Talk:Double planet
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Ok, so here's a proposed re-write:
A double planet is a set of two planets of comparable mass orbiting one another. There is no accepted definition on where to draw the line between a double planet and a system consisting of a planet and moon, so the term "double planet" is an informal one.
In most cases, a moon is of very small mass relative to its host planet. However, there are some examples of moon/planet mass ratios much closer than average: particularly, the Earth and its moon Luna (mass ratio 0.01230), and Pluto with its moon Charon (mass ratio 0.147). (Ignoring the debate over whether Pluto should be considered a planet, or simply a larger Kuiper belt object.)
Several approaches have been suggested for defining a double-planet. One method, suggested by Isaac Asimov, is to compute the force of gravitation exerted by a planet on its moon (Fp), and the force exerted by the Sun on that moon (Fs). The ratio Fp/Fs is the Tug Of War (TOW) value. A TOW value less than 1 indicates that the moon is more firmly gripped by the Sun than its planet. The TOW value for Earth and Luna is approximately 0.455. Two recently discovered satellites of Jupiter, and the outermost satellite of Neptune, also have TOW values less than one.
Charon was still undiscovered at the time Asimov proposed the TOW ratio. The TOW ratio for Charon is over 600, so by Asimov’s reckoning Charon would be a moon of Pluto.
However, astronomers prefer to regard Pluto and Charon as a binary system because (a) they are so close together and so close to the same size that the point they orbit about (the barycenter) is above Pluto's surface, and (b) they orbit with the same side facing each other. By the barycenter method, Pluto/Charon is a double planet, but Earth/Luna is not.
--Gary Puckering, 2004-03-15
Some excellent work on this subject was published in 1975 by Isaac Asimov in "Time and Space and Other Things", Avon Books, New York. Asimov's approach is to consider the tug-of-war (TOW) ratio between the satellite and it's primary, and the satellite and the sun. One would expect a satellite to be more firmly grasped (gravitationally speaking) by it's primary than by the sun, and such is the case for all the satellites in our solar system, including Charon. But is not the case for our Moon. For a good accounting of Asimov's treatise on the web, see [1] (http://www.jmp.com/news/jmpercable/fall97/just_mooning_around.html)
Asimov suggests that the appropriate cutoff points for "moon" status are the TOW distance and the Roche Limit. A true "moon" must lie between these two points. It should be fairly obvious that the Roche Limit is a more reliable minimum distance than the surface of the primary -- regardless of the location of the barycenter -- since it cannot exist inside the limit without being destroyed by tidal forces.
As for the maximum distance, I'm with Asimov -- the TOW distance makes the most sense. And by this line of reasoning, Earth and Moon would be classified as a double-planet, whereas Charon would be classified as a satellite of Pluto -- a conclusion that is quite the opposite of that drawn by Bryan Erickson (below).
--Gary Puckering, 2004-03-15
Not having actually read this article, I see that two instances of the phrase "I propose" occur and that it's signed. Wikipedia isn't the place to make proposals (i.e., to do original research). To do so is to ask the people here to act as peer reviewers and, worse, to pass off your proposals as the state-of-the-art research in the field. For all I know about you (Bryan Erickson), they might be, but I doubt it. But I feel strongly enough about this particular (proposed) rule that, on Nupedia, a very distinguished Classics editor quit when I refused to allow him to expound his latest theories on something or other. I'm not in the same position of authority on Wikipedia (because this is a wiki :-) ) but I do tend to get put out by this sort of thing nonetheless.
Beyond these concerns about the propriety of original research, there is also the very generally accepted rule about writing from the NeutralPointOfView.
So, how might we rewrite this article so as to remove idiosyncratic proposals? --Larry Sanger
Frankly, the whole thing is just an argument about the use of terminology--it belongs in a style guide for publications on Astronomy or something. I don't see that it has any purpose in an encyclopedia at all. An article entitled "double planet" should simply mention that the definition is necessarily vague, and then list a few and describe them. I want to learn something about double planets, not something about what astronomers argue about. --LDC
Following are the problematic paragraphs:
I propose two different quantitative analyses that could draw a sharp distinction between double planets and planet-moon systems. The first is the Center of Mass Method. If the center of mass of the two bodies lies under the surface of one body, then that body will be defined as a planet and the other body will be defined as a moon. This method has the following advantages: it's relatively easy to perform when the masses and volumes of the two planets are observable; it is logically sensible; and it provides a definite demarcation between the two classes of two-body systems in almost all cases. The disadvantages are: in a few hypothetical cases, the center of mass might lie almost exactly on the surface of the more massive body, so that the center of mass is below the surface when the maybe-moon happens to be passing over mountain ranges but above the surface when the maybe-moon is above a valley; this would require further definition of exactly where the surface is, is it sea level, or if the planet is among the great majority that don't have seas, is it above the "datum" or average radius; secondly, this method requires observation not only of the masses of the two bodies but also of their radii; in the near future, as observations of planets in other star systems improve, we will reach the point of measuring masses and tracing orbital paths of planets but remaining capable of imaging them only as individual pixels. This would be under the level of technology embodied in NASA's Terrestrial Planet Finder telescope. It will be a long time after that before our data gathering technology achieves the ability to make reliable measurements of the radii of these planets. Therefore, the Center of Mass Method for distinguishing double planet systems and planet-moon systems, while working fine for our own Solar System, would be face more difficulty for other star systems.
Which leads to the second analysis, the Lagrange Method. If the mass ratio between the two bodies is at least about 0.081, the 4th and 5th Lagrange points of the two-body system do not provide a stable equilibrium. If the ratio is less than that amount, Lagrange 4 and 5 do provide stable equilibrium. This provides a distinction that could be used to define the cutoff between double planet systems and planet-moon systems. It has the advantage that it requires only determination of the masses of the two bodies, which could be accomplished for extra-solar planets at a much lower level of technology; we already measure the minimum mass of extrasolar bodies, and with observations of their orbital paths, even with only single-pixel resolution, as with the proposed NASA Terrestrial Planet Finder orbiting telescope, we will be able to define their masses pretty well. This method also provides a definite cutoff between the two categories. However, it has the drawback of providing a less logically compelling definition. Therefore, I propose that the Center of Mass Method be adapted as the official analysis. It could still be applied, albeit with less precision, to extrasolar planets under the Terrestrial Planet Finder level of technology, if we use a model of a planet's density based on its mass and the semimajor axis - average distance - from its host star. This density model would provide a good prediction of the planet's radius, which could be used together with measured mass to determine whether the center of mass is beneath the surface of the larger mass or is in space between the two bodies.
Coming back to our solar system, under the preferred Center of Mass method, Pluto and Charon are clearly double planets, but Earth and its Moon are not; the center of mass is almost 2000 kilometers, or about 30%, of the way to the center of the Earth.
--Bryan Erickson
Double planets
Don't forget that the moon is slowly moving out in its orbit...in six or seven billion years, when the sun becomes a red giant, the center of mass of the Earth/Moon system should be outside of the Earth's surface, due to tidal evolution.
Tom Mazanec
A couple of notes on today's major edit of the Double Planet entry:
A couple of notes on today's major edit of the Double Planet entry:
1. Mr. Puckering's proposed definition forces a conclusion that is quite the opposite of that found in the professional literature by the scientists in the field as sampled in the references provided, since he concedes that his definition would exclude Pluto and Charon from double planet status while including tiny moons of Jupiter and Neptune as new double "planets", along with Earth's moon. He might also pay attention to the fact that Asimov was not defining limits for a double planet at all, but for a moon. The Roche Limit has no effect on the question of defining whether a moon qualifies as a double planet, rather than of how close a moon (or double planet) can get to the host planet before shredding apart. A double planet can get along just fine with its barycenter well under the Roche limit just as a moon get get along just fine with its planet-relative barycenter under the planet's surface.
2. The first, relatively minor edit corrected a factual error, consiting of a definition so broad it applied to all moons rather than double planet-systems as opposed to planet-moon systems. It said that:
"Hypothetically, the moon/planet mass ratios between two bodies could vary anywhere between zero through one, where two planets of exactly the same mass orbit each other..."
But a mass ratio approaching zero would include every conceivable planet-moon system, eliminating any possible difference between a double planet system and a planet-moon system. It would define literally thousands of orbiting debris objects to be in "double planet" systems with the Earth.