The Pluto Files: The Rise and Fall of America's Favorite Planet (6 page)

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.”

A
S 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:

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