Is Pluto a planet

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ASTRONOMY AND SPACE EXPLORATION

Is Pluto a planet?

Viewpoint: Yes, Pluto's status as a planet has been continually defended by the International Astronomical Union. Although its size and elliptical orbit do differ from those of the solar system's other planets, these criteria are arbitrary traits that do not discount its status as a planet.

Viewpoint: No, Pluto has more in common with the comets and asteroids discovered in the Kuiper Belt during the 1990s.

The controversy surrounding the planetary designation of Pluto sounds deceptively simple. While Pluto was identified as a planet upon its discovery in 1930, recent refinements in the taxonomy of orbital bodies have raised questions about whether Pluto is "really" a planet, or one of the smaller, more numerous objects beyond Neptune that orbit our Sun. Part of the controversy is essentially semantic: there is no rigid, formal definition of "planet" that either includes or excludes Pluto. The other eight planets are a diverse group, ranging greatly in size, composition, and orbital paths. Size is the primary distinction that sets them apart from the thousands of smaller objects orbiting the Sun, such as asteroids and comets. Pluto, however, is much smaller than the other planets but much larger than the bodies found in the asteroid belts. This fact alone has prompted some scientists to "demote" Pluto as a planet.

Size is not the only issue raised by astronomers who want to reevaluate Pluto's planetary status. For example, they point out that Pluto's orbit differs significantly from that of the other planets, and that its composition is more similar to comets than to the other planets. Scientific organizations, such as the International Astronomical Union, however, maintain that Pluto is a major planet, and that such distinctions are arbitrary.

Perhaps the most compelling aspect of this controversy is what it says about our understanding of the solar system. The image of our solar system consisting of one Sun and nine planets is elegant, easy to picture, and has been a staple of astronomy textbooks for more than 70 years. But as scientists learn more about the smaller bodies that orbit our Sun, and look far beyond Pluto and see a wide population of other orbital bodies, it seems simplistic and naive to view Pluto as the outer boundary of the solar system. If Pluto is re-assigned to the broader category of "Trans-Neptunian Objects," one of the small, solar system bodies orbiting beyond Neptune, it would become a recognizable exemplar of a group of far-off objects made mysterious by their distance from us, but nevertheless a part of our solar system.

—LOREN BUTLER FEFFER

Viewpoint: Yes, Pluto's status as a planet has been continually defended by the International Astronomical Union. Although its size and elliptical orbit do differ from those of the solar system's other planets, these criteria are arbitrary traits that do not discount its status as a planet.

Pluto, the last major planet of Earth's solar system, has been considered a planet since its discovery in 1930 by American astronomer Clyde W. Tombaugh at Lowell Observatory in Flagstaff, Arizona. Tombaugh was conducting a systematic search for the trans-Neptunian planet that had been predicted by the erroneous calculations of Percival Lowell and William H. Pickering. Some scientists maintain that the only reason Pluto is considered a planet today is because of the long, ongoing and well publicized search for what was then referred to as Planet X. When Pluto was discovered, media publicity fueled by the Lowell Observatory "virtually guaranteed the classification of Pluto as a major planet," according to Michael E. Bakick in The Cambridge Planetary Handbook.

Society of Astronomers Declare Pluto a Planet

However, it is not the public opinion that determines whether a celestial body is a planet or not. That responsibility rests with a scientific body known as the International Astronomical Union (IAU), the world's preeminent society of astronomers. In January of 1999 the IAU issued a press release entitled "The Status of Pluto: A Clarification." In that document the IAU stated, "No proposal to change the status of Pluto as the ninth planet in the solar system has been made by any Division, Commission or Working Group." The IAU stated that one of its working groups had been considering a possible numbering system for a number of smaller objects discovered in the outer solar system "with orbits and possibly other properties similar to those of Pluto." Part of the debate involved assigning Pluto an identification number as part of a "technical catalogue or list of such Trans-Neptunian Objects." However, the press release went on to say that "The Small Bodies Names Committee has, however, decided against assigning any Minor Planet number to Pluto."

Notwithstanding that decision, in year 2000 the Rose Center for Earth and Science at New York City's American Museum of Natural History put up an exhibit of the solar system leaving out Pluto. That action received press coverage and re-ignited the controversy. Alan Stern, director of the Southwest Research Institute's space studies department in Boulder, Colorado, criticized the museum's unilateral decision, stating, "They are a minority viewpoint. The astronomical community has settled this issue. There is no issue."

Still, the argument continues, occasionally appearing in journal articles and in the popular press. However, for every argument against Pluto's designation as a major planet, there seems to be rational counter arguments for retaining that designation. Scientists who argue against Pluto being a major planet stress Pluto's differences from the other eight planets—the four inner planets, Mercury, Venus, Earth, and Mars, and the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Supporters of Pluto as a major planet believe such arguments are fallacious because the two groups of planets could, as the Lowell Observatory put it in 1999, "scarcely be more different themselves." For instance, Jupiter, Saturn, Uranus, and Neptune have rings. Mercury, Venus, Earth, Mars, and Pluto do not. Mercury has an axial tilt of zero degrees and has no atmosphere. Its nearest neighbor, Venus, has a carbon dioxide atmosphere and an axial tilt of 177 degrees. Pluto has an axial tilt between those two extremes—120 degrees. (Earth's tilt is 23 degrees.)

Size Doesn't Matter

A main argument against Pluto being a major planet is its size. Pluto is one-half the size of Mercury, the next smallest planet in our solar system. In fact, Pluto is even smaller than the seven moons in our planetary system.

"So what?" is the response of Pluto's defenders. They point out that size is an arbitrary criterion for determining the status of orbiting bodies. Mercury, for instance, is less than one-half the size of Mars, and Mars is only about one-half the size of Earth or Venus. Earth and Venus are only about one-seventh the size of Jupiter. From the standpoint of giant Jupiter, should the midget worlds of Mercury, Venus, Mars, and Earth be considered planets?

The most commonly accepted definition of a planet, according to University of Arizona educator John A. Stansberry, is that a planet is "a spherical, natural object which orbits a star and does not generate heat by nuclear fusion." For an object in space to maintain a spherical shape it has to be large enough to be pulled into that shape by its own gravity. According to that definition, "Pluto is clearly a planet," concludes Stansberry.

Detractors have also pointed out that Pluto's highly eccentric orbit has more in common with comets that originate from the Kuiper Belt than with the other eight planets in our solar system. That's true, reply Pluto's supporters, but just because Pluto has an eccentric orbit doesn't mean that it isn't a planet. Besides, Pluto's elongated orbit is only slightly more "eccentric" than Mercury's.

Another argument against Pluto being a planet is that it has an icy composition similar to the comets and other orbital bodies in the Kuiper Belt. Supporters of Pluto's planetary status argue that planets are already categorized into two unlike groups: The inner planets—Mercury, Venus, Earth, and Mars—which are composed of metals and rock, and the outer planets—Jupiter, Saturn, Uranus, and Neptune—which are, essentially, giant gaseous planets. Why couldn't there be three kinds of planets: terrestrial, giant gas planets, and icy rocks? Pluto may simply be the first planet in a new category.

Pluto: The Dividing Line

Apparently Pluto is the largest of the 1,000-plus icy objects that have been discovered in the Kuiper Belt region of space. Since Pluto is the largest, that provides further support for Pluto retaining its "major planet" designation. There has to be a dividing line between "major" and "minor" planets, or as some call them, planetesimals. As Descartes reminds us, "No one of the sciences is ever other than the outcome of human discernment." In this context the quote reminds us that the distinctions we make between objects in orbit around the Sun are determined by our human decision. Planetary scientist Larry A. Lebofsky proposes that the IAU simply define Pluto's diameter of 1,413 miles (2,273 km) (almost half that of Mercury, the next smallest planet) as the smallest acceptable size for a major planet. It's an elegant solution to an argument that is fueled by differences in definitions. Let Pluto be the dividing line. In an age when we look forward to the development of technology that will soon allow us to spot orbiting bodies around other stars, it is a solution that would enable us to adopt a planetary classification scheme that would, as Stansberry puts it, provide "consistent and rational answers whether applied to our solar system, or to any other star system."

—MAURY M. BREECHER

Viewpoint: No, Pluto has more in common with the comets and asteroids discovered in the Kuiper Belt during the 1990s.

Overview

In the year 2000 when Hayden Planetarium at New York City's American Museum of Natural History opened its shiny new Rose Center for Earth and Science, the astronomy hall was not the only new phenomenon. Its planetary display depicted a new view of the solar system: Mercury, Venus, Earth, and Mars—the rocky planets—were grouped together as the "terrestrials"; Jupiter, Saturn, Uranus, and Neptune as the "gas giants"; and Pluto … well, where was Pluto? Pluto was assigned to the Kuiper Belt, or perhaps more accurately, the Trans-Neptunian Belt, which the museum simplistically described as a disk of small, icy worlds beyond Neptune.

Pluto is now generally accepted among astronomers as being one of the three-to-four hundred objects discovered in the Kuiper Belt during the final decade of the 1900s. These objects are commonly known as Trans-Neptunian Objects (TNOs). Although Pluto's composition is unknown, its density indicates it is a mixture of ice and rock, as is the case with other TNOs. All the rocky planets are closest to the Sun, and those furthest from the Sun are all gaseous—except Pluto, which lies furthest of all planets from the Sun. How can this be if it is truly a planet? In light of this, can Pluto still be called a planet?

Brian Marsden, associate director for Planetary Sciences at the Harvard-Smithsonian Center for Astrophysics and director of the International Astronomical Union (IAU), the worldwide hub for recording sightings of comets and asteroids (astronomical bodies given minor planet designation), believes Pluto is a comet. In a radio interview with Robyn Williams entitled "Comets versus Planets," Marsden describes how astronomers suspect TNOs are dragged out of the Kuiper Belt (named for Gerard Kuiper who first suggested it as the source of short-period comets) when they pass close to Neptune. Neptune, in turn, may drag them somewhere else where they become influenced by the forces of other planets. Some TNOs get thrown further toward Earth to become short-period comets, circling the Sun approximately every six years. Others get thrown far out into the solar system into the region of the Oort Cloud where passing stars throw them back at us. These are the long-period comets, which often display spectacular tails when their icy component vaporizes as they approach the Sun. Others are proto comets—TNOs that remain in orbit in the solar system but have an unstable orbit. "Pluto being the biggest comet, if you like … Imagine bringing Pluto in to the distance of the Earth from the Sun. That would be a comet of the millennium as all the ice vaporized and made a great long tail," explains Marsden.

The Question Arises

Although its planetary status has been questioned by some since its discovery in 1930, Pluto remained relatively secure in its ninth-row seat until 1992. It was then that David Jewitt of the University of Hawaii, and Jane X. Luu of Harvard, discovered "a curious object called '1992 QB1'," writes Tony Phillips in "Much Ado about Pluto." This small, icy body, Phillips explains, is about the size of an asteroid, orbiting 1.5 times further away from the Sun than Neptune, and was the first indication there might be "more than just Pluto in the distant reaches of the solar system." Phillips notes that, apart from its comparatively large size, Pluto is almost indistinguishable from other Kuiper Belt objects (KBOs) and even the short-period comets. What does distinguish Pluto from other KBOs, however, is its luminosity—a 60% higher albedo (reflective power) than anticipated for other KBOs. This phenomenon is attributed to Pluto's mass and gravitational quality, both of which are significant enough for the planet to "retain a tenuous atmosphere, from which bright frosts may be deposited on the surface," says Jewitt.

Utilizing an 8,192 x 8,192 pixel CCD camera, and experimenting with an even larger 12,000 x 8,000 pixel camera, Jewitt and colleagues have discovered KBOs one-third the diameter of Pluto—all in just the tiny portion of the sky (50 sq. degrees) examined thus far from their vantage point at the University of Hawaii. In 1996 they discovered TO66, calculated to be 497 miles (800 km) in diameter. "It would be incredible in its own right if Pluto proved to be the only 1,250-mile (2,011-km) object. I think we'll have Pluto II, Pluto III … within a few years," Jewitt told Phillips. Jewitt, speculating on the possibility of discovering KBOs larger than Pluto, wonders what happens then to Pluto's planetary designation.

Defining a Planet

Also complicating matters is the scientific definition of a planet: There is no formal one, and therefore the question of Pluto's planetary status, like the above definition of the Trans-Neptunian Belt, also may be simplistic. In his article entitled "Is Pluto a Giant Comet?" Daniel W.E. Green, associate director of the IAU's Central Bureau for Astronomical Telegrams, writes: "The issue was/is over 'major planet' status, not 'planet' status. Indeed, with apparently nonstellar companions being discovered at an increasing rate around other Milky Way stars, the issue about how to define the word 'planet' is becoming more complex, and it is obvious that the word planet needs, in almost all cases, to have accompanying qualifier words ("major," "minor," "principal," etc.) for usage of the word 'planet' to make much sense in any given context."

"It's very difficult to come up with a physically meaningful definition under which we'd have nine planets," said Hal Levison, astronomer at the Southwest Research Institute in Boulder, Colorado, to David Freedman for his article entitled "When is a Planet not a Planet?" Freedman writes: "One generally accepted [definition] is a 'non-moon, sun-orbiting body large enough to have gravitationally swept out almost everything else near its orbit.' Among the nine planets, Pluto alone fails this test… . The second is a 'non-moon, sun-orbiting body large enough to have gravitationally pulled itself into a roughly spherical shape.' Pluto passes this test—but so do Ceres, a half dozen or so other asteroids, and possibly some other members of the Kuiper Belt." Also, the latter definition can be applied to stars, which are self-luminous—something a planet, by definition, is not.

Ceres, the small, rocky body discovered between the orbits of Mars and Jupiter in 1801, was declared a planet shortly thereafter. Another similar body, Pallas, discovered in the same orbital zone a year later, was also designated a planet. However, when these two discoveries were followed by numerous similar findings, their planetary status was changed to asteroid, even though they were certainly the largest members of what is now known as the Asteroid Belt. Similarly, Pluto is undoubtedly the largest member discovered in the Kuiper Belt thus far. However, it is just one of 60, and possibly hundreds of thousands, of comet-like objects in the fascinating Belt that extends far beyond our planetary boundary. "If you're going to call Pluto a planet, there is no reason why you cannot call Ceres a planet," Freeman quotes Marsden as saying.

Also relating to the size issue is how small can a planet be and still be called a planet? Some astronomers say anything larger than 620 miles (998 km) in diameter. This would put Pluto "in" and Ceres (at 580 miles, or 933 km) "out." However, is this arbitrary designation because Pluto was "in" long before anyone knew its true size? When first discovered, scientists believed it was about the size of Earth. By 1960 that estimate was adjusted to about half the diameter of Earth. By 1970, when its moon Charon was discovered and scientists realized the object they were seeing was not one but two, Pluto's size diminished again to one-sixth that of Earth. By this time it had become entrenched in literature and history as planet number nine, yet had diminished in size to less than half that of Mercury and smaller even than the seven satellites in our solar system commonly called moons. These include the Moon, Io, Europa, Ganymede, Callisto, Titan, and Triton. In his article "Pluto is Falling from Status as a Distant Planet," Rick Hampson quotes Michael Shara, curator of the American Museum of Natural History, as saying: "If Pluto is a planet then so is Earth's moon and hundreds of other hunks of debris floating around the Sun." Complicating the size issue is the fact that Pluto's moon is proportionately larger to Pluto than any other moon is to its respective planet. In fact, many astronomers who still consider Pluto a planet say it and Charon are, in fact, a double planet.

And what of Pluto's eccentric orbit—highly elliptical, tilted 17 degrees from that of the eight planets, and orbiting the Sun twice for every three times Neptune does? "About a third of them [TNOs] have Pluto-like orbits, and all of them appear to be, like Pluto, amalgams of ice and rock," writes Freedman. "Also, Pluto actually crosses Neptune's orbit, swapping places with Neptune every 248 years to spend 20 years as the eighth instead of the ninth planet. But does that mean it should not be considered a planet?"

"If Pluto were discovered today," writes Green, "it would be handled by the IAU's Minor Planet Center and given a minor-planet designation, as has happened for the hundreds of other TNOs discovered since 1992." Echoing this belief, Levison says in Kelly Beaty's article "Pluto Reconsidered," "I firmly believe that if Pluto were discovered today, we wouldn't be calling it a planet."

Changing History

So, can a planet's designation be changed? Let's return to Ceres and Pallas, whose discoveries were followed by two more "planets" in 1804 and 1807. Because they floated about between Mars and Jupiter, these four bodies were designated as the fifth, sixth, seventh, and eighth planets, with Jupiter, Saturn, and Uranus moved to ninth, tenth, and eleventh positions. When, in the late 1800s, many more such bodies were discovered occupying the same area of space, astronomers reclassified the four newcomers as minor planets, or asteroids. This included Ceres, almost twice the size of the next largest asteroid.

More than 100 years later and roughly 50 years after the designation of Pluto as a major planet, discoveries of TNOs continue. While most astronomers agree Pluto is a TNO—albeit the largest one discovered thus far—the IAU officially announced in 1999 they had no intention of changing Pluto's planetary status. A formal poll taken at the 2000 General Assembly of the IAU and published in Northern Lights found that only 14% of astronomers from around the world now consider Pluto a major planet, 24% consider it a TNO and not a major planet, and 63% give it "dual status" as both planet and TNO. Green points out that many years went by before astronomers finally accepted Copernicus's heliocentric concept of the universe, and suggests it may be years yet before new astronomers "fully accept the poor logic behind viewing Pluto as the ninth major planet."

While the IAU sticks with tradition, the undeniable facts surrounding the dispute are leading others in a different direction. Green's article includes a small list of the plethora of scientific publications arguing against Pluto's status as a major planet. Freedman writes in his article, "Pluto doesn't need any official ruling to move into minor planethood. It could happen on a de facto basis, and it probably will… . Some of the newest astronomy textbooks, in fact, are already openly questioning Pluto's status."

Marsden believes that keeping Pluto as a planet misleads the public, and particularly school children, by presenting an archaic perspective of the solar system "that neatly ends in a ninth planet, rather than trailing off beyond Neptune into a far-reaching and richly populated field of objects."

In a brief article entitled "Pluto and the Pluto-Kuiper Express," Jewitt writes: "Bluntly put, one has two choices. One can either regard Pluto as the smallest, most peculiar planet moving on the most eccentric and most inclined orbit of any of the planets, or one can accept that Pluto is the largest known, but otherwise completely typical, Kuiper Belt object. The choice you make is up to you, but from the point of view of trying to understand the origin and significance of Pluto it clearly makes sense to take the second opinion… . Our perception of Pluto has been transformed from a singularly freakish and unexplained anomaly of the outer solar system to the leader of a rich and interesting family of Trans-Neptunian bodies whose study will tell us a great deal about the origin of the solar system. So, we have discovered-1 planets and +1 Kuiper Belt. It seems a fair trade to me."

—MARIE L. THOMPSON

KEY TERMS

ASTEROIDS:

Rocky, inert, irregularly shaped, airless objects found in great abundance in Earth's solar system. Most are located in an "Asteroid Belt" located between Mars and Jupiter.

COMETS:

Very small, but often spectacular, visitors that create amazing celestial shows when they enter the inner solar system and come close to the Sun. The heat from the Sun releases water from their surfaces. The water carries some of the comet substances, dust or dirt, along with it and creates huge tails and streamers in the sky.

KUIPER BELT:

A disk-shaped region past the orbit of Neptune roughly 30 to 100 AU from the Sun containing many small icy bodies called Trans-Neptunian Objects (TNOs), and considered to be the source of the short-period comets. (AU—astronomical unit: 1 AU is the average distance of Earth from the Sun.)