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Space Topics: Pluto and CharonAnd Then There were Eight: IAU Vote Shakes Up the Solar SystemBy Amir Alexander It has been the better part of a century since our seemingly familiar solar system has received such a jolt. That was on February 18, 1930, when 24 year-old Clyde Tombaugh noticed a blinking point of light in the photographic plates he was examining. As Tombaugh well knew, this indicated that somewhere beyond the orbit of Neptune, a sizable object was moving through space, in orbit around the Sun. By the time the public announcement was made two months later, the mysterious object had a name: it became Pluto, the latest addition to the exclusive club of solar system planets. As far as the general public was concerned, Tombaugh's discovery was the final word on the composition of the solar system. As we all learned in school, our Sun – a medium-sized star on an arm of the Milky Way galaxy – is orbited at increasing distances by nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Smaller objects such as asteroids and comets, which occasionally pass near the Earth and light up our skies, spice up the "nine planet" picture but in no way alter it. To most people the nine-member planetary system is as familiar and reliable as their own neighborhood. But now, 76 years after Tombaugh's discovery, the members of the International Astronomical Union (IAU) have overturned what seemed to be the irrefutable judgment of history. At their general meeting in Prague on August 24, 2006, the IAU membership voted to strip Pluto of its planetary status, making it a member of a new class of objects called "Dwarf Planets." The solar system is now left with only eight planets, and an indeterminate number of smaller objects such as asteroids, comets, and KBOs, which will now be called "Small Solar System Objects." In voting to scratch Pluto from the ranks of true solar planets the IAU rejected a proposal drawn up by special "Planet Definition" committee set up by the organization. The committee's proposal would have recognized 12 planets in the solar system right now, with dozens of other planets likely to join the ranks in the coming years. The old nine-planet neighborhood we have grown accustomed to would have been replaced by a frequently changing crowd of planetary strangers. Instead, the IAU chose to eliminate the most questionable member of the planetary club and fix the number of solar planets once and for all on eight. How Big and How Small?The demand to come up with a clear definition for the term "planet" has been mounting in recent years, as new discoveries have thrown the traditional understanding what a planet is into doubt. In the past, no term in astronomy had seemed as clear cut as a "planet." There were nine, six of which had been known for several millennia, and while no formal definition was available, none seemed necessary. Planets seemed intuitively obvious, and if more were discovered somewhere in the universe, both professionals and lay people confidently assumed that they will know one when they see one.
This seemingly well-earned confidence, however, was undermined in the past decade by two separate lines of research. One was the search for planets outside the solar system, which after decades of frustration was finally bearing fruit. The discovery of the first extrasolar planet orbiting 51 Pegasi in 1995 was quickly followed by the detection of many other planets, with the count now approaching 200. But in what came as a surprise to both astronomers and the general public, most of these newly found planets were real oddballs when compared with the familiar nine of our own planetary neighborhood. Instead of moving in stately orbits around their stars, taking anywhere from months to centuries to complete each cycle, these newcomers seemed to orbit in the very shadow of their home star, racing along at a dizzying speed and completing each orbit in a matter of days. Even stranger was the new planets' size: they were huge. To get an idea of just how big, consider that Jupiter, the largest planet in our solar system, is more than 300 times the mass of the Earth. Some of the new found planets had a mass dozens of time that of Jupiter, or thousands of times the mass of the Earth. Astronomers believe that the abundance of such "hot Jupiters" among the extrasolar planets discovered so far is not necessarily an indication that they are particularly common in the universe, but rather that they are the easiest to detect with current methods. Nevertheless these giants are out there, and they raise the inevitable question: how large can a planet be and still be considered a planet? While this question was hanging in the air, an even more pressing challenge to the concept of a planet came from another line of research: the search for large objects at the edge of our own solar system. Back in the 1950s, astronomer Gerard Kuiper, studying the trajectories of short-period comets, concluded that a belt of rocky debris populated the far reaches of the solar system around the orbit of Pluto and beyond. Finally, in 1992, the first of these "Kuiper belt objects" (KBO's), also known as "trans-Neptunian objects," was observed directly from Earth. The observation confirmed what had long been suspected: Pluto, far from being the solitary planet it was thought to be at the time of its discovery, was in fact the brightest representative of a large class of KBOs.
Since the observation of the first KBO 14 years ago, many other objects have been observed in that region of space and given scientific designations. While most were significantly smaller than the acknowledged planet Pluto, they have been getting bigger. Over the past five years, a group of planetary scientists consisting of Michael Brown of Caltech, David Rabinowitz of Yale, and Chad Trujillo of the Gemini Observatory have registered, among other objects, "Quaoar," with a diameter of around 1,000 kilometers (600 miles), "Sedna," with an estimated diameter of 1200 to 1800 kilometers (700 to 1,100 miles), and the as yet unnamed 2005 FY9, with a diameter similar to that of Sedna. If one considers that Pluto, by far the smallest of the "official" planets, is only 2,300 kilometers (1400 miles) in diameter, one inevitably begins to wonder why it alone should have the honor of being a planet, while its obvious peers are merely "objects." Then, in July of 2005, Brown and his colleagues made the announcement that seemed inevitable: an object was discovered in the far reaches of the solar system that was larger than Pluto. Measurements by the Hubble Space Telescope later confirmed that the object, designated 2003 UB313, is around 2,400 kilometers in diameter, beating out Pluto's 2,300 kilometers by the narrowest of margins. Now the question could no longer be avoided: If Pluto is a planet, then clearly 2003 UB313 is one as well, and other giant KBO's such as Sedna and Quaoar should be considered serious candidates. But if 2003 UB313 is only an "object," then it follows Pluto, which is in any case much smaller than the other planets, should also be demoted to "object" status. More generally, the question was the reverse of the one that emerged from the study of extrasolar planets: how small could a planet be and still be considered a planet?
An Open Society of Planets?Faced with this double edged assault on the concept of a planet, the International Astronomical Union (IAU) set out to find a workable definition of the suddenly evasive term. For the past two years, the question had been raised and discussed in various IAU forums and committees in an effort to reach a consensus. In the last stage, a "Planet Definition Committee," composed of historians and writers as well as astronomers, and chaired by Harvard astronomer and historian Owen Gingerich, met in July of this year to draft a new definition of what a planet is. The meeting took place at the Royal Observatory in Paris, where Jean-Dominique Cassini, who discovered the gap in Saturn's rings, had been director under Louis XIV, and where Urbain Leverrier predicted the discovery of Neptune in 1846. In this historic setting the committee worked closely and amiably, says writer Dava Sobel who took part in the deliberations. After two days of intense deliberations, they voted unanimously for the following definition:"A Planet is a celestial body that (a) has sufficient mass for its self gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and (b) is in orbit around a star and is neither a star nor the satellite of a planet." In other words, for a celestial object to be considered a planet, it has to orbit a star, be approximately spherical in shape, and be neither a star nor a moon. The committee's formula addresses both sides of the challenge to the concept of "planet." To place an upper limit on the size of a planet, the committee inserted the seemingly self-evident clause that a planet cannot be a star. As the committee explains in accompanying documents, a "star" is an object that is large enough to ignite a fusion reaction in its core. The very smallest stars, according to this definition, are "brown dwarfs," whose lower mass limit is around 12 Jupiters. The bulk of the Committee's definition, however, is aimed at answering the opposite concern: how small can a planet be, and still be worthy of its name? The criterion chosen by the committee is that a planet has to be massive enough to be round. The solar system is full of small space rocks of various sizes which are clearly not "planets" by any common understanding of the term. For a planet to be recognized as such, it must therefore have sufficient mass that its own self-gravity will mold it into the shape of a sphere. On average, objects of around 1/10,000 Earth masses with radii of around 800 kilometers (500 miles), are the smallest whose own gravity produces a spherical shape, though the specifics vary from case to case. This, according to the Committee, is the minimum mass of a planet. The choice of "roundness" as a criterion for planetary status also reflects the Committee's sensitivity to the broad cultural significance and use of the term. After all, a "planet" is not just a technical term used in scientific communications, but also a word in the spoken language, signifying a commonly known object. It is undeniable that in common parlance and the popular imagination planets are always, always round, as anyone who has watched an episode of Star Trek well knows. The committee endorsed this popular understanding of the term and used it to scientifically define what a planet is. The same sensitivity to cultural implications is also in evidence in the Committee's requirement that a planet cannot be a moon of another planet. There are, after all, many satellites in our solar system that look very much like planets. Jupiter's moon Europa, for example, or Saturn's Titan, would be very strong candidates for planetary status if they happened to be orbiting the Sun. But since they are in orbit around already established planets, tradition alone recommends that their status be preserved and they be referred to as "moons."
All the criteria proposed in the Committee's definition therefore seem quite natural and in line with existing tradition and the common understanding of the term. But when one applies the definition to the composition of the solar system, the results are quite startling. Spherical Pluto, unsurprisingly, retains its planetary status, and the slightly larger 2003 UB313 is designated a "planet" as well. But so is Ceres, the largest member of the asteroid belt between Mars and Jupiter. When it was discovered in 1801 by Giuseppe Piazzi, Ceres was indeed considered a planet, but was later designated an "asteroid" when many similar (though smaller) objects were observed orbiting the Sun in the same region of the sky. Now, thanks to its 950-kilometer (570 mile) diameter and round shape, Ceres is set to regain its former status as a fully accredited solar planet. Equally surprising is the rise of Charon to planetary status. Since it is most commonly known as Pluto's moon, designating Charon as a planet seems to violate the second term of the Committee's definition. Charon, however, is not a normal moon: for all other moons in the solar system, the center of gravity of their "moon-planet" system resides within the body of the planet itself. Charon, in contrast, is close enough in mass to its planet, Pluto, that the center of gravity of the Pluto-Charon system is located in the empty space between them. This, according to the committee makes Pluto-Charon a "double planet," and since Charon, like Pluto, is round and orbits the Sun, it too must be considered a planet. If the addition of 2003 UB313, Ceres, and Charon were approved, the planet count in our solar would system stand at 12. But what about Quaoar, Sedna, 2005 FY9, and other giant objects in the Kuiper belt? And what of Vesta, Pallas, and Hygiea, Ceres' large companions in the asteroid belt? All of these are, for the moment, planet candidates, whose status will be decided in the coming months and years. And with the ongoing pace of discovery of new objects, and better and more precise measurements of known objects, there is no doubt that the population of planets orbiting our Sun will reach several dozen in the near future. Eight is EnoughThe Planet Definition committee's proposal would have transformed the exclusive planetary club into an open society, where newcomers of different sizes and characteristics regularly join in. "That's how it should be," said Alan Stern, Principal Investigator of the New Horizons mission, currently on its way to Pluto. "In all of science, categories are open; we don't put a limit on the number of clouds or the number of biological species. Why do so for planets?"But the majority of IAU members disagreed, criticising the expansion of the term "planet" to include an unlimited number of objects that look nothing like traditional planets. Pluto and its neighbors, argued critics of the committee's proposal, are too small to be considered planets. Even Mercury, the smallest of the traditional planets, has a diameter more than twice that of the largest KBO. Furthermore, the orbits of Pluto, 2003 UB313, Sedna, Quaoar and their peers are qualitatively different from those of the eight inner planets. For one thing, the "true" planets paths are only slightly elongated, and close to circular, whereas the KBO's orbits are highly elongated and clearly elliptical. For another, the traditional planets all orbit on the same plane, or very close to it. In contrast, Pluto deviates from it by a substantial 17 degrees, and 2003 UB313 by as much 45 degrees. Most significantly, perhaps, the eight large planets orbit the Sun alone, accompanied only by their moons. This is because early in the history of the solar system, the planets' massive gravitational pull sent other space rocks out into interstellar space, or hurtling into the Sun, leaving the planets to travel through space in regal isolation. In contrast, Pluto, 2003 UB313, Sedna, and the other large trans-Neptunian objects, were too small to clear their neighborhoods of debris left over from the formation of the solar system. As a result they are all members of the Kuiper belt – a region of space filled with rocks and boulders of all shapes and sizes. The same is true of Ceres, which is merely the largest asteroid in the asteroid belt between Mars and Jupiter. The true planets, argue proponents of the eight-planet solar system, are naturally distinguishable from the plethora of large and small objects at the edge of the solar system, and our definition of "planet" should reflect that. If Pluto were discovered today, they argue, no one would consider designating it a planet since it is obviously a rather typical large KBO. Rather than implausibly stretch the term planet to accommodate Pluto, wouldn't it be better to correct a 76 year old mistake and exclude Pluto from the family of planets?
The IAU's resolution reflects this view. Similar to the Planet Definition Committee's proposal, the resolution states that "a planet is a celestial body that (a) is in orbit around the Sun and (b) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape. But unlike the original proposal the resolution adds another criterion, stating that (c) a planet has cleared the neighborhood around its orbit. Significantly, the IAU did not extend the definition of a planet beyond the solar system, limiting it to objects orbiting the Sun. Even though it deprives him of the honor of being the discoverer of a planet, Michael Brown likes the IAU resolution. If aliens came upon the solar system, he suggested on his website, "they would quickly come to the conclusion that there are 8 major bodies orbiting the Sun." Isn't it time, he argues, that we did the same? The majority of the IAU membership seemed to think that it is: as of August 24, 2006, officially at least, the only true planets in our solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Little Pluto, long a favorite among the general public and especially children, has been left out in the cold. How Many Planets Should the Solar System Have?
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