Well, perhaps not. But the possibility sure does exist. And the possibility has been discussed for decades (for example, see Fred Whipple's 1964 paper in Proc. National Acad. Sci. 52, 583). Well, then, is Pluto a planet? Yes, it appears to be a planet. But it is reasonable to qualify that by saying that Pluto is not a major planet because it is simply too small to be a major planet (though the pride of many American astronomers causes them to continue to fight politically for Pluto to be called "one of the nine major planets"). It is a planet of some sort, and many planetary scientists in the last couple of decades of the 20th century began speaking of Pluto as a "planetesimal" -- and no longer as a "major planet" -- in the astronomical literature (note the several dozen authors in the documented list below). There is no need to have an "edict" saying that Pluto is not the ninth major planet, as astronomy doesn't really work that way, and the field is gradually but steadily moving toward an eventual universal acceptance of a new picture of the solar system that places Pluto in a different (but still significant) light. And, yes, it might be that we will want to consider placing Pluto in different categories, such as (minor) planet and comet. Such "dual status" already exists for some comets and minor planets, which are given formal numbers and names in both kinds of catalogues.
In recent years, some ill-informed discussion in the print and electronic news media has centered around the question of whether Pluto is a planet or not; such discussions about Pluto lost sight of the fact that the issue was/is over "major planet" status, not "planet" status. Indeed, with apparently non-stellar 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. But do large committees need to be created to determine a "definition" of the word "planet"? Probably not. An easy definition of a planet is an interplanetary body orbiting the sun (or located anywhere in the universe, perhaps) that is larger than a meteoroid (which are small interplanetary rocks with a size of, say, 10 meters and smaller) but smaller than a star or proto-star (or brown dwarf -- perhaps 0.01 to 0.1 solar mass). This webpage was constructed as a result of popular demand, purely to correct a lot of misinformation regarding Pluto; for example, in the mid-1990s, a few planetary astronomers still seemed unaware that dozens of their colleagues had ceased calling Pluto a "major planet" in the astronomical literature (see the documented list below).
Simply because Pluto has been shown to not be a major planet does not in any way "demote" the object; astronomy does not work that way -- things are often re-classified or discussed in different ways in the light of new knowledge (and "demotion" is simply a political, not realistic, word used by Pluto-is-ninth-planet proponents in the United States). Pluto is a very significant and interesting object, with many clues about the origin and evolution of our solar system, and it would be wonderful to have a spacecraft mission to this trans-Neptunian object to learn much more about it. The minor objects of the solar system are just as significant in many ways, and more significant in other ways, as/than are the major planets!
But the real problem is saying that "there are nine planets" in the solar system, as if this has any real meaning. As Hal Levison has said [in D. H. Freedman's 1998 article "When is a Planet Not a Planet?", Atlantic Monthly 281(2), 22], "it's very difficult to come up with a physically meaningful definition under which we'd have nine planets", because at least a half-dozen main-belt asteroids (along with several recently discovered transneptunian objects) are large, spherical objects that would also satisfy definitions of "major planethood" if sphericity is the criterion. Teachers will be doing their students a big service now to stop talking about total "numbers of planets" in the solar system as anything definitive, and rather discuss the solar system as a complex region with the following key components of both large and small objects (note that 1 astronomical unit = 1 AU is close to the mean earth-sun distance of about 93 million miles or 150 million km):
Note that when (1) Ceres was first discovered in 1801, it was presumed to be a "regular" planet (after it was initially considered as a comet, that is; Uranus had also been thought to be a comet when it was first discovered, because two centuries ago the concept of new "planets" was novel). This presumption held true also for the next three main-belt asteroids (which were found in 1802, 1804, and 1807): they were all considered new planets of the solar system, and were counted as the 8th, 9th, 10th, and 11th planets (or 5th, 6th, 7th, and 8th -- with Jupiter, Saturn, and Uranus moved to the 9th, 10th, and 11th spots). The fifth asteroid was not found until late 1845, and the sixth not until mid-1847, and astronomy publications and textbooks for nearly half a century referred to "eleven primary planets" of the solar system [e.g., P. A. Hansen (1837), in Jahrbuch fuer 1837, ed. by H. C. Schumacher (Stuttgart and Tuebingen: J. G. Cotta'schen Buchhandlung), p. 83; D. Olmsted (1847), An Introduction to Astronomy; Designed as a Text Book for the Students of Yale College (New York: Collins and Brother), p. 174; J. H. Wilkins (1833), Elements of Astronomy (Boston: Hilliard, Gray, Little, and Wilkins), p. 4], until a rush of asteroid discoveries in the late 1840s and 1850s encouraged astronomers to refer to them as "minor planets" or "asteroids" instead of "primary planets". Olmsted had the four newest planets as numbers 5, 6, 7, and 8, and Jupiter was the ninth planet. (Note that the other seven primary planets were Mercury-Uranus, until Neptune was discovered in 1846.) The eminent British astronomer Sir John Herschel wrote a series of famous textbooks on astronomy in the first half of the 19th century; he, too, counted 11 planets, with Jupiter as the ninth, and referred to Ceres, Juno, Pallas, and Vesta as "ultra-zodiacal planets" in his 1833 edition of A Treatise on Astronomy.
So the business of "ninth planet" being assigned to Pluto has historical discrepancies, as well. Many of the "Pluto-as-ninth-planet" proponents today claim that textbooks and "history" should not be altered in the case of Pluto, but these same individuals seem unfamiliar with astronomical history prior to 1930; in fact, prior to Copernicus, there were seven different "planets", and the earth wasn't one of them: the moon, the sun, Mercury, Venus, Mars, Jupiter, and Saturn --- and in ancient times, Apollonius of Myndos said that even comets were planets, according to Seneca (Nat. Ques. VII, 17). [Readers interested in this issue of arbitrary numbers of planets in our solar system can look at the delightful 1990 article by I. Bernard Cohen, "G. D. Cassini and the Number of the Planets" in Nature, Experiment, and the Sciences, ed. by T. H. Levere and W. R. Shea (Dordrecht: Kluwer Academic Publishers).] As an interesting historical aside, in 1844 William Henry Smyth wrote (in A Cycle of Celestial Objects, Vol. 1, p. 153): "Sir William [Herschel's] own definition of asteroids is that they are celestial bodies, moving in orbits the plane of which may be inclined to the ecliptic in any angle; their motion may be direct or retrograde; and they may or may not have considerable atmospheres, very small comas, discs, or nuclei." It was Herschel himself who proposed the term "asteroids" for Ceres, Pallas, Vesta, and Juno, and it was also Sir William (the father of Sir John) who reported Uranus initially as an apparent comet when he first discovered it.
Freedman (op.cit.) notes that the sixth-grade curriculum in his hometown school now critically examines Pluto's planetary status in a valuable way that can really teach pupils and students how science works. As can be ascertained from the list of citations below, one can determine how outdated an astronomy textbook is by whether or not it simply refers to Pluto as the ninth major planet; properly updated textbooks should discuss Pluto as a member of the minor trans-Neptunian objects of the solar system, and note that the erroneous categorizing of Pluto merely as the ninth major planet (championed by Lowell Observatory) upon its discovery in 1930 is slowly but steadily being corrected. Pluto was only called a major planet in 1930 because of a search for a mythical ninth major planet that was supposedly perturbing Neptune and Uranus (due to the lack of sufficient precise positional observations for those giant planets back then). If Pluto were discovered today, it would be handled by the International Astronomical Union's Minor Planet Center and given a minor-planet designation, as has happened for the hundreds of other trans-Neptunian objects discovered since 1992. Basically, Pluto would not be considered a major planet if it were discovered today!
Indeed, it was proposed in 1998 that minor planet number 10000 be assigned to Pluto, so that it would get more frequent orbital analyses and useful astrometric cataloguing (noted in Freedman, op.cit.), but this suggested was shouted down by numerous American astronomers (who likely would not have cared, had Pluto been discovered in Europe, a point not missed by some non-Americans; this underlies the issue that politics and not science is largely responsible for a push to view Pluto as "the ninth planet"). Note that some three dozen other TNOs (not Pluto, yet) have been given permanent "minor-planet" numbers by the Minor Planet Center (as of early 2003). Indeed, in 1947 the Minor Planet Center published astrometric observations of Pluto under the title "minor planets" (MPC 13)! The newly adopted IAU "Terms of Reference" (2000) that define the Minor Planet Center "officially" allow it to handle astrometry and orbital calculations for Pluto, and the Minor Planet Circulars are now situated to publish such data on Pluto as the worldwide refereed publication to do this routinely (there was a need to collect and archive data on Pluto).
If it would not be considered a major planet if discovered today, why should we continue to accept the errant assumptions of the distant past?
Of course, it took many years after Copernicus' publication in 1543 of his heliocentric theory before astronomers were generally inclined to teach that planets move about the sun, not about the earth. And the reasons were similar to the reasons for calling Pluto a major planet in the 1930s: observations that were greatly inferior to the observations that we have today. (Had more precise observations of the planets been available to Copernicus, the problem would have been solved much more quickly; the poor nature of the observations led many to pause on adopting heliocentrism in the 16th century.) So perhaps the politics and social pressures of the 20th century are not that different in overall character from the politics and social pressures of the 16th century, and some more years must pass before new generations of astronomers fully accept the poor logic behind viewing Pluto as the ninth major planet. Even Galileo (who turned astronomy upside down with his astronomical discoveries in the early 17th century using early telescopes) was unable to accept the assertion by Tycho Brahe and Johannes Kepler that comets orbit the sun (Galileo followed Aristotle's assertion that comets are part of our earth's upper atmosphere, closer to us than the moon). Astronomers are human, too, and many of them have trouble accepting change.
The history of astronomy is full of examples of things assumed to be one way, then shown to be another way, with public acceptance being slow because of comfort in the status quo [examples include the earth revolving about the sun (instead of vice versa); the motions of the planets and comets following physical laws (rather than spelling out "readable" predictions of events on Earth, via astrology); comets as solid bodies (rather than loose collections of meteoroids); stars as distant suns (rather than reflecting dots on the ceiling of the solar system); galaxies as distant island "universes" like our own Milky Way (rather than nebulae in our own galactic universe); etc.]. As Freedman (op.cit.) writes, the chief holdouts who still regard Pluto as "the ninth planet" are those who are afraid of "changing history", for fear that astronomy will look bad; but he adds that gradual and inevitable universal rejection of the concept of nine major planets will arrive over the coming decades as younger astronomers replace the older ones now working in this field. After all, change is an accepted part of astronomy (and all science), and it is understood by even the general public that even major concepts and ideas must sometimes be abandoned to further progress; the editor of one of the world's leading science journals put it this way: "Science is, by its very nature, a process open to the questioning and overthrowing of currently accepted ideas" (2005, Nature 436, 1). Indeed, changes are already filtering into the mindsets of scientists in the field, as seen at the 2000 General Assembly of the International Astronomical Union, where an informal poll of several dozen solar-system astronomers from around the world yielded the following results among voters: those who consider Pluto only as a major planet, 14 percent; those who consider Pluto only as a trans-Neptunian object (TNO) and not as a major planet, 24 percent; those who consider Pluto as having "dual status" [like (2060) = 95P/Chiron, a comet that was given a minor-planet number as a Centaur] as both a planet and a TNO, 63 percent [results published in Northern Lights, 24th IAU General Assembly, No. 7]. Textbooks and encyclopedia articles are being re-written to reflect the changing view of Pluto, and the American Museum of Natural History in New York City now correctly displays only eight major planets (Mercury-Neptune) in our solar system.
Even shortly after discovery, there was some concern that Pluto should not be so readily called a major planet (because of its faintness and implied small size), including published concerns by two prominent astronomy textbook writers. John Charles Duncan (1935, Astronomy: A Text Book, 3rd revised ed.; New York: Harper & Brothers Publ., p. 271) wrote that Pluto "seems to belong in the class of the smaller planets or even of the asteroids"; even years later, in Duncan's 1946 edition of his textbook (p. 287), he retained the same stance. Similarly, Forest Ray Moulton (1931, Astronomy; New York: The MacMillan Co., p. 244) observed: ``Perhaps [Pluto] is the most remote member of the solar family, but more probably it is related to the planetoids . . . (or asteroids)". And R. A. Lyttleton suggested in 1936 that Pluto might be an escaped satellite of Neptune (MNRAS 97, 108). It is interesting that, after their initial press releases, the Lowell Observatory astronomers had some second doubts and announced publicly that they "thought that it may be proved to be a unique asteroid or an extraordinary comet-like object", this in response to an announcement by A. O. Leuschner at Berkeley that the new object could be "a large asteroid . . ., . . . one of many long-period planetary objects yet to be discovered, or a bright cometary object" due to its "high [orbital] eccentricity and small mass" --- the small mass being obvious from the object's apparent brightness and calculated large distance from the earth [New York Times, 1930 April 14, p. 23].
Below is a partial list showing how Pluto is now described/mentioned as not being termed a "major planet" by astronomers in the scientific literature, textbooks, and popular science magazines (note that Chiron is a large comet-like minor planet that crosses the orbit of Saturn, Titan is a large satellite of Saturn, Triton is a large moon of Neptune, and Charon is Pluto's satellite). The list below is only a partial list (hopefully a realistic cross-section) of what has been published recently in terms of Pluto's non-major-planet status; it is provided here by popular demand, in response to both astronomers and non-astronomers who have asked that this information be made available publicly. So, then, we find Pluto now described as follows (this list was completed in 1997-1998 and has not been updated):
0) "Pluto may thus be a cometary accumulation" or "a large comet" -- F. L. Whipple (1964), Proc. Nat. Acad. Sci. 51, 711; ibid. 52, 583. 1) "Now we picture a relatively small, low-density methane snowball... ...demotion to 'minor planet' may be in order. . . . Pluto fits in well with the low-density comets. Is it a supersize comet nucleus?" -- R. S. Harrington and B. J. Harrington (1980), "Pluto: Still an Enigma After 50 Years", Sky and Telescope 59, 452-454. 2) we should "[drop] the apellation 'ninth planet' and [classify] Pluto . . . as an unusual minor planet" -- B. G. Marsden (1980), "Planets and Satellites Galore", Icarus 44, 29. 3) "interplanetary, small bodies: Pluto, Chiron" -- G. W. Wetherill (1980), in EARLY SOLAR SYSTEM PROCESSES AND THE PRESENT SOLAR SYSTEM, Proceedings of the International School of Physics "Enrico Fermi", Course LXXIII (D. Lal, Ed.; Amsterdam: North-Holland Publishing Co.), p. 83. 4) "[Pluto is] certainly not a planet in the accepted sense of the term. It may be asteroidal, and perhaps of the same basic nature as Chiron . . . It may even be the brightest member of a whole swarm of similar bodies in those remote regions of the Solar System." -- P. Moore (1984), "Pluto", J. BRIT. ASTRON. ASSN. 94, 224. 5) "[we] doubt that Pluto deserves the status of a planet. It seems more sensible to categorize it as a stray asteroidal-cometary object." -- J. T. Wasson and M. G. Kivelson (1986), in THE SOLAR SYSTEM: OBSERVATIONS AND INTERPRETATIONS (M. G. Kivelson, Ed.; Englewood Cliffs, NJ: Prentice-Hall), p. 3. 6) Pluto "appears to fall into this category [of] outer solar-system satellites . . . whose surfaces are dominated by frozen volatiles" -- F. P. Fanale (1986), in THE SOLAR SYSTEM: OBSERVATIONS AND INTERPRETATIONS (M. G. Kivelson, Ed.; Englewood Cliffs, NJ: Prentice-Hall), p. 145. 7) "icy planetesimal"; "we do not regard Pluto as a planet, but rather as among the largest of the accreted cometesimals" -- P. R. Weissman (1986), in THE GALAXY AND THE SOLAR SYSTEM (Univ. of Arizona Press), p. 236 8) classed in orbital computations as a "smaller particle (an asteroid, comet, meteoroid, or Pluto)", as opposed to a major planet -- D. I. Olsson-Steel (1988), Astron. Astrophys. 195, 327-328. 9) "large outer Solar System planetesimal" -- W. B. McKinnon and S. Mueller (1988), Nature 335, 240-243. 10) "planetesimals" (Pluto and Charon) -- D. P. Simonelli, J. B. Pollack, C. P. McKay, R. T. Reynolds, and A. L. Summers (1989), Icarus 82, 10. 11) "planetesimal" -- M. E. Bailey, S. V. M. Clube, and W. M. Napier (1990), in THE ORIGIN OF COMETS (Pergamon Press, Oxford), p. 394. 12) "astronomers question whether it deserves the title `planet'. . . Pluto should perhaps be called a 'minor planet' or 'planetesimal'" -- P. Flower (1990), UNDERSTANDING THE UNIVERSE (textbook) (St. Paul: West Publishing Co.), p. 310. 13) "much more like the minor bodies [of the solar system]" -- B. Bertotti and P. Farinella (1990), PHYSICS OF THE EARTH AND THE SOLAR SYSTEM (Dordrecht: Kluwer Academic Publ.), p. 275. 14) ". . . Pluto belongs neither to the inner planets [nor] to the giant planets [in classification], but to the icy satellites of the latter. . . . Pluto may belong to a family of icy minor planets." -- T. Encrenaz and J.-P. Bibring (1990), in THE SOLAR SYSTEM (Springer Verlag), transl. by S. Dunlop, p. 232. 15) "large planetesimal" with inference of relationship to the Whipple belt of comets beyond Neptune -- J. A. Fernandez and W.-H. Ip (1991), in COMETS IN THE POST-HALLEY ERA (Kluwer Academic Publishers), 1, 520. 16) "large outer solar system planetesimals" (Pluto and Triton, both) -- S. R. Taylor (1992), in SOLAR SYSTEM EVOLUTION: A NEW PERSPECTIVE (Cambridge University Press), p. 264. 17) "Pluto, Triton and Titan: three small bodies with atmospheres" -- T. C. Owen (1992), in OBSERVATIONS AND PHYSICAL PROPERTIES OF SMALL SOLAR SYSTEM BODIES, ed. by A. Brahic et al. (Universite de Liege: Institut d'Astrophysique), 151. 18) "another example" of a satellite-like "body" like Triton -- M. Podolak, W. B. Hubbard, and J. B. Pollack (1993), in PROTOSTARS AND PLANETS III (Univ. of Arizona Press), p. 1121. 19) "Pluto-Charon . . . and the cometary nuclei apparently represent samples of those ice-dominated planetesimals" -- M. J. Gaffey, T. H. Burbine, and R. P. Binzel, Meteoritics 28, 163. 20) "icy planetesimals" (Pluto and Triton, both) -- T. Owen (1994), Astrophys. Space Sci. 212, 5. 21) not a major planet -- D. W. E. Green, J. E. Chambers, and G. V. Williams (1994), "Focal Point", Sky Telesc. 88(2), 6-8. 22) Pluto and Charon are "often described as the largest icy planetesimals to have grown (and still be preserved) in the Kuiper belt" -- P. R. Weissman (1995), Ann. Rev. Astron. Astrophys. 33, 330. ["Kuiper belt" = region of transneptunian objects] 23) "an especially large planetesimal" -- R. A. Kerr (1995), Science 268, 1704. 24) "Pluto-Charon are best viewed as the largest end-members of the Kuiper belt population, rather than as an independent planet-satellite system" and "Pluto proved to be only the largest end-member of a huge population of comets in the trans-Neptunian region, remnant icy planetesimals from the origin of the solar system." -- P. R. Weissman and H. F. Levison (1995), "The Population of the Trans-Neptunian Region", submitted for the book PLUTO-CHARON (Univ. of Arizona Press) ["Kuiper belt" = region of transneptunian objects] 25) "planetary embryos", or "giant comets" (Pluto and Triton) -- W. B. McKinnon, J. I. Lunine, and D. Banfield (1995), in NEPTUNE AND TRITON, ed. by D. P. Cruikshank et al. (Tucson: Univ. of Arizona Press), 846. 26) "two huge icy planetesimals" (Pluto and Charon) -- T. L. Roush, D. P. Cruikshank, J. B. Pollack, E. F. Young, and M. J. Bartholomew (1996), Icarus 119, 214. 27) "Pluto now appears special only because it is larger than any other [known] member of the Kuiper belt. One might even question whether Pluto deserves the status of a full-fledged planet. . . . attempts to find a 10th planet may . . . have succeeded in reducing the final count to eight." -- J. X. Luu and D. C. Jewitt (1996), Scientific American 274(5), 52. ["Kuiper belt" = region of transneptunian objects] 28) "This . . . establishes Pluto as the largest of a hitherto unknown family of dynamically similar [trans-Neptunian] bodies, and effectively changes its status from that of 'smallest planet' to 'largest known Kuiper Belt object." -- D. Jewitt, J. Luu, and J. Chen (1996), "The Mauna Kea-Cerro Tololo (MKCT) Kuiper Belt and Centaur Survey", Astron. J. 112, 1238. ["Kuiper belt" = region of transneptunian objects] 29) numerous Pluto-like "planetesimals . . . may make a significant contribution to the total 'cometary' mass" of the solar system -- M. E. Bailey (1996), Earth, Moon, and Planets 72, 57. 30) "Perhaps Pluto . . . and Chiron and Triton, along with a few other recently discovered objects beyond Neptune . . . [are] planetesimals formed in the solar nebula more than 4 billion years ago . . ." -- R. C. Bless (1996), DISCOVERING THE COSMOS [astronomy textbook] (Sausalito, CA: University Science Books), p. 681. 31) ". . . [Pluto] is most meaningfully considered as the largest of the known Kuiper Belt objects, rather than as an independent . . . planet." -- D. Jewitt and J. Luu (1996), in COMPLETING THE INVENTORY OF THE SOLAR SYSTEM, ASP Conf. Ser. 107, 256. ["Kuiper belt" = region of transneptunian objects] 32) "ice dwarf planets" (Plutos) in the 30-50 AU zone -- S. A. Stern and J. E. Colwell (1997), Astron. J. 114, 848. 33) "planetesimal" -- S. Mueller and W. B. McKinnon (1997), article on Pluto in ENCYCLOPEDIA OF PLANETARY SCIENCES (London: Chapman and Hall), 649.
In addition, one will see that most serious discourses on the evolution of objects in the outer solar system mention crossing the orbit of Neptune, the outermost major planet (Pluto is not usually so mentioned as a reference point, for obvious reasons). See, for example, P. R. Weissman (1991, in COMETS IN THE POST-HALLEY ERA, Vol. 1, p. 477); Fernandez and Ip (1991, reference given above); G. W. Wetherill (1991, in COMETS IN THE POST-HALLEY ERA, Vol. 1, p. 539); M. J. Mumma et al. (1993, in PROTOSTARS AND PLANETS III, p. 1188). Be cautious of the reliability of any text in which the author discusses objects in the solar system "beyond Pluto".
Getting recognition for an inflated categorization for political benefit of an individual or group (as Lowell Observatory did for Pluto in 1930) is not uncommon. Recently, Vermont politicians got Congress to recognize Lake Champlain as one of the "Great Lakes" so that Vermont could benefit from legislation directed at the Great Lakes [Seife 1998, New Scientist 157(2125), 13].
Ian Ridpath [1978, Astronomy 6(12), 6] put it well when he recited Abraham Lincoln's riddle, in which Lincoln asked somebody, 'If you call a tail "a leg", how many legs does a dog have?' The person offered, 'Five?', to which Lincoln replied, 'No, four --- [merely] calling a tail "a leg" does not make it a leg!' To which Ridpath added: 'If you call Pluto "a [major] planet", how many [major] planets does the sun have?'
Index to the CBAT/MPC/ICQ pages.