International Comet Quarterly

What is improper about the term "Kuiper belt"?

(or, Why name a thing after a man who didn't believe its existence?)

The second "trans-Neptunian" object (TNO) orbiting the sun to be discovered (the first being Pluto, in 1930) was found at Mauna Kea in 1992 -- ushering in a new era of solar-system studies, forever altering our view of the outer solar system, and putting Pluto finally into proper perspective. By "trans-Neptunian", we here mean objects with most or all of their orbits outside that of Neptune and whose eccentricities are generally well below that of typical comet eccentricities; a more formal (and technical) definition of TNO might be the following: an object that orbits the sun and either (a) has a perihelion distance > 30 AU (the mean heliocentric distance of Neptune), or (b) has a semi-major axis > 30 AU but is in a short-order resonance with respect to (and exterior to) Neptune. Soon after additional TNOs were found in the following months, some astronomers started circulating the term "Kuiper belt" (a term unknown until the 1990s) for these objects. Had they done a little more research, they probably would not have chosen the term "Kuiper belt", and it may be time to now stop using this term.

Some astronomers were much too hasty in calling the new trans-Neptunian objects "Kuiper-belt" objects, both because that's too simple a term for dynamically-different objects in that region and because Kuiper was the wrong person to recognize for reasons cited below. Kuiper did briefly speculate on the possibility of "one or more small planets, like Ceres" beyond heliocentric distance r = 38 AU, and on "remnants" of a circular ring of comets being still left beyond r = 50 AU. However, Kuiper presumed that there was no longer any significant population of small objects between r = 30 and 38 AU, and he remarked that it would be "puzzling" if there "were asteroidal bodies" today between 30 and 50 AU [Kuiper 1951, Astrophysics: A Topical Symposium (New York: McGraw-Hill Book Co.), p. 402)! (There are now hundreds of known "asteroidal bodies" orbiting the sun between 30 and 50 AU.) Harvard astronomers Fred Whipple and Al Cameron, meanwhile, conjectured separately in the 1960s that even today "there must be a tremendous mass of small solid material" between r = 40 and 50 AU, and Whipple illustrated his hypothesis by showing some members of the "comet ring" as close as r = 30-35 AU. Whipple stated clearly in a 1972 paper that his thinking on this was spawned independent of the work by Cameron and by Kuiper, and Whipple pleaded with astronomers to not forget that there might be now a detectable belt of objects beyond Neptune. Whipple's concept may have germinated as early as the 1930s, when his undergraduate mentor at UCLA proposed just such a group of object beyond Neptune (see Leonard 1930, Leaflet Astron. Soc. Pacific No. 30, pp. 121-124), when Frederick C. Leonard questioned, "Is it not likely that in Pluto there has come to light the *first* of a *series* of ultra-Neptunian bodies, the remaining members of which still await discovery but which are destined eventually to be detected?" [Later in Kuiper's career, he may have been heavily influenced by the work of Cameron and especially that of Whipple, whereby he was more vague about the "comet zone" around heliocentric distances 35 to 60+ AU in terms of the current situation -- but Kuiper never came out and claimed emphatically that there must now be many objects orbiting the sun in the tens of AU just beyond Neptune's orbit, as Whipple and Cameron very much did, and as Leonard and Kenneth Edgeworth had done years before Kuiper published anything on cosmogony.]

Regarding the early prediction of (what we now call) cubewano objects, besides Leonard (1930), see K. E. Edgeworth 1943, J.B.A.A. 53, 186; A. G. W. Cameron 1962, Icarus 1, 13; F. L. Whipple 1964, Proc. National Acad. Sci. 51, 711; Whipple 1964, A.J. 69, 563; Whipple 1972, in The Motion, Evolution of Orbits, and Origin of Comets (ed. G. A. Chebotarev et al.), p. 401; E. I. Kazimirchack-Polonskaya (1972, ibid., p. 252); and Cameron 1978, in The Origin of the Solar System (ed. by S. F. Dermott; Chichester: John Wiley and Sons), pp. 61ff. Writing a second time on the topic, Edgeworth (1949, MNRAS 109, 609) wrote: "It is not unreasonable to suppose that this outer region [just beyond the orbit of Neptune] is now occupied by a large number of comparatively small clusters [condensations], and that it is in fact a vast reservoir of potential comets". Note again that -- whereas Kuiper mainly spoke of this region as being a region of comet formation and existence early in the solar system (where the objects would have been largely depleted over billions of years by the planets, so that there should not be much of anything there today) -- the other authors (cited above) all spoke of such a belt as currently being likely to still include a great many comets --- which is relevant to the discussion of Pluto being the first known apparent member of this trans-Neptunian comet family. [The reference to Leonard was noted in 2000 by B. G. Marsden (after evidently being long forgotten), and Leonard's comments were perhaps the most prescient of the 1990s discoveries other than Whipple's many comments over decades in the literature.]

The over-crediting of Kuiper evidently started with a much-cited 1980 paper by Julio Fernandez (MNRAS 192, 481), in which Fernandez unfortunately appears not to have done very extensive searching of the literature on the matter (though he did more than most others that followed him); to his credit, Fernandez also adopts the term TNO in his recent book as being preferable to "Kuiper-belt object", generously acknowledging that he "accepts part of the blame for this unjust situation since [I] overlooked Edgeworth's work in [my] 1980 paper" [Fernandez 2005, Comets: Nature, Dynamics, Origin, and Their Cosmogonical Relevance (Dordrecht, The Netherlands: Springer), pp. 198-199]. In fact, many others (e.g., Duncan et al. 1987, A.J. 94, 1330; Torbett 1989, A.J. 98, 1477; Jewitt and Luu 1993, Nature 362, 730) writing on the topic of a possible belt of comets beyond Neptune during the 1980s and early 1990s followed suit -- merely citing a single 1951 reference of Gerard Kuiper's that only briefly touches on objects beyond Neptune in a lengthy chapter discussing all aspects of solar-system evolution (and that does not state that there should be many small objects now situated between 30 and 50 AU!). The flood gates apparently opened in earnest due to the paper by Duncan et al. in 1987; prior to that, authors actually looked at the earlier papers and realized that the proper credit for saying that comets may still, now be orbiting the sun in a region just beyond Neptune should, in fact, be given to Cameron (1962) and Whipple (1964, 1972), as was done properly by, e.g., Hamid et al. (1968, A.J. 73, 727), Marsden (1974, Ann. Rev. Astron. Astrophys. 12, 1), Eneev (1980, Sov. Astron. Let. 6, 163), and Bailey (1983, MNRAS 205, 47P).

The real problem was that Fernandez (and, thus, the authors following Fernandez's lead without checking to see what Kuiper actually wrote) did not look beyond this one early paper that Kuiper had written on the speculations about the origins of the solar system for a general book [Astrophysics: A Topical Symposium, p. 400; noted above], in which Kuiper said nothing that would warrant his name being attached to a group of objects beyond Neptune. Indeed, Kuiper himself, in several later publications, stated that his comments on objects beyond Neptune were preliminary (though his revisions were both slight and vague). There was never any detailed expansion by Kuiper of the idea of small objects beyond Neptune -- indeed even less than was made by other authors -- and those later papers by Kuiper (e.g., 1951, Proc. Nat. Acad. Sci. U.S.A. 37, 13; 1953, ibid., 39, 1156; 1956, JRASC 50, 115; 1974, Cel. Mech. 9, 324 and 346) all reinforce the notion that he was not in synch with numerous other pre-1992 authors on this topic (who all differed with Kuiper in their speculation that such objects were likely to now be populating that region beyond Neptune; Kuiper believed that any comets or small planets that formed in the region 30-50 AU in the early history of the solar system would have been dispersed, ejected, or dissolved by Pluto so that nothing substantial should be there today).

It seems that many authors typically do not do their historical homework, preferring to rely on the homework of those authors that they cite; such practice leads to a propagation of errors, sometimes where even the citation is in typographical error (cf. Ball 2002, Nature 420, 594)! The tell-tale sign of this blind "following suit" by recent authors is that Kuiper published a few other papers in which he made brief mention of possible objects beyond Neptune, but nobody cites Kuiper's other work pertaining to the topic, showing that they do not understand the situation regarding Kuiper's stance. In fact, it is remarkable that, of all the numerous pre-1992 authors mentioning the possibility of objects in low-eccentricity orbits just beyond the orbit of Neptune, Kuiper appears to have been absolutely alone in predicting that there should NOT NOW be objects there! So why name a "belt" after a man who actually believed it should not be there?!

That Kuiper's thoughts on a region of small bodies beyond Neptune were very minor parts of Kuiper's work is reflected in the lack of citing Kuiper on this topic by most other pre-1987 authors, such as T. Yamamoto (1984, A.Ap. 142, 31), Fred Whipple, and Al Cameron. [When Kuiper was cited on this topic prior to 1987 (e.g., Biermann and Michel 1978, Moon and Planets 18, 447), it was in reference to his placing comets as having formed originally at or beyond Neptune, without comment on Kuiper's stance on the evolution of the placement of those original comets.] As another example, notably absent in the transcribed discussion at a key 1952 meeting on the topic of origins of small objects in the outer solar system, involving most of the major contemporary players (including Kuiper), is any reference to Kuiper's work on the topic [cf. La Physique des Cometes 1953, Extrait des Memories in-8o de la Societe Royale des Sciences de Liege Quatrieme Serie, Tome XIII, Fasc. I-II (Louvain: Imprimerie Ceuterick), pp. 386ff]. Furthermore, none of his obituary writers -- mostly contemporary planetary scientists who should have known the situation well -- made any mention to Kuiper speculating on objects in a region just beyond Neptune, despite including lengthy lists of notable accomplishments by Kuiper in his planetary-science career [see Cruikshank 1974, Sky Telesc. 47, 159; Owen and Sagan 1974, Mercury 3(2-3), 16; Whitaker 1974, Physics Today 27(3), 85; Sagan 1974, Icarus 22, 117; Anonymous 1974, Nature 248, 539]; indeed, though numerous of Kuiper's significant publications were cited in the obituaries, there is no mention of Kuiper's chapter in Hynek's book that is so highly cited by "Kuiper-belt" proponents today. The "Kuiper-belt" proponents are attempting to re-write history without looking at the facts, and even one historian has fallen into this trap [Doel 1996, Solar System Astronomy in America, Cambridge Univ. Press], again because he did not look into all of Kuiper's writings on the topic (Doel simply accepted that many have taken the term "Kuiper belt" as proper, without looking into the matter).

The fact that many astronomers, soon after the discovery of Pluto, were considering its probably small mass and its non-major-planet-like orbit as indications that this was an unusual object akin to comets or minor planets shows that this was a logical way of thinking (and it is logical to assume that numerous astronomers may have independently thought that there might well be a belt of asteroidal or comet-like objects beyond Neptune, of which Pluto was only one member). Astronomers certainly realized that "major planet" categorization did not make complete sense, and this sensibility evidently remained through the 1930s, 1940s, and 1950s -- as one astronomer after another made brief comments inferring the meaning of Pluto in terms of a family of objects beyond Neptune. So it may be unwise to call these objects anything other than "trans-Neptunian objects", or "cubewanos", "plutinos", and "scattered-disk objects". If astronomers' names are attached to the population, the names of Leonard and Whipple likely would be most historically correct and proper ("Leonard-Whipple belt"), but perhaps we could settle with something catchy like the "LEdgeKWhip belt(s)", to "honor" Edgeworth and Kuiper as well as Leonard and Whipple. Given that historians have found Kuiper to have likely plagiarized the work of others (cf. Doel 1996, op.cit., pp. 142ff), including on this very topic (by not acknowledging Edgeworth's two prominent articles in the 1940s and the ideas of Leonard; cf. Green 1999, ICQ 21, 45), it is especially improper to give Kuiper the only credit through the ill-conceived name "Kuiper Belt". [Kuiper has numerous other things named in his honor (including craters on the moon, Mars, and Mercury; a research aircraft; a minor planet; etc.), so he hardly "needs" something named for him that is actually inappropriate, in that it supposedly recognizes something that he didn't claim exists, while others -- who go largely unrecognized -- did strongly believe to exist.]

Unfortunately, some people strangely also refer to *all* TNOs as "Kuiper-belt objects" -- even those outside the region that might be truly called a "belt" with semi-major axes in the range 37-50 AU. The "scattered" objects, some of which are entirely beyond Neptune's orbit and others of which cross Neptune's orbit, are mostly thought to be cousins of the centaurs -- the latter having simply been perturbed closer toward the sun over time -- but some of the scattered TNOs with very large aphelia or semi-major axes may be in their current orbits due to a ancient assist from a passing star. At any rate, to say that the scattered TNOs are part of a belt is nonsense. If speaking of a belt, the proper thing is to include only the cubewanos but perhaps also the Neptune-resonant objects with semi-major axes > 30 AU (including the plutinos). Also, the term "transneptunian object" was recommended over "Kuiper-belt object" or "Edgeworth-Kuiper object" by the Scientific Organizing Committee of the ESO Workshop on "Minor Bodies in the Outer Solar System" (1998 Nov. 2-5, Garching, Germany), with it noted that the term TNO is less controversial than KBO or EKB [see Davies 2000, Minor Bodies in the Outer Solar System (Berlin: Springer), p. 9].

Additional remarks on this topic (in more detail, with more references) are published in the January 1999 issue of the ICQ (pages 44-46) and the July 2004 issue (pages 117-118).

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