| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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skywatcher Pluto's Identity Crisis Poor little Pluto is the most picked-on planet in the solar system. Ever since its discovery, Pluto has been the runt of the planetary litter. Neither a rocky body orbiting near the Sun like Mercury, Venus, Earth, and Mars, nor a giant gaseous world like Jupiter, Saturn, Uranus, and Neptune, small, icy Pluto seems isolated in the frigid depths of deep space, four billion miles from the Sun. When Clyde Tombaugh discovered Pluto on February 18, 1930, astronomers thought it was roughly the size of Earth.
But as astronomers observed the planet over the following decades, their estimates got smaller and smaller. Some astronomers joked that at the rate Pluto’s estimated size was shrinking, it would eventually disappear. By the late 1980s, astronomers had pinned down Pluto’s diameter at a mere 1,430 miles. It was by far the smallest planet, less than half the diameter of the next smallest planet, Mercury. Still, few astronomers questioned whether Pluto should be considered anything other than a planet. But all of that changed in 1992, when University of Hawaii astronomers David Jewitt and Jane Luu discovered a smaller version of Pluto orbiting the Sun beyond Neptune. Like the proverbial child who pulled his finger out of the dike, Jewitt and Luu’s discovery opened the floodgates to Pluto-like bodies orbiting beyond Neptune. Since then, astronomers have discovered 540 of them. These are the worlds of the Kuiper belt, a disk of small, icy objects that many consider the source of some comets. To some astronomers, Pluto suddenly appeared to be not a planet, but just the largest known member of the Kuiper belt. The cautious and polite reserve of many astronomers gets thrown out the door when a discussion turns to Pluto’s planetary status. For unfathomable reasons, this little ice ball in the outer solar system stirs the deepest of passions. For some, maintaining Pluto’s planetary status takes on the air of a sacred duty, while others seem to have an almost maniacal urge to boot it from planethood. And there are those who just want to stay out of the fray. When University of California at Berkeley astronomer Geoff Marcy was asked by David Letterman on TV whether or not Pluto should be called a planet, Marcy replied, “I’m going to try to stay neutral because I’ll get a lot of hate mail if I say Pluto is or isn’t a planet.” The problem with Pluto is that both sides can summon valid arguments to support their cause, and there is no standard definition of a planet to meet. In the following debate, hypothetical astronomers Dr. Major and Dr. Minor duke it out over whether Pluto is a “major planet” like Earth, or simply another asteroid, or “minor planet.” Dr. Minor: With a diameter of 1,430 miles, Pluto is a pip-squeak of a planet. In fact, seven planetary satellites, including our Moon, are bigger than Pluto. Dr. Major: Oh yeah? Two of those moons, Jupiter’s Ganymede and Saturn’s Titan, are bigger than Mercury. Yet nobody is calling for Mercury’s demotion. Pluto might be smaller than the other eight planets, but it’s still big enough, and has gravity strong enough, to shape it into a sphere. If an object is dense and massive enough for the forces of nature to mold it into a sphere, and it freely orbits the Sun, it should be called a planet. Dr. Minor: Hold on a minute. Several dozen asteroids are at least 125 miles across, large enough to be roughly spherical. If we start calling all of these spherical objects planets, we’ll soon have more planets than Elvis impersonators! Dr. Major: Let’s stop this anti-Pluto crusade in its tracks. Do you want to rewrite all the textbooks? We’ve been calling Pluto a planet for 70 years. Why change now? There is historical precedence for calling Pluto a planet. Dr. Minor: There is historical precedence for downgrading Pluto. When Italian astronomer Giuseppe Piazzi found Ceres between Mars and Jupiter in 1801, everyone called it a planet. But by the mid-1840s, after astronomers had found a bunch of similar objects between Mars and Jupiter, they realized that Ceres was just the largest member of a new population. So they started calling these small objects “asteroids,” or “minor planets.” The story is the same with Pluto. Dr. Major: But Ceres is only 580 miles across. Pluto’s diameter is two-and-a-half times bigger. In fact, if all the asteroids in the belt condensed into one body, it would be only 800 miles across, much smaller than Pluto. We could define planets to be objects more than 600 miles across that directly orbit the Sun. In that case, Pluto is in and Ceres is out. Dr. Minor: That’s totally arbitrary. If Pluto were discovered today, nobody would be calling it a planet. Pluto should thank its lucky stars that Tombaugh had the patience to sift through hundreds of photographs and 90 million star images to pinpoint its faint dot amidst a crowded star field.
Dr. Major: The fact is that Pluto was bright enough to be detected in 1930, when astronomers were still using crude photographic plates. Its discovery didn’t have to wait until the 1990s, when astronomers armed with cameras ten times more efficient at capturing light are finding Kuiper belt objects, or KBOs, by the cartload. Most were too faint to be seen in 1930. Dr. Minor: Ah, the Kuiper belt. Thank you for bringing that up. The 540 KBOs found to date are all small iceballs, just like Pluto. All these objects are basically the same: building blocks left over from the formation of the solar system. I admit that Pluto reigns supreme as the king of the Kuiper Belt....for now. Astronomers have already found several KBOs that are a significant fraction of Pluto’s size. Working with a telescope in the Andes in May, Lowell Observatory Director Robert Millis and colleagues found KBO 2001 KX76 which at about 870 miles across, is more than half the diameter of Pluto. Millis and other astronomers predict that it is only a matter of time before they find a KBO larger than Pluto. Such a discovery will drive the final nail into its planetary coffin. Dr. Major: It’s certainly true that astronomers might find a KBO larger than Pluto. If they do, we’ll call that a planet also. Ten planets would make a nice round number. Dr. Minor: The Kuiper belt’s very existence shows that Pluto shouldn’t be called a planet. Major planets clear out lanes around the Sun; minor planets like Ceres and Pluto do not. Astronomers estimate that tens of thousands of KBOs are at least 100 miles across, and billions are at least ten miles across. Like Ceres, Pluto is such a lightweight object that its gravity isn’t strong enough to sweep its region of space clean. Dr. Major: Earth hasn’t completely cleaned out its region of space! An estimated 900 asteroids at least half a mile in diameter have orbits that cross Earth’s orbit. Some of these objects will eventually smash into Earth. Even Jupiter suffers the indignity of having the so-called Trojan asteroids littering its path in an orbit 60 degrees ahead and behind the planet. Dr. Minor: Yeah, but neither the near-Earth asteroids nor the Trojans constitute a “belt” of objects, like the asteroid belt or the Kuiper belt. And if I were you, I would not have brought up Jupiter. You could fit 252,000 Plutos inside Jupiter, which is 160,000 times more massive. It’s ridiculous to give Pluto the same designation as Jupiter; the two worlds have nothing in common other than the fact they both orbit the Sun. Pluto has much more in common with KBOs in terms of size and composition than with Jupiter. Are we going to call all KBOs planets too? Dr. Major: It’s a matter of perspective. If you were an astronomer living on Jupiter, you probably wouldn’t consider Earth a planet. Jupiter is 1,400 times more voluminous and 318 times more massive than Earth. Jupiter is much bigger relative to Earth than Earth is to Pluto. To someone living on Jupiter, there isn’t much difference between Earth and Pluto. Both represent rocky debris that wasn’t able to form a “real” planet. Do you want to go on record saying that Earth shouldn’t be called a “real” planet? Dr. Minor: But what about Pluto’s screwy orbit? It circles the sun along a path tipped 17 degrees from the plane of the rest of the planets in our solar system. And unlike the other planets, its orbit is oval. In fact, Pluto spends 20 years of its 248-year orbit inside Neptune’s orbit, as it did from 1979 to 1999. No other planet has an oddball elliptical orbit that crosses the path of another planet. Dr. Major: Pluto’s orbit may be different from the others, but that doesn’t mean it’s not a planet. Let’s consider the 80 or so planets that have been found orbiting other stars. Most have orbits far more elliptical than Pluto’s, yet astronomers worldwide are calling these objects planets. And if Earth had an inclined orbit like Pluto’s, we’d still call it a planet. Dr. Minor: But remember, virtually all extrasolar planets found so far are about the size of Jupiter, if not many times more massive. With its icy composition and egg-shaped orbit, Pluto has more in common with comets than planets. Like a comet, Pluto’s ices evaporate to form an atmosphere when it travels close to the sun, then freeze back onto its surface when it moves away again. Neil de Grasse Tyson, the director of New York’s Rose Center for Earth and Space, likens Pluto to an “overgrown comet.” Dr. Major: But how many asteroids have atmospheres? None! An atmosphere is a hallmark of a major planet. Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune all have atmospheres. Dr. Minor: For all intents and purposes, Mercury has no atmosphere, yet it’s very close to the Sun. Another argument against Pluto’s planethood is its large moon, Charon. Charon is 780 miles across, about half the size of Pluto and therefore much larger relative to its “planet” than any other moon in the solar system. And consider Pluto’s and Charon’s rotational patterns. Earth’s gravity is so much greater than the Moon’s that it has gravitationally locked its satellite. The Moon’s rotation is slowed down so much that it shows only one hemisphere to us. Charon and Pluto, on the other hand, have slowed one another so much that both are gravitationally locked onto each other; the two worlds always show the same hemisphere toward each other. No other planet suffers such humiliation at the expense of its moon! And the two are so close in size that Charon doesn’t even revolve around a center of mass within Pluto; instead, the two worlds revolve around a point in space 800 miles above Pluto’s surface. Pluto isn’t a planet; it’s a binary KBO. Dr. Major: Surely you jest. Our Moon is a full one-quarter the size of Earth. If you’re going to call Pluto a binary system, then the Earth-Moon system is also a binary system. And the fact that Charon is locked on to Pluto has more to do with their close distance than Charon’s size. The two are only about 12,000 miles apart, one-twentieth the distance between the Earth and the Moon. The mere fact that Pluto has a moon eloquently supports its planetary status. Dr. Minor: Having a satellite isn’t such a big deal. Astronomers have found at least nine asteroids that have moons, as well as another binary KBO system in addition to Pluto-Charon. Besides, Pluto doesn’t fit into either of the two current planet categories: the rocky, terrestrial worlds, or the gas giants. Dr. Major: Those two classes have very little in common with each other. Millis argues that Pluto should be the prototype of a third type of planet. We should also honor Clyde Tombaugh’s memory, and his exhaustive search, by keeping Pluto a planet. Think of all the disappointed people around the world if you get your way. Dr. Minor: You’re basing your viewpoint on sentiment rather than science. As Tyson says, it would be more dignified for Pluto to be called the king of the Kuiper belt than the runt of the planets. Dr. Major: When a few astronomers tried to reclassify Pluto in 1999, the International Astronomical Union (IAU), the official arbiter in such matters, ruled firmly in Pluto’s favor. Until the community of professional astronomers agrees on what constitutes a planet, it makes no sense to demote Pluto. Pluto's planethood continues to be a hot-button issue. In fact, there was a recent report of the IAU working group in charge of defining a planet, but they sidestepped the Pluto issue. It discussed the definition of large planets orbiting other stars, but said it would not “address the question of a possible lower mass limit for ‘planets’ in our solar system.” The chair of the group, Alan Boss, admitted that “the IAU prefers to avoid controversy.” Clearly, our current system of nomenclature is out of date, and we need a more sophisticated planetary classification scheme to keep up with recent discoveries. As Brian Marsden of the IAU’s Minor Planet Center says, “The word ‘planet’ has in some sense outlived its usefulness.” In the meantime, Alan Stern and Hal Levison of the Southwest Research Institute in Boulder, Colorado, have developed what may be a more rational classification system. They subdivide what we now call “planets” into five broad categories based on size. The largest asteroids and Pluto would be called subdwarfs. The four terrestrial planets would be called dwarfs. Medium-sized planets like Uranus and Neptune would be subgiants. Jupiter and Saturn would fall into the giant category. And some of the particularly large planets found orbiting other stars would be called supergiants. But until astronomers can settle on a definition of what makes a planet a planet, the argument will likely rage on. One thing astronomers on both sides of the debate would agree upon is the need for a mission to Pluto. A visit to this strange little world will give us the opportunity to study pristine material from which the solar system formed. By observing Pluto up close, we will learn more about conditions in the solar system at the time Earth and the other planets formed. This is also a good opportunity to study Pluto’s atmosphere before it freezes back onto the surface. Otherwise, we’ll have to wait 200 years before Pluto sprouts an atmosphere again. Hubble Space Telescope images have already shown us that Pluto has one of the most varied surfaces in the solar system. Congress recently allocated $30 million to initiate development of a Pluto mission to be launched by 2006. The mission doesn’t yet hsve funding for subsequent years and, though unlikely, Congress could pull the plug at any time. Launching before 2006 will allow NASA to use Jupiter’s gravity to accelerate the spacecraft out to Pluto by 2020. After 2006, Jupiter won’t be in the right position for many decades. Robert Naeye is editor of Mercury, the magazine of the Astronomical Society of the Pacific. He wrote “Superman’s Telescope,” in the Summer 2001 California Wild. www.astrossociety.org |
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