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Definition of a Planet (and other stuff)

  1. Jun 4, 2012 #1
    I searched for "definition" and "planet" but found no thread which matched this purpose; if one already exists then it is significantly old, but I will apologize anyways.

    It used to be that our solar system had Nine planets. Then some trans-Neptunian object (Eris) forced some astrological society to draw a line in the sand between "planet" and "almost but not quite". For the most part, I enjoy the definition; but I still find it lacking. It's very specific to our solar system. I would suggest some alterations that uphold the spirit but also improve upon the concept. I'm looking for constructive criticism, but positive feedback is okay too. Being that I have not been an Astronomy major, and my knowledge of the subject comes from my general thirst for knowledge; I can't pretend that I'll propose a "final solution" to the planet definition debacle.

    In laymen's terms, a planet is a large object that orbits a star. Most people aren't worried much further than that. A satelite orbits a planet and a star produces energy via fusion, asteroids are those small objects nearby or the large objects that wander around (where a comet is a specific kind of asteroid). My assertions will spring from these (simplified)assumptions. Later stated definitions shall override any prior definitions.

    An asteroid should be defined as any object massive enough to hold its composition (ie: not a dust cloud). A special asteroid (with what name, "rogue planet"?) would be any object massive enough to fulfill the shape requirement of being a planet, but does not orbit any star.
    A satelite (aka moon) should be defined as any object that fulfills the above and orbits a planet.
    A planet should be defined as any object: (1) massive enough to form/maintain a generally spherical shape, (2) orbits a star (including binary, etc), and (3) holds the majority of mass within its orbit by a ratio of at least [10] to 1 (where '1' is everything else combined).
    Special types of planets include: a dwarf planet lacks either the conditions or mass required to maintain a generally spherical shape; a binary planet is two objects that together hold the requisite ratio to be a planet, and may or may not directly orbit each other around their star; a gas giant (for purposes of being a type of planet) is composed mostly of gasses but lacks sufficient mass to conduct fusion; ...and otherwise meets the other conditions of being a planet.
    A star is any object massive enough, and is currently observed, to conduct fusion. A dead star would be any object that is not currently but had at any time enough mass to fuse at least Deuterium (ie: Brown dwarf). Specific kinds of stars (dwarf, giant, etc) would still hold their definitions, but to be in the "star" catagory at all I suggest this definition.
    A black hole is any "object" massive enough to trap light in its orbit/gravitational field. Other objects may also orbit black holes, without a star being present, and maintain their titles.
    Galaxies, nebulas, and universes are beyond my scope, but that would be a fun thought exercise as well. How would one explain galaxies to a lay person without simply stating "a collection of stars"?

    We still wouldn't consider Pluto a planet, but this way we can easilly classify any object quickly and intuitively. I see no problem with moons never being considered "planets" in their own right, even when they show all the characteristics of what we would expect from a planet. Popular culture agrees; when the Death Star targeted Yavin IV, it needed to first clear a planet and would be targeting a moon. Clearly, the general populace isn't concerned with how we label habital objects, as long as they make intuitive sense. Should I begin lobbying astrological organizations to push for these laymen definitions or do they need refinement first?
  2. jcsd
  3. Jun 4, 2012 #2


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    Uh ... good luck with that.
  4. Jun 4, 2012 #3
    Astrological organizations???
  5. Jun 4, 2012 #4
    Seriously people, I am looking for constructive criticism, not pedantic repetition of key segments of text.
  6. Jun 5, 2012 #5
    You are in violation of the forum guidelines which specifically states:
    " Generally , in the science discussion forums we do not allow [...] Personal theories or speculations that go beyond or counter to generally-accepted science."

    This is not the place to get opinions on your personal theories unless published in a peer reviewed journal.

    And you may want to start here: en.wikipedia.org/wiki/IAU_definition_of_planet
    Last edited: Jun 5, 2012
  7. Jun 5, 2012 #6
    What part of this goes beyond or runs counter to generally accepted science? Quite literally, the definition of a planet is open to interpretation outside of our solar system and I'm using the International Astronomical Union (IAU)'s definition of a planet (which self-imposes its definiton to only affect objects within our solar system) to suggest how we should define solar systems we explore (in person or via instruments) in the future.

    I'm not sure if you're trolling on purpose or if you're one of those "don't change anything" types of people and will use pedantics to assert your desires. Either way, please contribute refutations which describe aboslutely how I am wrong or otherwise provide constructive criticism; or you can unsubscribe this thread so you won't have to see future updates. Your choice.
  8. Jun 5, 2012 #7


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    Paldin, spelling and grammar errors aside, I generally like your definitions. The problem is that I also like the IAU's definition as well! The real problem is that people can't decide what to call what because they all have different opinions and see things differently, and until very recently there hasn't been a need to classify things outside of the solar system other than stellar objects and gas/dust clouds.

    A good example is what to classify Titan, Ganamede, Callisto, and other very large moons that orbit gas giants. I find it very easy to accept that there are Earth size moons orbiting exosolar gas giants, similar to Pandora from the movie Avatar. So, do you call it a moon or a planet? While it does orbit a planet, like our own moon does, it is also much more massive than any moon here in the solar system. If you also have planets of similar or smaller masses in their own orbits around the star some people think you should call it a planet instead.

    Honestly we could argue all day about what would be the "best" definition, but we still wouldn't be any closer to anything because it's all pure opinion, which is the worst thing to argue about. In my own opinion of course.
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