The Pluto Files: The Rise and Fall of America's Favorite Planet (3 page)

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.

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 mnemonic, 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.

B
EFORE THERE WAS
P
LUTO THERE WAS
P
LANET
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 motion 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.

Figure 2.2.
Clyde Tombaugh, age 22, poses proudly next to his homemade reflecting telescope. Two years later he would discover Pluto.

One clever method to estimate Pluto’s size, which gets you a little closer to its mass, is to time your observation for when Pluto moves against a background star, temporarily blocking the star’s light. When you combine the distance and orbital speed of Pluto with how long the star has dimmed, you can get a good estimate of Pluto’s width on the sky. As more and more stars passed nearer and nearer to Pluto, but without any dimming, astronomers were forced to continually downsize previous guesses for how large Pluto really is.

Figure 2.3.
The original plot from Dressler and Russell (1980) in which they show the run of estimates for Pluto’s mass, dating back from when it was Planet X. The mathematical equation for M
_P
(the mass of Pluto) is the best-fit model to the data, which carries the distressing news that if the trend continues, Pluto will disappear from the solar system by 1984. (From A. J. Dressler and C. T. Russell, “The Pending Disappearance of Pluto,” EOS 61, no. 44 [1980]: 690.)

In 1978, Pluto was discovered to have a relatively large, close-orbiting moon named Charon, allowing a quality estimate for Pluto’s mass. Thanks to a simple application of Isaac Newton’s laws of gravity, Pluto dropped precipitously from about Neptune’s mass to less than 1 percent the mass of Earth. A 1980 tongue-in-cheek article published in the geology newsletter
EOS
by A. J. Dressler, of Rice University, and C.T. Russell, of UCLA, plotted the mass estimates for Pluto, from its days as Planet X through the 1970s, and predicted that at the rate Pluto’s mass was dropping, it would disappear completely from the solar system by 1984 (Figure 2.3).
¹⁰

At this level, Pluto’s mass was far too small to account for Uranus’s and Neptune’s orbital oddities. Planet X still had to be lurking, undiscovered, in the outer limits of the solar system.

That was the prevailing belief until May 1993, when E. Myles Standish Jr., of the Jet Propulsion Laboratory in Pasadena, California, published a paper in the
Astronomical Journal
titled “Planet X: No Dynamical Evidence in the Optical Observations.” Standish used the updated mass estimates for Jupiter, Saturn, Uranus, and Neptune that had become available from the Voyager flybys; in the case of Neptune, the mass difference amounted to nearly 0.5 percent—quite large by today’s standards. Assuming that the masses derived from the Voyager missions were accurate (a wise move), and discounting a single set of suspicious measurements made at the U.S. Naval Observatory between 1895 and 1905 (another wise move), Standish recalculated all the orbital parameters. The result? The misbehaving trends in the paths of Uranus and Neptune disappeared completely, and their orbits could be explained entirely within the gravitational landscape of the presently known solar system. In plain English: Planet X was dead. The inventory of large objects, as decided by the gravity budget of the solar system, was complete.

It seems quite
obvious what a planet is, or ought to be. If an object orbits the Sun but is not itself a comet and does not orbit another object the way moons do, then all is well. William Herschel discovered Uranus in 1781. And Johann Galle, of the Berlin Observatory, discovered Neptune in 1846. But few people know that on January 1, 1801, the Italian astronomer Giuseppi Piazzi discovered the planet Ceres happily and silently orbiting the Sun between Mars and Jupiter. The suspiciously large gap between Mars and Jupiter had finally been filled. But astronomers rapidly determined that Ceres was much, much smaller than any other planet. Then on March 28, 1802, the German astronomer Heinrich Wilhelm Olbers discovered the planet Pallas in the same orbital zone as Ceres. For these two new planets, William Herschel could not identify a visible surface, even through the optics of his powerful telescopes. Apart from their obvious motion across the field of view, the telescopic appearance of Ceres and Pallas was otherwise indistinguishable from that of a distant star. In an expression of sentiment that echoes modern-day debate over what to call Pluto, Herschel wrote, in an 1802 letter to his friend, physician and scientist William Watson: