-------------------------------------------------------------- Planetary taxonomy: An inclusive and multidimensional approach -------------------------------------------------------------- by Margo Schulter ABSTRACT: This article presents an inclusive approach to the classification of both solar and extrasolar planets. A planetary mass object or planemo is any celestial body larger than a meteoroid and less massive than a fusor (i.e. a main sequence star or brown dwarf). Any planemo is classified as macro/micro depending on whether it has sufficient mass for self-gravitationally constrained near-sphericity or hydrostatic equilibrium. Stellar system planets (planemos orbiting fusors) are additionally classified as major/minor based on a test of dynamical dominance or "clearing the neighborhood." The macro/micro and major/minor dimensions are taken as orthogonal. Unbound or free-floating planemos may optionally also be called "free-floating planets," and subtyped according to their histories (if ascertainable) as "sub-brown dwarfs" formed by primary core accretion or "ejected planets" formed in protoplanetary disks. Satellites, non-fusors orbiting other non-fusors, may be classified in a scheme combining a macro/micro dimension (as with other planemos) and a major/minor dimension focusing on dynamical dominance within the satellite's orbital region of the planet-satellite system (e.g. a moonlet in a ring system situated in a gap or embedded within a ring). Binary/multiple planet systems as defined by a barycenter test (e.g. Pluto-Charon) and "quasi-binary" planet-satellite systems with comparably sized members (e.g. Earth-Moon) might usefully be placed on a continuum of "companion planemo relations" which considers both barycenter location and mass ratios. The wealth of microplanet- microsatellite or binary/multiple microplanet systems in our own Solar System (e.g. the asteroid belt) should help to enrich our understanding of this continuum of possibilities for extrasolar planetary systems. -------------------------------------- I. Planetary mass objects or planemos. -------------------------------------- A _planetary mass object_ or _planemo_ is a celestial body larger than a meteoroid[1]; and insufficiently massive at any time in its life history to initiate significant nuclear fusion reactions. As a non-fusor, a planemo must have a mass smaller than that of a fusor or star: either a star reaching the main sequence (~75-90 Jupiter masses or jupiters, or greater); or a brown dwarf or infra-main-sequence star which does not reach the main sequence but initiates the fusion at least of deuterium (~13-75 jupiters). Planemos, whatever their cosmogony or circumstances, may be placed in two categories based on the characteristics of mass and composition: A. Macroplanemos. A _macroplanemo_ is a non-fusor with sufficient mass to attain a near-spherical or "globose" shape constrained by self-gravity, so that it is essentially in hydrostatic equilibrium.[2] B. Microplanemos. A _microplanemo_ is a non-fusor larger than a meteoroid, and thus at least 10-100 meters across, but not sufficiently massive to attain a near-spherical or globose shape constrained by self-gravity.[3] --------------------------- II. Stellar system planets. --------------------------- A _stellar system planet_ is a planemo orbiting a fusor (main sequence star or brown dwarf), as distinct from a satellite orbiting another non-fusor with the barycenter of the orbit located within the radius of that other body.[4] Stellar system planets may be classified as macroplanets or microplanets based on characteristics of mass and composition; and as major or minor planets based on orbital or dynamical circumstances. These two dimensions of classication, macro/micro and major/minor, are in principle orthogonal, although all possible combinations may not occur, at least in a given stellar system. A. The Macroplanet/Microplanet distinction. 1. A _macroplanet_ is a macroplanemo orbiting a fusor and not the satellite of another non-fusor: that is, a stellar system planet sufficiently massive to attain a near-spherical or globose shape constrained by self-gravity, so that it is essentially in hydrostatic equilibrium. 2. A _microplanet_ is a microplanemo orbiting a fusor and not the satellite of another non-fusor: that is, a stellar system planet with insufficient mass to attain a near-spherical or globose shape constrained by self-gravity. B. The Major/Minor Planet distinction. 1. A _major planet_ is a stellar system planet which has become the dynamically dominant body in its region, typically by clearing the neighborhood around its orbit, so that its mass greatly exceeds the combined masses of all other bodies in that neighborhood which are not in orbit around it or otherwise under its gravitational influence.[5] 2. A _minor planet_ is a stellar system planet which shares its region with a substantial population of other bodies not under its gravitational influence, and thus has not cleared the neighborhood around its orbit so that its mass greatly exceeds the combined masses of these other bodies.[6] C. Composite or two-dimensional categories. 1. A _major macroplanet_ is a stellar system planet sufficiently massive to attain a near-spherical shape constrained by self-gravity which has become the dynamically dominant body in its region. In our own Solar System, where all eight major planets are also macroplanets, this category coincides with that of the major planets. 2. A _minor macroplanet_ is a stellar system planet sufficiently massive to attain a near-spherical shape constrained by self-gravity which shares its region with a substantial population of other bodies. In Solar System astronomy, a synonymous term is _dwarf planet_, with 1 Ceres in the asteroid belt and 134340 Pluto in the Kuiper Belt as famous historical examples.[7] 3. A _minor microplanet_ or _small stellar system body_ (SSSB) is a stellar system planet insufficiently massive to attain a near-spherical shape constrained by self-gravity, and which shares its region with a substantial population of other bodies. In our Solar System, where all major planets are macroplanets, this category coincides with that of the microplanets (e.g. most asteroids, comets, Kuiper Belt Objects).[8] 4. A _major microplanet_ is a stellar system planet insufficiently massive to attain a near-spherical shape constrained by self-gravity which nevertheless has become the dynamically dominant body in its region. Such a case, although as yet unobserved in nature, has received notice in the recent theoretical literature.[9] ----------------------------------------------------------------- III. Free-floating, unbound, or interstellar planemos or planets. ----------------------------------------------------------------- A _free-floating_, _unbound_ or _interstellar_ planemo or planet is a planemo which neither orbits a fusor nor is the satellite of another non-fusor. People taking the view that "planet" properly means a stellar system planet may prefer the usage "free-floating planemo," while others may speak more broadly of a "free-floating planet," etc. Both styles of usage should be readily and mutually intelligible. Although the cosmogonic history of a given free-floating planemo or planet may not always be ascertainable from available data, certain conceptual categories are useful and common in the literature: A. A _primary planemo_ or _primary sub-brown dwarf_ is a free-floating planemo or planet formed like main sequence stars and some brown dwarfs "by primary core accretion in molecular clouds."[10] B. A _rogue planet_ or _ejected planet_ is a planemo formed by secondary accretion in a protoplanetary disk like typical stellar system planets and some brown dwarfs, but at some point ejected from the system by dynamical processes (e.g. the gravitational influence of another planet like our Jupiter "clearing its neighborhood").[11] ------------------------------------------------------------------ IV. Satellites, or planemos orbiting other planemos or non-fusors. ------------------------------------------------------------------ A _satellite_ is a planemo orbiting another planemo or non-fusor, with the barycenter of the orbit located within the radius of that other body. A satellite may orbit either a stellar system planet or a free-floating planemo or planet. Satellite systems around planets are dynamically somewhat analogous to planetary systems around fusors (main sequence stars or brown dwarfs), so that a satellite, like a stellar system planet, can be classified in a two-dimensional scheme based both on characteristics (mass and composition) and orbital circumstances. A. The Macrosatellite/Microsatellite distinction. 1. A _macrosatellite_ is a macroplanemo in orbit around another planemo: that is, a satellite massive enough to attain a near-spherical or globose shape constrained by self-gravity.[12] 2. A _microsatellite_ is a microplanemo in orbit around another planemo: that is, a satellite insufficiently massive to attain a near-spherical or globose shape constrained by self-gravity. B. A Major/Minor Satellite distinction -- along a continuum? 1. A _major satellite_ is a satellite which has become the dynamically dominant body of its region, for example by clearing the neighborhood around its orbit, so that its mass greatly exceeds the combined masses of all other bodies in that neighborhood (e.g. in a ring system) which are not under its gravitational influence. 2. A _minor satellite_ is a satellite which shares its region with a substantial population of other bodies not under its gravitational influence, and thus has not cleared the neighborhood around its orbit so that its mass greatly exceeds the combined masses of these other bodies. The major/minor distinction, while quite clear in our Solar System for planets, might be more nuanced for microsatellites or "moonlets" in ring systems, for example -- or indeed for planets in other stellar systems, which might invite more of a spectrum or continuum approach for this dimension. C. Composite categories -- along a two-dimensional continuum? 1. A _major macrosatellite_ is a satellite massive enough to attain a near-spherical shape constrained by self-gravity which is the dynamically dominant body of its orbital region. Our Moon is a familiar example. 2. A _minor macrosatellite_ is a satellite massive enough to attain a near-spherical shape constrained by self-gravity which shares its orbit with a substantial population of other bodies. I am not sure if this case, or something approaching it, has been observed in nature. 3. A _major microsatellite_ is a satellite insufficiently massive to attain a near-spherical shape constrained by self-gravity which is the dynamically dominant body of its orbital region -- for example, a microsatellite of a gas giant orbiting in a discrete gap in a ring system.[13] 4. A _minor microsatellite_ is a satellite insufficiently massive to attain a near-spherical shape constrained by self-gravity which shares its orbit with a substantial population of other bodies. A "propeller moonlet" at a scale of around 100 meters situated within a ring where it cases local "propeller-shaped" disturbances is one familiar example.[14] Given the diversity of satellite systems and the rich variety of dynamical associations found in planetary ring systems, for example, these categories may be more of a starting point for a continuum approach. ------------------------------------------------ V. Binary or Multiple Planemo or Planet Systems. ------------------------------------------------ Under one common technical criterion, a _binary planemo_ system is a system of two planemos orbiting each other with the barycenter located outside the radius of either body. A _trinary_ system likewise has three planemos orbiting each other with the barycenter located outside the radius of any of them, and so on for multiple planemo systems with four or more members.[15] The term _multiple planet system_ would apply in a narrower sense if the members are stellar system planets -- that is, if the system is orbiting a fusor; and in a broader sense if the members are free-floating planemos or planets (e.g. two or more primary planemos or "sub-brown dwarfs"). In our Solar System, the binary planet system of Pluto and Charon is the most celebrated example, and the only known example involving two macroplanets -- more specifically, minor macroplanets or dwarf planets of the Kuiper Belt. In the asteroid belt, however, many microplanet associations have been catalogued -- which, depending on the location of the barycenter, fit the definition of either a microplanet-satellite or a binary/multiple microplanet system. A. Companion planemo relations: the barycenter test and mass ratios We can use the convenient term "companion planemo" to describe either a satellite or a member of a binary/multiple planet system; and the term can be useful in situations like some of those we are about to consider where classifying a body as either a "satellite" or a "planet" in a binary/multiple system might have its attractions. One such situation occurs where the barycenter test indicates a binary/multiple planet relationship between bodies of vastly disparate scale or mass where we find it intuitive to regard the smaller member as a "quasi-satellite." Thus the Pluto-Charon system is actually a quadruple system because it also includes the recent discovered microplanemos Nix and Hydra, which orbit the common barycenter located outside the radius of any member. Describing these two smaller companions as "quasi-satellites" -- technically microplanets under the barycenter test, but "virtual microsatellites" when we consider the drastic disparities of mass -- may convey both sides of the situation. What we might want to say here is that the location of the barycenter outside the radius of Pluto, the largest member, owes far more to the mass of Charon than to that of Nix or Hydra. We also encounter situations where the barycenter test indicates planet-satellite relationship between bodies of comparable mass, say within two or three orders of magnitude of each other, that we might want to consider to some degree also as a "quasi-binary system," like the Earth-Moon system, where the ratio of masses is about 81:1.[16] Considering both the barycenter test and comparability or drastic disparity of masses, we have these four types of situations illustrating a two-dimensional continuum of possibilities: 1. _Straightforward binary/multiple system_. The barycenter indicates a binary/multiple planet system, and the members are of more or less comparable size (e.g. Pluto-Charon with a strikingly low mass ratio of about 6.8:1). 2. _Quasi-satellite situation_. The barycenter test indicates a binary/multiple planet system, but vast disparities of mass make it attractive to classify one or more members as "quasi-satellites" (e.g. Nix and Hydra in the quadruple system formed with Pluto and Charon). 3. _Quasi-binary/multiple planet situation_. The barycenter test indicates a planet-satellite system, but the comparability of masses makes a "quasi-binary planet" concept attractive (e.g. Earth-Moon system, where either view has its appeal). 4. _Straightforward planet-satellite system_. The barycenter test indicates a planet-satellite system, and the vast disparity of masses reinforces this view (e.g. Saturn-Titan). This typology is indeed a list of four situations on a continuum, since it is possible to construct cases where the result of the barycenter test varies as the bodies move through their orbital circuits; or where the disparity of masses between two members is greater than in the Earth-Moon example (not quite two orders of magnitude), but less than that otherwise obtaining in our Solar System between a major planet and any of its satellites (at least some three orders of magnitude, or about 4000:1). B. Binary/multiple systems and the major/minor planet distinction Since the major/minor planet distinction usually addresses the dynamical dominance or otherwise of a single body, possibly one with satellites, applying this concept to a binary/multiple planet system is an interesting question. The Earth-Moon system in a scenario where it is permitted to evolve (undisturbed by changes associated with solar aging, or other cosmic vicissitudes) to the point where its barycenter moves outside's Earth radius, or even at present in a "quasi-dual planet" perspective, presents a possible example of a binary major planet system. One intuitive approach would be to say that with a binary/multiple planet system, the major/minor classification belongs to the system as a whole rather than to any member of it: thus we would ask if the combined mass of the system greatly exceeds the combined mass of all other bodies sharing its orbital neighbhorhood and not under its gravitational influence (e.g. through orbital resonance). If so, its members might be described as "co-major planets" -- or as members of a "major dual/multiple planet system." We might also take an approach of individually comparing the mass of each member of the system with the combined masses of all other bodies in the region not members of the system or under its gravitational evidence, possibly viewing the system as an ensemble of major and minor planets (an especially intuitive approach if some of the members differ dramatically in size and mass). Suppose, for example, that the Pluto-Charon system were moved to a much less crowded neighborhood than the Kuiper Belt where its combined mass -- or the mass of either macroplanet taken alone -- greatly exceeded the combined masses of all bodies not members of the system or under its gravitational influence, say by about three orders of magnitude for the combined system or Pluto alone, and some two orders of magnitude for Charon alone at about 0.147 the mass of Pluto. Under the first approach, we would regard Pluto and Charon as "co-major planets" or "members of a major multiple planet system"; and recognize that Nix and Hydra might also technically share that status as "quasi-satellites" of this major system. Under the second approach, we would individually classify both Pluto and Charon as major planets, but Nix and Hydra as minor planets, since neither of the latter alone would in this scenario greatly exceed in mass the combined masses of all other bodies not members of the system or under its gravitational influence. Either approach lends its own perspective, so that we might do well to combine the insights of both. While Pluto-Charon as the only known "double macroplanet" in our Solar System by the barycenter test, and also the "quasi-double" nature of the Earth-Moon system, have drawn special attention, we should focus also on the rich assortment of microplanet systems to be explored in a region such as the asteroid belt. There we may find many illustrative cases to help flesh out the continuum of companion planemo relations ranging from clear planet-satellite systems to binary/multiple systems with members sharing a near-parity of scale and mass. Enthusiastically studying and appreciating these associations between the planemos of our own Solar System, large and small, may better prepare us for the surprises to be found in other stellar systems -- or between them, in the realm of "free-floating" associations of non-fusors. VI. Conclusion. Since whatever virtues this approach to planetary taxonomy may have should speak for themselves, I would like to conclude by emphasizing one critical limitation, and then offering some acknowledgements. The critical limitation is that this typology is anything but exhaustive. As Bill Ferris would put it, the focus is only on families and genera of planets -- not on species or subspecies such as "major terrestrial planet" or "planetesimal-sized asteroid, say about 1 km across" or "gas giant" or "ice dwarf just massive enough to be a macroplanet."[17] Now for the acknowledgements. I would first like to thank Professor Gibor Basri both for his illuminating articles and for his generous encouragement to place our local Solar System in a larger context. George Dishman offered invaluable insights and dialogue in analyzing the recently adopted Resolution 5A of the International Astronomical Union (24 August 2006), and sharing his ideas in a frank, friendly, and enlightening way on Usenet's sci.astro newsgroup. I would also like to thank many other Usenet participants in this and related newsgroups who have contributed their ideas and helped me to formulate or refine my own. John W. Weiss, or the "Cheshire Cat" as he is styled on his Web page , lent me a critical and catalytic insight during a friendly discussion about the new IAU resolutions at . He reminded me that the term "planet" has a traditional if less commonly used meaning embracing both "major planets" and "minor planets" -- a simple, well-documented, and elegant starting point that I hadn't considered! His affable hint had on me an energizing impact rather like being gently massaged with a velvet clue-by-four. Later, in searching on the Web for earlier uses of some terms I was considering for a planetary typology, I discovered that Bill Ferris, a premier minor planet observer and discoverer, had arrived at the same insight and shared it in a post to the Minor Planet Mailing List (MPML) of Yahoo! Groups (message #17628) of 27 August 2006 -- three days after the IAU decision, and a good week before my encounter with John Weiss. Further, I learned that the term "microplanet" had been proposed for a usage much like that followed here by Michael Paine in an essay entitled "Small is Beautiful" (25 September 2000) available on CCNet . The main difference is that Paine's proposal of 2000 sets as an upper limit a radius of 1000 km -- in distinction to the "hydrostatic equilibrium" test widely advocated since and adopted here, thus making an asteroid such as Ceres at about 960 km a very small macroplanet. Of course I am also indebted to authors of the literature cited or alluded to here, and to participants on all sides of the IAU process as well as commentators on that process. Last but not least, I would like to thank my mother Rose Donaldson for her devoted interest, encouragement, and wise sense of humor in seeing me through this curious endeavor. ----- Notes ----- 1. The upper limit of scale for a meteoroid, and thus the lower limit for a planemo, might be placed somewhere around 10-100 meters. The placing of this limit at 10 meters is proposed in Beech, M., and Steel, D. I., (1995) "On the Definition of the Term Meteoroid," _Quarterly Journal of the Royal Astronomical Society_ 36(3), 281-284, . cited in . 2. On the criterion of near-sphericity constrained by self-gravity or hydrostatic equilibrium, see, e.g., Stern, S. A. and Levison, H. F. (2002), "Regarding the Criteria for Planethood and Proposed Planetary Classification Schemes," _Highlights of Astronomy_ 12: 205-213 ; Basri, G. B. (2003), "What is a `Planet'?" _Mercury_ 32 (6) (Nov./Dec.), ; Basri, G. and Brown, M. E., "Planetesimals to Brown Dwarfs: What is a Planet?" (2006) _Annual Review of Earth and Planetary Science_ 34, 193-216 ; and International Astronomical Union (IAU) Resolution 5A (24 August 2006) . The adjective "globose" may convey the idea of a globe-like shape brought about by the massiveness of the body. 3. Thus the lower limit for a microplanemo agrees conceptually with that traditionally accepted for a minor planet in our own Solar System. As we approach the upper limit, there is a transitional region where an object's degree of "near-roundness" might be debatable; see Soter, S., "What is a Planet?" submitted to the _Astrophysical Journal_ 16 August 2006 , pp. 2-3. Possibly a transitional category like _mesoplanemo_ might accommodate this debatable region of what Soter aptly terms "a continuum of sizes and shapes," ibid. at p. 2. For the term "microplanet" I am indebted to Michael Paine who proposed it in an essay of 25 September 2000, "Small is Beautiful" . He suggests its application to bodies with a diameter or semi-major axis of less than 1000 km as a term to replace "minor planet" -- or possibly more narrowly to sub-200 km objects only, with those in the range of 200-1000 km styled "miniplanets." Here, as in Paine, the term "microplanet" (Section II), and thus also the broader "microplanemo," relates to an intrinsic characteristic of the body, its scale or mass, but with hydrostatic equilibrium rather than any absolute size as the upper limit. 4. The term "stellar system" is meant to generalize the familiar "solar system." As discussed in Section V, the barycenter test to distinguish between a planet-satellite system and a binary/multiple planet system is one convenient criterion rather than a full basis for appreciating the rich continuum of companion planemo relations that obtain in our Solar System or elsewhere. 5. On this "clearing the neighborhood" concept, see for example Stern and Levison (2002), n. 2 supra; Brown, M. E. (2004), "Is Sedna a Planet?" ; Basri and Brown, n. 2 supra; Soter, n. 3 supra; and IAU Resolution 5A, n. 2, supra. Soter, p. 9, suggests a "provisional" interpretation that a body has effectively cleared its neighborhood if its mass exceeds the combined masses of all other bodies in that neighborhood not under its gravitational influence by a ratio of 100. He notes that in our Solar System, where there is a jump of about "five orders of magnitude" between any of the eight major planets and Ceres, one might acceptably set this ratio at "anywhere between about 10 and 1000." However, other stellar systems may present us with situations calling for refinement of the major/minor distinction into more of an express continuum (e.g. "demi-major" planets where the ratio is around 1-10); or indeed a catalogue of "dynamical classes" tailored to fit a range of extrasolar realities. 6. As it happens, this definition of "minor planet" nicely fits the customary use of the term for planets within our own Solar System which occur as members of substantial populations: notably the asteroids discovered beginning with 1 Ceres in 1801; and the Kuiper Belt Objects or KBO's beginning with 134340 Pluto in 1930. The distinction suggested by Stern and Levison (2002), n. 2 supra, p. 6, between an _überplanet_ "that is dynamically important enough to have cleared its neighboring planetesimals in a Hubble time" and an _unterplanet_ "that has not been able to do so" is closely equivalent to the major/minor distinction here. We can also speak of minor planets evocatively as "belt planets," a usage suggested to me by Basri (2003), supra n. 2, who playfully proposes for "planets without orbital dominance" the term "beltway planets" with its humorous political allusions in the USA. 7. The criteria for a minor macroplanet are essentially equivalent to those adopted for a _dwarf planet_ in IAU Resolution 5A. We might regard "dwarf planet" as a felicitous provincialism for "minor macroplanet" in stellar systems such as ours (as presently known) where such planets are notably smaller than any of their major counterparts, and also in absolute terms not too far from the lower limit of size for macroplanets. However, an expressly two-dimensional concept like "minor macroplanets" may better fit systems where these macroplanets sometimes overlap in size with major ones, or in absolute terms attain a size near the massive end of the planetary spectrum, as in the hypothetical example of "untergiants in Oort Clouds" presented by Stern and Levison (2002), p. 9 (see previous note for their use of über/unter categories, equivalent to major/minor here). 8. The criteria for an SSSB are similar to those of IAU Resolution 5A, except that here an SSSB is specifically a _minor_ microplanet, as opposed to any microplanet. Since in our Solar System, the limited domain addressed by Resolution 5A, all microplanets are minor planets occurring as members of substantial orbital populations, the distinction might seem moot. However, I would be inclined to say that should a major microplanet turn up in some other stellar system, it might best be placed in a category other than SSSB because of its dynamical prominence. See also the following note and accompanying text. 9. Thus Soter, n. 3 supra, at p. 10, who uses the term "planet" as more or less equivalent to "major planet" here, remarks: "A potato-shaped body would be classified as a planet if it dominated its orbital zone." 10. Soter, ibid., p. 10. For Soter, a planet is "an end product of disk accretion around a primary star or substar," so that "I suggest we classify as planets" any brown dwarfs (mass greater than 13 jupiters) formed by such a process. This is a fine example of a definition oriented to cosmogony, in contrast to the characteristics-oriented distinction of fusor/non-fusor followed here. The Working Group on Extrasolar Planets (WGESP) of the IAU in its _Position Statement on the Definition of a Planet_ (28 February 2001; last modified 28 February 2003), , takes what is called a "compromise" approach combining characteristics and circumstances: a "planet" is a non-fusor, however formed, orbiting a fusor -- in contrast to "[f]ree-floating objects in young star clusters" with masses below the limit for fusion of deuterium, which are not "planets" but "sub-brown dwarfs" or the like. Where the WGESP usage is preferred, "free-floating planemo" might be a congenial style. Personally I am comfortable with "free-floating planet" or "unbound planet," here an optional usage. See, e.g., Stern and Levison (2002), n. 2 supra, at pp. 5-6, "An _unbound planet_ is any planetary body not bound to a single or multiple star system." For a fascinating recent survey of this and related definitional questions, exploring both current views on stellar system formation and some cultural considerations, see Basri and Brown (2006), n. 2 supra. 11. Soter, n. 3 supra, p. 11, defines "rogue planet" as "a term applied to a low-mass object" [i.e. a non-fusor] "accreted in a disk and expelled to interstellar space by gravitational perturbations," noting that "simulations of planetary formation suggest that [these bodies] may outnumber planets bound to stars." Basri, in a section discussing his own views at the conclusion of Basri and Brown (2006), n. 2 supra, 213, remarks that some free-floating planemos "might indeed be ejected planets, but one would not call them that unless there were empirical reasons for doing so." Under his definition, demonstrated "ejected planets" would indeed be planets, i.e. planemos "whose primary orbit is now, or was in the past around" a fusor, ibid. at 214. Soter, ibid. at p. 11, likewise observes that rogue or ejected planets may be common but "difficult to detect, and, if detected, may be indistinguishable from sub brown dwarfs, which accreted as primary objects." 12. "Macrosatellite" as used here is synonymous with _planetary-scale satellite_ as defined in Stern and Levison (2002), p. 5, where "planetary bodies" are synonymous with macroplanemos here: "A _planetary-scale_ satellite is any planetary body orbiting a larger planetary body on a bound orbit; examples of planetary-scale satellites include Luna, the Galilean satellites, Titan, and Triton." 13. A different and intriguing example may be provided by the exquisitely beautiful microplanet-microsatellite system of 243 Ida and its tiny moon Dactyl, as captured in 1993 by the Galileo probe. According to Geissler, P., Petit, J.-M., Durda, D. D., Greenberg, R., Bottke, W., Nolan, N., and Moore, J. (1996), "Erosion and Ejecta Reaccretion on 243 Ida and its Moon," _Icarus_ 120:140-157, available on the Web at , 151, "Gravitational scattering by Dactyl efficiently clears the space surrounding Ida and makes it highly unlikely that another nearby satellite orbits the asteroid." They remark in their conclusion, "Dactyl clears the space surrounding Ida both by capture of orbiting debris and by gravitationally scattering particles to escape Ida orbit," ibid. at 155. Dactyl, with a radius of about 1.5 km, also shows how a very small planemo can attain a strikingly near-spherical shape by means other than self-gravity -- possibly evolving "from a presumably irregular fragment to an almost spherical figure" by "collision with a population of impactors too small to be detected by usual methods of crater counting," ibid. at 154. 14. Spahn, F. and Schmidt, J. (2006), "Saturn's bared mini-moons," _Nature_ 440:614-615 (30 March); available on the Web at . 15. For the barycenter test, see, e.g., Stern annd Levison (2002), pp. 5-6, n. 5, "A double planet would be a system consisting of a planet and a satellite which is massive enough to place their primary barycenter outside of the primary, and between the two bodies." 16. On the question of whether the Earth-Moon may be considered as a "double planet," or at least a quasi-binary system, see a surveying of views in and . The European Space Agency (ESA) page "Welcome to the double planet" (5 November 2003), , espouses a "binary system" perspective. 17. Thus Stern and Levison (2002), n.2 supra, p. 8, present a proposed classification scheme for planetary bodies "that loosely parallels astrophysical nomenclature for stars" using a "two-parameter" system based on size and composition. Basri (2003), n. 2 supra, likewise advocates "compositional and size-based adjectives," as well as others addressing cosmogony. Such "higher resolution" typologies, as Stern and Levison, ibid., describe them, seem highly congenial to the basic approach advocated here. We might, for example, develop a set of categories spanning the spectrum of planetary sizes or masses from the smallest microplanemos (e.g. "subplanetesimals" up to about 1 km; "planetesimals" at around 1-100 km) to the most massive macroplanemos ("supergiants" of 2-13 jupiters supported by electron degeneracy pressure). Such finer categorizations of characteristics such as size and composition should be combined with a dynamical description, as advocated by Stern and Levison (über/unter) and Basri (major/minor, as here). Especially for minor planets with their belt environments, this dynamical description could expand into a brief "demographic" categorization based, for example, on the ratio of a minor planet's mass to the total mass of its belt; and how it is situated in the distribution of sizes among belt population members. Most respectfully, Margo Schulter mschulter@calweb.com 20 September 2006