[meteorite-list] THE ALTERNATIVE DEFINITION OF PLANET
From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat Aug 19 16:21:39 2006 Message-ID: <001c01c6c3c9$a27d20b0$947f4b44_at_ATARIENGINE> Hi, All, Here's the text of the alternative definition of planet, containing in the following article: http://www.space.com/scienceastronomy/060819_new_proposal.html Commentary first, but short. When the outline of this proposal was put forward, I pointed out that it was probably based on a widely held view in the various theories of planetary formation first put forward by the late A. G. W. Cameron 45 years ago, of zonal sweeping that "outlawed" highly mixed sources of planetesimals. And sure enough, the longest of the three paragraphs of justification attached to the "definition" do just that. The ironic thing is that considerable evidence has piled up in the last 5-8 years that this condition cannot be true, so the theory on which this definition is based may be about to fall. Mostly, it's a bad idea to base a fundamental definition of the much larger question of "what's a planet?" on a single theoretical possibility in cosmogony, particularly if it looks like it might be in trouble. How closely this definition is tied to airy theory is demonstrated in the notes: "Pluto, Ceres and other large Trans-Neptunian objects... should be not considered as planets, since they never were the dominant bodies in their accretion zones." This implies that the signers KNOW where and how Pluto and Company accreted. If so, they're the only ones. Next, "local" is defined as orbits that cross or "close approach" each other. Venus and Earth come within 0.255 AU of each other. Mars and the Earth come nearly as close. Now, you know that if this were in the other solar system, that would more than enough to disqualify them as planets. There was no past formal definition of planet. We found'em, we named'em, until there were Nine. When we discovered that Pluto was different and small, discontent grew. That is the origin of the Gang of Eight (Planets): to rectify the lazy mental habits of the Nine Worlds Organization and make Them pure in theory. Why stage their coup now? They are threatened unexpectedly by the multiple discoveries of objects they resolutely asserted (for a decade or so) could not exist because Their Theory cannot account for them. Oh, this'll fix it... OK, I'll let them talk now. Sterling K. Webb (text follows) ----------------------------------------------------------- New proposal for Resolution 5: Definition of a Planet (1) A planet is a celestial body that (a) is by far the largest object in its local population[1], (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) does not produce energy by any nuclear fusion mechanism [3]. (2) According to point (1) the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane are the only planets of our Solar System. All the other objects in orbit around the Sun are smaller than Mercury. We recognize that there are objects that fulfill the criteria (b) and (c) but not criterion (a). Those objects are defined as "dwarf" planets. Ceres as well as Pluto and several other large Trans-Neptunian objects belong to this category. In contrast to the planets, these objects typically have highly inclined orbits and/or large eccentricities. (3) All the other natural objects orbiting the Sun that do not fulfill any of the previous criteria shall be referred to collectively as 'Small Solar System Bodies'.[4] Notes: [1] The local population is the collection of objects that cross or close approach the orbit of the body in consideration. [2] This generally applies to objects with sizes above several hundreds km, depending on the material strength. [3] This criterion allows the distinction between gas giant planets and brown dwarfs or stars. [4] This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets. Further Considerations: There has been a long discussion about what a planet is. This problem appears at both ends: for the very massive bodies and for the smaller ones. At the large end, the limit seems to be clearer; it is now widely accepted that planets must not generate any energy from nuclear fusion, while brown dwarfs generate some nuclear energy from the fusion of deuterium. More problematic is the small end. We think that the definition should be kept as simple as possible and based on physical and cosmogonic reasons. There is a wide consensus that planets formed by the accretion of small bodies "the planetesimals." The accretion process led to the formation of embryo planets that, as they grew in size and acquired more powerful gravitational fields, went to a process of runaway accretion in which the size of a few of them detached from the rest of the bodies of their neighboring zones. Given the powerful gravitational fields of these massive bodies - that we can call at this stage protoplanets - they were able to clean the population that had close encounters with them. The bodies interacting with the protoplanets were finally incorporated to the planets or scattered to other regions. >From a cosmogonic point of view, it therefore makes more sense to consider a planet as an object that acquired a mass large enough to clean a zone around its orbit. According to this definition, only eight planets, Mercury (perhaps marginally), Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune fulfill this condition. It is obvious that, at least for our solar system, this cosmogonic definition implicitly carries the condition of objects with a roundish shape determined by self-gravity. >From our definition, Pluto, Ceres and other large Trans-Neptunian objects in quasi-hydrostatic equilibrium [1] should be not considered as planets, since they never were the dominant bodies in their accretion zones. It is suggested that Pluto be kept unnumbered by historical reasons. Is may be possible that in the near future cases of objects not foreseen at present could appear beyond our solar system, as for instance free-floating planets, stray planets, or double planets. We think that we should not advance definitions at this point for these exotic cases and leave their discussion when if they became a part of the observed world. [1] From our present knowledge of the Solar System, we know that objects as small as Mimas (D~400km) are roundish. If this were the lower limit for an icy body to be in hydrostatic equilibrium, then we would already have several tens of bodies fulfilling this requirement. List of adherents to the above proposal: Name/Country Julio A. Fernandez Uruguay Marcello Fulchignoni France Daniela Lazzaro Brazil Gonzalo Tancredi Uruguay Alessandro Morbidelli France Mario Di Martino Italy Paolo Paolicchi Italy Antonella Barucci France Giovanni Gronchi Italy David Vokrovhlicki Czech Rep. David Nesvorny USA Fernando Roig Brazil Hugo Levato Argentina Steven Chesley USA Alsonso Sena Mexico J. E. Arlot France I. Shevchenko Russia Patrick Michel France Received on Sat 19 Aug 2006 03:56:59 PM PDT |
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