Rover keeps preventing amorous encounters between Mickey and Minnie, angering Mickey. Rover makes amends by using his tail as a windshield wiper when Mickey and Minnie drive home in a rainstorm.

  At last, on May 3, 1931, Disney releases “The Mouse Hunt,” in which the playful bloodhound first appears as Pluto, Mickey’s dog. In a press release issued by Mickey Mouse, the rodent recalls Walt Disney suggesting the alliterative Pluto the Pup:

  Walt decided that I should have a pet and we decided on a dog. All the writers at Disney tried to come up with a name. We tried the “Rovers” and the “Pals”, but none seemed to fit. Then one day, Walt came by and said, how about Pluto the Pup? And that’s what it’s been ever since.4

  After twenty or so cartoon appearances, Pluto finally stars in his own production. On November 26, 1937, Disney releases “Pluto’s Quin-Puplets,” in which Pluto is left in charge of five puppies as his Pekingese wife, Fifi, goes out for food. The puppies wreak puppy-havoc at home while Pluto gets drunk on moonshine. When Fifi returns, they all get kicked out of the doghouse.

  Such are the humble beginnings of a cartoon icon.

  While there is no unambiguous link between Pluto the Disney character and Pluto the planet, the connection has always been assumed.5 We can bet that Walt Disney was not thinking about constipation when he suggested the name for Mickey’s dog; before the release of “Mouse Hunt,” Pluto the planet had already spent a full year wooing the hearts and minds of the American public. Whether or not Walt Disney was thinking about the cosmos when he named his dog is not important here. What matters is that the seeds were sown for planet Pluto to receive a level of attention from the American public that far exceeds its astrophysical significance in the solar system. The New York Times science writer Malcolm W. Brown, in a February 9, 1999, article on Pluto, quoted an unnamed astronomer who made a similar observation:

  If Pluto had been discovered by a Spaniard or Austrian, I doubt whether American astronomers would object to reclassifying it as a minor planet.

  Over the decades to follow, as the size, influence, and wealth of the Walt Disney conglomerate grew, now a $30 billion company, so, too, did the name Pluto in the collective sentiment of Americans. Indeed, the corporation had achieved a kind of control over our Plutonic emotions, leaving me with no choice but to label the Disney empire what it is:

  Plutocracy |pl-tä-kr-s| (noun) Government by the wealthy.

  1) a country or society governed in this way.

  2) an elite or ruling class of people whose power derives from their wealth.6

  As a scientist at New York City’s American Museum of Natural History, I sustain an osmotic link with colleagues whose expertise draws from the entire animal kingdom. We’ve got herpetologists, paleontologists, entomologists, and mammalogists, to name a few. So while I cannot claim fluency on all subjects of natural history, I do claim sensitivity. This leads me to ask how it came to be that Pluto is Mickey’s dog, but Mickey is not Pluto’s mouse.

  Something is awry in the taxonomic class of mammals in the Disney universe.

  I would later learn that if you are a Disney character who wears clothes, no matter what your species, you can then own pets, who themselves wear no clothes at all, except perhaps for a collar. Pluto runs around naked except for a collar that says “Pluto.” Mickey runs around with yellow shoes, pants, white gloves, and the occasional bow tie; The haberdasheral hierarchy is clear.

  One never knows fully how and why some words, names, ideas, or objects penetrate culture, while others fade to insignificance. In straw polls that I persistently take of elementary school children, their favorite planet is Pluto, with Earth and Saturn a distant second. At some level of cognition, the simple sound of a word on the ear or an exotic meaning can make or break a word’s popularity and prevalence. Among all planet names, for example, Pluto sounds the most like a punch line to a hilarious joke: “…he thought he was on Pluto!” And while the names of all other planets are traceable to mythical gods whose talents or powers one might envy, Pluto is, of course, named for the god of a dark and dank residence for the dead. That’s just funny.

  Figure 1.8. The cultural juxtaposition of Pluto the dog and Pluto the planet makes irresistible content for cartoonists. Top: Cartoonist Bill Day, of the Commercial Appeal , parodies America’s ongoing scientific illiteracy. Bottom: Pluto, the most misbehaved of all planets, gets sent to the interstellar doghouse in a comic by Dick Locher, of the Chicago Tribune .

  In most times and at most places throughout history, the greatest measure of cultural penetration comes not from what sociologists discuss but what artists draw. It may be a while, if ever, before we see a Pluto exhibit at New York’s Museum of Modern Art, but that doesn’t stop the creative urges of comic strip illustrators from comingling the affairs of Pluto with the affairs of state.

  Maybe we shouldn’t stand in denial of the provinciality of it all. Disney is an American company. Mickey Mouse is cartoon royalty. Pluto is Mickey’s dog. Pluto the planet was discovered by a farm boy from middle America, on a search conducted from the mountains of Arizona, initiated and funded by a descendant of blue-blooded Bostonians.

  We have further made a cottage industry of memorizing the sequence of planets from the Sun.

  My Very Easy Method Just Simplifies Us Naming Planets.

  My Very Excellent Mother Just Served Us Nine Pickles.

  My Very Educated Mother Just Stirred Us Nine Pies.

  My Very Excellent Man Just Showed Us Nine Planets.

  My Very Easy Memory Jingle Seems Useful Naming Planets.

  My Very Excellent Monkey Just Sat Under Noah’s Porch.

  My Very Early Mother Just Saw Nine Unusual Pies.

  Mary’s Velvet Eyes Makes John Sit Up Nice and Pretty.

  Mary’s Violet Eyes Makes John Stay Up Nights Pondering.

  Many Very Eager Men are Just Sissies Under Normal Pressure.

  Man Very Early Made Jars Stand Up Nearly Perpendicular.

  My Very Elegant Mother Just Sat Upon Nine Porcupines.

  For most of these mnemonics, the word substituted for Pluto represents the principal subject of the sentence, leaving the sentence vulnerable to collapse if the P-word ever disappeared.

  From the late 1980s onward, the most popular planet mnemonic has been “My Very Educated Mother Just Served Us Nine Pizzas,” associating Pluto with pizza, a favorite food in America,7 especially among schoolchildren. No other mnemonic has come close to its popularity, in spite of the many clever ones that circulate.

  On reflection, I may have strongly influenced the choice of the word pizza for the planet mnemonic. In my early years of graduate school (begun at the University of Texas at Austin, but finished at Columbia University, in New York City), I had only ever heard Pluto associated with prunes in the mn
emonic, which is surely what your educated mother, who is interested in your gastrointestinal well-being, would serve you, not to mention the distant connection prunes have with Pluto Water as a laxative. I dislike prunes but love pizza. Given that Americans eat 100 acres of pizza a day, I am not alone in that sentiment, and I did not worry about how absurd a serving of nine pizzas would be, compared with being served nine prunes. And so, while I was a teaching assistant in Texas, I remember changing “Prunes” to “Pizza” beginning in 1980 for all the large introductory astronomy classes I taught, which totaled thousands of students by the time I left Texas. I also introduced pizza for the planet mnemonic in my first book, Merlin’s Tour of the Universe, published in 1988. And I have not once heard prunes associated with Pluto since the early 1990s.

  The perennial classroom exercise of memorizing planets in sequence from the Sun allowed the enumeration of the nine planets to take on mythical significance in the minds of students and educators alike. Every printed introduction to the solar system, no matter the grade level of the curriculum, began with a list of the nine planets, in order from the Sun, accompanied by a table or diagram of their relative sizes. This tradition became the pedagogical equivalent of eating comfort food. You somehow knew that all was right with the universe as you learned the planetary sequence, with little Pluto rounding out the list of nine. Even the Planetary Society, an organization founded in 1980 by Carl Sagan and two colleagues, Lou Freidman and Bruce Murray (both from NASA’s Jet Propulsion Laboratories in Pasadena), chose as its toll-free phone number 1-8 0 0-9 W O R L D S.

  Figure 1.9. Cartoon postcard by Paul McGehee. Although he drew one for each of the planets, Pluto’s cultural popularity surpasses them all.

  Meanwhile, the Voyager 1 and Voyager 2 spacecraft, launched in the 1970s but executing their outer-planet flybys in the 1980s, revealed that the moons of Jupiter, Saturn, Uranus, and Neptune may be as interesting as the planets themselves—maybe more so. It was soon clear that the number of intriguing worlds in the solar system vastly exceeds nine, including seven moons that measure larger than Pluto itself: Earth’s Moon; Jupiter’s Io, Ganymede, Callisto, and Europa; Saturn’s Titan; and Neptune’s Triton. The grade school tradition to rote memorize planet names (usually one’s first encounter with the solar system) unwittingly concealed a staggeringly rich landscape of objects and phenomena.

  2

  Pluto in History

  BEFORE THERE WAS PLUTO THERE WAS PLANET X.

  Planet X was the “undiscovered” object in the outer solar system whose gravity was needed to fully account for the motions of the known planets. Heard about it lately? Probably not. That’s because it’s dead. But widespread belief in the existence of Planet X is what led directly to the systematic search and discovery of what would become Pluto.

  The rise of Planet X begins with the German-born English astronomer Sir William Herschel, who more or less accidentally discovered the planet Uranus on March 13, 1781. The episode was an exciting moment in eighteenth-century astronomy. Nobody in recorded history had ever actually discovered a planet. Mercury, Venus, Mars, Jupiter, and Saturn can each be seen relatively easily with the naked eye, and all were known to the ancients. The bias against finding additional planets was so strong that Herschel, even in the face of contrary evidence, assumed he discovered a comet. He even titled his discovery paper “Account of a Comet.”8 Other astronomers were in denial as well. Charles Messier, the eighteenth century’s king of comet hunting, noted on April 29, 1781, “I am constantly astonished at this comet, which has none of the distinctive characters of comets.”9

  Archival records of star positions show that several observers had seen Uranus before Herschel did, but each one had mistakenly classified the planet as a star. In an embarrassing example from January 1769, the French astronomer Pierre Charles Lemonnier did not discover Uranus six times. When Herschel finally noted that the mysterious object moved, the avail-ability of nearly a century’s worth of “prediscovery” data on its position in the sky enabled astronomers to calculate its orbit with good precision. Those calculations showed that the object’s orderly, near-circular path, far from the Sun, had nothing in common with the eccentric trajectories of all known comets. At this point, you would have had to be both blind and boneheaded to resist calling the new object a planet.

  But all was not orderly in the solar system. Uranus was behaving badly. This new planet’s trajectory around the Sun was not following the path Newton’s law of gravity would have it take after all known sources of gravity were accounted for. Some astronomers suggested that Newton’s laws might be invalid at such large distances from the Sun. Not so crazy: under new or extreme conditions, the behavior of matter can, and occasionally does, deviate from the predictions of the known laws of physics. Only if Newton’s theory of gravity had been nascent and untested would one have good reason to question it. By the time Herschel had discovered Uranus, Newton’s laws were on a 100-year run of successful predictions. Most famous among them was Edmond Halley’s predicted return in 1759 of the comet that would be named in his honor.

  The simplest conclusion? Something was lurking undiscovered in the outer solar system—something whose gravity was unaccounted for in the expected orbital path of Uranus.

  Beginning in the late eighteenth century, the French mathematician Pierre-Simon de Laplace developed perturbation theory, which he published in his influential multivolume treatise Mécanique Céleste. Laplace’s new math gave astronomers an indispensable tool to analyze the small gravitational effects of an otherwise undetected celestial object. Mathematicians and astronomers across Europe, armed with these new tools of analysis, continued to investigate what might be perturbing Uranus. In 1845, a young, unknown English mathematician, John Couch Adams, approached Sir George Airy, Britain’s astronomer royal, with a request that he search the sky for an eighth planet. But neither looking for planets nor following the leads of young, spunky mathematicians were part of the astronomer royal’s job description, so Adams’s request was dismissed. The next year, the French astronomer Urbain-Jean-Joseph Leverrier independently derived similar calculations. On September 23, 1846, he communicated his prediction to Johann Gottfried Galle, who was then assistant director of the Berlin Observatory. Searching the sky that same night, Galle found the new planet, soon to be named Neptune, within a single degree of the spot Leverrier had predicted.

  But once again, all was not orderly in the solar system. Uranus was still behaving badly, although less so now that the gravity from Neptune had been accounted for. Meanwhile, Neptune’s orbit had some peculiarities of its own. Could yet another planet be awaiting discovery?

  Figure 2.1. An 1895 portrait of Percival Lowell looking dapper. Lowell, the founder of Arizona’s Lowell Observatory, launched the search for Planet X, which led to the discovery of Pluto.

  In his early years, Percival Lowell indulged a fanatical, even delusional fascination with Mars, claiming that intelligent civilizations were in residence there, digging networks of canals to channel water from the polar ice caps to the cities. He imagined a diminishing water supply, leaving them on the brink of extinction, which fed the War of the Worlds, Martian invasion fever of the day. But he devoted most of the rest of his life to the search for the object he called Planet X (X for the algebraic unknown)—the mysterious body in the outer solar system that continued to perturb Neptune. By this reckoning, of course, one might have previously identified Neptune as the Planet X to Uranus

  All efforts to predict the location of Planet X based on perturbations to Neptune came up empty. Any discovery would require a large-area survey of the sky.

  When looking for a planet, nobody wants to pore over images of the sky that contain countless millions of dots, hoping to spot the one that moved between one photo and the next. Fortunately, an ingenious mechanical-optical device known as a blink comparator would come to the rescue, streamlining the task. Blink comparators exploit the remarkable ability of the human eye to detect change or m
otion amid an otherwise unchanging field: Place two photographic images of the same section of the sky, but taken at different times, side by side in precise alignment. Next, flash the two images back and forth in rapid succession. Against the background star field, any speck on the two photographs that brightens, dims, or shifts position from one image to the other becomes immediately apparent.

  Percival Lowell died in 1916, but Clyde W. Tombaugh would later be hired by the observatory to carry on this arduous search, which led to the discovery of Planet X in 1930. The young fellow had been looking at a pair of photographic plates he took on January 23 and 29 of the region around Delta Geminorum, the eighth brightest star in the constellation Gemini. Tombaugh became the third and last person ever to discover a planet in our very own solar system.

  In any well-designed, well-conducted survey, you don’t stop just because you’ve discovered something. By completing the survey, you might discover something else. So for the next thirteen years Tombaugh scoured more than 30,000 square degrees of sky (out of a total of 41,253 square degrees). He didn’t find any objects as bright as or brighter than Pluto. But the time wasn’t wasted. The survey discovered six new star clusters, hundreds of asteroids, and a comet and would stand for decades as the most thorough search of the outer solar system.

  But was newly discovered Pluto the Planet X of everybody’s suspicions? Pluto was first presumed to be of commensurate rank in size and mass with Neptune, itself about 18 times Earth’s mass. If Pluto were to perturb Neptune with its gravity, as people suspected it was doing, then Pluto must be at least that size. But Pluto’s distance was far beyond the power of available telescopes to see anything other than an unresolved point of light. In fact, Pluto’s size and mass could only be guessed at based on Pluto’s brightness after you make an assumption about how reflective its surface is.