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

by Neil DeGrasse Tyson

  The fastest hunk of hardware ever launched, the New Horizons spacecraft left Cape Canaveral, Florida, on January 19, 2006, atop the powerful Atlas V rocket. After the second-and third-stage rockets fired, the half-ton, piano-sized spacecraft was endowed with enough speed to pass the Moon’s orbit in 9 hours (the Apollo astronauts took 31/2 days) and to reach Jupiter for a gravity assist in just over a year. After Jupiter’s gravity assist, the craft will be traveling at a breakneck 53,000 miles per hour, nearly 15 miles per second.

  Figure 3.7. Masked MIT planetary scientist Richard Binzel gives a thumbs-up for the New Horizons spacecraft, shown behind him. The craft is in a clean room at the Applied Physics Laboratories of Johns Hopkins University, being prepared for launch. Binzel appears unmasked in Figure 3.10.

  Figure 3.8. Patch for the New Horizons mission, a joint venture of the Southwest Research Institute, NASA, and the Johns Hopkins University’s Applied Physics Laboratory. Notice that it has nine sides—one of many occasions where the number nine appears in mission materials. A coincidence? Or a subliminal ploy to sway the emotions of the viewer?

  Figure 3.9. Alan Stern (left), principal investigator of the New Horizons mission to Pluto, poses next to the author, both in a desperate attempt to show that astrophysicists can be cool. Photographed at Kennedy Space Center just before the scheduled launch of the New Horizons mission to Pluto, January 2006.

  Figure 3.10. James Christy (left), the discoverer of Pluto’s moon Charon, stands with Richard Binzel, the first person to measure Charon passing in front of Pluto, which allowed significant conclusions to be drawn about the Pluto-Charon orbiting system.

  The lead scientists packed the New Horizons spacecraft with seven scientific experiments to answer fundamental questions, such as: What is Pluto’s atmosphere made of, and how does it behave? What does the surface of Pluto look like? Are there big geological structures? How do particles ejected from the Sun (the solar wind) interact with Pluto’s atmosphere? How empty of dust is the space between Earth and Pluto?

  I was kindly invited to the launch by Southwest Research Institute’s Alan Stern (Figure 3.9), a Pluto expert, the mission’s principal investigator, and a lifelong Plutophile. A magnanimous gesture on Alan’s part, knowing my spotty public position regarding Pluto’s planethood. I was honored to be asked and I gladly accepted. Also in attendance at Kennedy Space Center that day were the discoverers of Pluto’s moon Charon, James Christy and Richard Binzel, as well as Bill Nye the Science Guy® (Figures 3.10 and 3.11). Bill was a student at Cornell when Carl Sagan was a professor there. While known primarily for his expositions of every-day science, the classes he took with Carl imprinted him with a love of the solar system and the rest of the universe that persists to this day.

  One of the stated goals for the New Horizons mission was to “complete the reconnaissance of the solar system.” This marching order never sat well with me. It conveys a needless tone of finality to the mission. One can just as easily assert that we are “beginning the reconnaissance of a new part of the solar system, previously unvisited,” as I consistently conveyed in my public appearances.

  In the meantime, the Hubble Space Telescope, famous for its detailed, high-resolution images of gossamer gas clouds across our Milky Way galaxy and of galaxies that reside in the distant universe, was trained on the surrounding environs of Pluto itself. This enabled the Pluto Companion Search Team, led by Hal Weaver and Alan Stern (seen in Figure 3.9), to discover in June 2005 two additional moons in orbit around Pluto (Figure 3.13). A year later, the International Astronomical Union (IAU) officially named them Nix (or Pluto II, the inner of the two moons) and Hydra (or Pluto III, the outer moon).15

  Figure 3.11. Like the Academy Awards, but for science, celebrities abound at the January 2006 New Horizons launch to Pluto. One of the nation’s leading educators, Bill Nye the Science Guy ® poses with the author. In this rare photo, Bill Nye appears without his trademark bow tie. And the author wears an embarrassingly loud necktie that displays eight planets in full view, with Pluto buried in the knot.

  Figure 3.12. Four onlookers stand agape at the Atlas V rocket, posed to launch the New Horizons spacecraft on a fast track to Pluto and beyond. The bulbous nose cone contains the space probe itself. Everything else—the copper cylinder and the white, strap-on boosters—are all rocket fuel. New Horizons successfully launched on January 19, 2006. At a peak speed of about 35,000 miles per hour (10 miles per second), it is the fastest-traveling spacecraft ever sent anywhere.

  Figure 3.13. Hubble Space Telescope images of two additional moons of Pluto, visible in each of the long-exposure photographs. Here identified as “candidate satellites,” but later confirmed and named Nix and Hydra, they were discovered in orbit around Pluto, allowing Plutophiles the world over to now refer to the “Pluto system” of host planet and three moons, Charon included. Alan Stern (see Figure 3.9), one of the world’s leading Pluto researchers, and Hal Weaver were the lead members of the Pluto Companion Search Team responsible for the discovery.

  The amount of brain energy invested in these names knew no bounds. The first letters of the two new moons, N and H, offer respect to the New Horizons mission to Pluto, echoing the fortuitous coincidence that the first two letters of the name Pluto offer tribute to Percival Lowell. Remembering that the Greek beast Hydra sports nine heads offers a battery of nods to Pluto’s 76-year tenure as the ninth planet. Meanwhile, Hydra’s first letter H honors the Hubble Space Telescope, used for the moons’ discovery. Nix (the Egyptian variant of Nyx) is named for the Greek goddess of darkness and night. She also happens to be mother of Pluto’s other moon Charon, creating a happy orbiting family in the depths of space that many justifiably call the “Pluto system.”

  4

  Pluto’s Fall from Grace

  AS ALREADY NOTED, OUR BEST ESTIMATES FOR THE mass of Pluto had been shrinking since the day it was discovered.

  Up through the 1970s, the typical astronomy textbook began with a section called “The Solar System” that would profile, chapter by chapter, each planet in sequence from the Sun, ending with Pluto. This approach presupposes that enumerating the nine planets, in order from the Sun, is of fundamental interest and of scientific importance and that it’s worthwhile for students to memorize their names in “My-Very-Educated-Mother…” order. But by the 1980s, as we discovered more and more comets, asteroids, and moons, and as we continued to characterize their detailed properties, it became clearer and clearer that the Sun’s planets are only part of the solar system’s story. The outward-bound Voyager 1 and Voyager 2 spacecraft, both launched in 1977, played a starring role in that drama. As they separately approached Jupiter in 1979 and the rest of the outer planets over the decade to follow, one of the welcome surprises was that the outer planets’ moons turned out to be as fascinating as the planets themselves—maybe more so. Beginning with the Voyager era, objects other than planets began to enjoy their rightful day in the Sun.

  As a direct consequence, textbooks began to organize the solar system into scientifically suggestive categories; Pluto, the comets, the asteroids, and other small bodies with interesting features, such as the moons of the outer planets, became parts of chapters whose titles featured words such as “Debris,” “Interlopers,” and “Vagabonds.” This grouping—or regrouping—began in the late 1970s and persisted through the 1980s. Gradually, Pluto and its properties were being taught differently from the rest of the planets in the solar system.

  After Pluto’s moon Charon was discovered in 1978, defensive Plutophiles were keen to note that only planets have moons, clearly distinguishing Pluto from comets and asteroids. Of course, Mercury and Venus do not have moons, and nobody was rushing to reclassify them. Clearly, then, a planet does not lose status for not having a moon—but surely if an object has a moon, what else could it be but a bona fide planet? Even Merlin, my pen name for two question-and-answer books on the universe, valued this distinction:

  Dear Merlin,

  What is Pluto, a pl
anet, planetoid, or comet? How will it be determined if Pluto should be demoted to asteroid status?

  Roy Krause

  Shaw AFB, South Carolina

  Merlin has noticed over the years that many people would like to demote Pluto to an “–oid” status.

  But Pluto is twice the size of Ceres, the largest known asteroid, and 50 times the size of the largest comets. When we consider that Pluto has a satellite of its very own it certainly gets Merlin’s vote for full rank and privileges of “planet”.16

  Plutophiles grabbed for Pluto’s moon as immediate proof of Pluto’s planethood—a criterion invented more or less on the spot, but which left them at risk that we might one day discover a moon around an asteroid. What do you do when that happens? This conundrum reveals a deeper truth in science: When your reasons for believing something are justified ad hoc, you are left susceptible to further discoveries undermining the rationale for that belief.

  Sure enough, on February 17, 1994, the Galileo space probe opportunistically imaged the aseroid Ida while en route to Saturn. While examining the data, mission member Ann Harch discovered that Ida has a small (1.4-kilometer) orbiting moon that came to be called Dactyl. Idaho-potato-shaped Ida is only 30 miles long and about 12 miles across (Figure 4.1). The thing is unimpeachably asteroidal. And in the time since Dactyl’s discovery, detailed observations of many more asteroids suggest that asteroid moons are common. Furthermore, some asteroids may not be solid at all. Many are composed of loosely assembled rubble, some the size of Dactyl itself, which undermines the concept of moon altogether.

  Figure 4.1. Galileo spacecraft image of asteroid Ida taken 14 minutes before closest approach in 1993. Ida’s tiny moon Dactyl orbits a short distance away to its right. At a mere 30 miles long and with an irregular potato shape, Ida is clearly an asteroid with a moon, undermining the moon criterion for planet status among Plutophiles after Pluto’s moon Charon was discovered in 1978. (NASA Jet Propulsion Laboratory Planetary Photojournal; http://photojournal.jpl.nasa.gov/jpeg/PIA00069.jpg.)

  Two kinds of scientists populate the world: those who see what is similar among objects and explore how they differ from one another, and those who see what is different among objects and explore how they’re all similar. To arrive at a deep understanding of the natural world often requires a sustained but resolvable tension between the two camps. Even after the shift in the 1980s, Pluto was still a planet, by anybody’s reckoning. But behind closed doors, planetary geologists recognized that Pluto possessed many properties that resemble those of comets and asteroids.

  This new approach to teaching the solar system didn’t just affect Pluto. The rest of the planets were grouped as well. Mercury, Venus, Earth, and Mars became the terrestrial planets, treated as a conceptually coherent subject: they are all small, rocky, and dense. Meanwhile, Jupiter, Saturn, Uranus, and Neptune became the Jovian planets, all of which are large, ringed, gaseous, low density, and fast rotating. Meanwhile, the rest of the Astro 101 textbook began to fill up with discoveries regarding the Big Bang, galaxy formation, galaxy collisions, black holes, the births and deaths of stars, and the search for life.

  The astrophysics community, primarily the planetary scientists, simply shifted how they thought about the contents of the solar system. Of course, Saturn is still very different from Jupiter, and Earth is very different from Venus. But Earth and Venus have much more in common with each other than either Earth or Venus has with Jupiter or Saturn. And Jupiter and Saturn have much more in common with each other than either they or the terrestrial planets have with Pluto. With Pluto’s properties (size, orbit, composition) sitting alone among the planets, can you justify a class of one? No. Classification schemes require at least two similar objects to define a class. Until that happens, you must find something else to do with your unusual object.

  Yes, in a sense, Pluto had no class. But that would shortly change.

  In 1992, University of Hawaii astrophysicist David Jewitt and his graduate student Jane Luu used their 2.2-meter optical telescope at Mauna Kea to discover an icy object, cryptically labeled 1992 QB1,17 orbiting the Sun out beyond Neptune, just where (forty years) earlier the University of Chicago planetary astronomer Gerard Kuiper had hypothesized such objects would live.

  Figure 4.2. Discovery images from 1992 of the first icy body discovered in the outer solar system since Pluto in 1930. Taken by University of Hawaii astrophysicist David Jewitt and his graduate student Jane Luu, using the 2.2-meter telescope on Mauna Kea, this object, labeled 1992 QB1 and identified by the shifting arrow, was the first of many to be discovered in a new region of the solar system called the Kuiper belt.

  One of the biggest problems you face when observing objects in the solar system is that they don’t radiate their own light. The most distant ones are so far away, the feeble sunlight that reaches them must then reflect from their surface and make it all the way back to the inner solar system before reaching our telescopes here on Earth. A reflective surface helps. Out there in the cold depths of space, clean ice can satisfy this need. But nobody knew for sure what 1992 QB1 was made of. All they knew was that it orbited the Sun beyond Neptune and that it was small, maybe a fifth the size of Pluto. 1992 QB1 was not a threat to Pluto’s regional prominence, but it nonetheless made you go “hmmm.”

  Jewitt and Luu looked for more. And they found more. One after another after another, all with orbits a bit tipped out of the plane of the solar system, just like Pluto’s, some with orbits so elongated that they crossed the orbit of Neptune, just like Pluto. This growing family of objects populated a new swath of real estate that orbits the Sun—a belt, analogous to the band of objects between Mars and Jupiter that came to be known as the asteroid belt.

  Gerard Kuiper had proposed that beyond the outermost planet in the solar system (perhaps in any star system) lies a reservoir of slowly orbiting debris—leftovers from the formation epoch that never got “vacuumed” up by a planet’s gravity or, more importantly, never coalesced to form a planet in the first place. By comparison, planetary orbits from Mercury to Neptune are relatively free of debris. Even though Earth plows through hundreds of tons of meteors a day—the source of our nightly display of shooting stars—this pales compared with what floats in the outer solar system. So imagine how much sweeping up a massive planet in the outer reaches could do. But once you pass Neptune, you’ve run out of big planets. And there’s so much space among the debris that it all stays there, in orbit, and in vast quantities.

  At those distances, 5 billion miles from the Sun, temperatures dip below -400ºF and stay there. Plenty of cosmically common ingredients that would evaporate when brought close to the Sun, such as water, carbon dioxide, ammonia, and methane, stay forever frozen at those temperatures, becoming a basic constituent of solid matter. Within a few years of Jewitt and Luu’s discovery of 1992 QB1, enough additional objects had been found to confirm that the solar system indeed contains a “Kuiper belt” of icy bodies. Looking at the distribution of sizes found and the rate at which these objects were being discovered, astrophysicists knew that it was just a matter of time before hundreds or even thousands of Kuiper belt objects would be discovered and cataloged. Makes you wonder: What happens the day we find something bigger than Pluto? Do we call it a planet, because Pluto is a planet, or do we use the opportunity to come up with modified nomenclature for this new class of objects, including Pluto?

  Figure 4.3. Clyde Tombaugh, discoverer of Pluto, seen here at age 90, the year before his death. Rarely seen without his cane, it served not only as a walking aid but as a means of punctuating his comments about why Pluto should stay a planet forever.

  Clyde Tombaugh was still alive in the early 1990s. He saw the Kuiper belt omens, but fought them tooth and nail with cane in hand, using his cane not only as a walking aid but also as punctuation for the aggressive arguments he would make. Tombaugh had the most to lose if Pluto were classified as anything other than a full, red-blooded planet. In a December 1994 letter to the editor of
Sky & Telescope magazine (the monthly bible for amateur astronomers), Tombaugh declared:18

  I’m fascinated by the relatively small “ice balls” in the very outer part of the solar system. I have often wondered what bodies lay out there fainter than 17th magnitude, the limit of the [photographic] plates I took at Lowell Observatory. May I suggest we call this new class of objects “Kuiperoids”?

  Not knowing that objects larger than Pluto awaited discovery in the Kuiper belt, Tombaugh was unwittingly suggesting that Pluto become a Kuiperoid as well. In any case, astronomers are not likely to adopt a word that sounds like a contagious skin disease.

  Clearly frustrated by all the talk of reworking a time-honored classification scheme, Tombaugh proceeds to attack other astronomical traditions that carry historical concepts into the present, including our entrenched and arcane classification system for the spectra of stars:

  While we are considering reclassifying astronomy, how about revamping the Hertzsprung-Russell diagram so the spectral types [of stars] are alphabetically ordered? No, that would wreck extensive catalogs of stellar spectra. Or let’s throw out the awkward constellation system! Alas, that would discard our beautiful mythology.

  Tombaugh now raises “cane” as he goes in for the kill:

  Pluto started out as the ninth planet, a supported fulfillment of Percival Lowell’s prediction of Planet X. Let’s simply retain Pluto as the ninth major planet. After all, there is no Planet X. For 14 years, I combed two-thirds of the entire sky down to 17th magnitude, and no more planets showed up. I did the job thoroughly and correctly. Pluto was your last chance for a major planet.

  CLYDE W. TOMBAUGH

  Mesilla Park, New Mexico