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When is a planet a planet?

  1. Oct 10, 2004 #1


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    There are many more objects in our solar system besides planets. The astroid belt is full of big rocks. Why do we not consider them planets?
    Or what is the minimum size of a planet.

    And what is the minimum size of a moon?
    There are a lot of small rocks around saturnus, how big must they get to become a moon?
  2. jcsd
  3. Oct 10, 2004 #2
    That's a very good question JV. There have been several attempts to create a definition of a planet that would allow us to make the distinction more clear but I don't believe any of them has any general acceptance.

    It's alot like asking when does a pond become a lake.
  4. Oct 10, 2004 #3
    The asteroids of the asteroid belt are very light compared to the planets: Ceres is the most masive of all, but its mass is only 1/100th of Moon's mass.
    An upper limit to what can be considered a planet is put by brown dwarfs. I don't think that brown dwarfs should be considered planets. There are two classes of brown dwarfs: T dwarfs, with a mass of 20-50 jupiter
    masses, and L dwarfs, with 50-80 jupiter masses
  5. Oct 11, 2004 #4


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    Pluto seems to set the mimimum size for a planet. Sedna isn't that much smaller, but it was never dubbed a planet. So I guess a planet >= pluto and < brown dwarf. But a Brown Dwarf only sets a limit for mass. I've heard that they're not much bigger than Jupiter, despite being at least 20 times as massive.
  6. Oct 11, 2004 #5


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    JV - It's a gray area with no absolute technical definition. In general, planets are big, round, and orbit a star. "Round" is significant in that it means the object has enough gravity (mass) to smooth out the surface. Many comets & asteroids are oddly shaped because they don't have enough mass/gravity to overcome that structure. Moons are small-to-large objects that orbit planets. Consider Saturn's moon Titan - - it's big enough to be a planet, but its primary orbit is around Saturn, so it's a moon. Things that are called moons can be quite small (astronomically speaking) - - like on the kilometer-sized scale. Tiny stuff like the debris/particles that makes up Saturn's rings just doesn't rank up there with a moon (mass/size-wise).

    There's still somewhat of a debate whether Pluto should still be called a planet or not. For now, it is a planet, perhaps largely due to habit. If it were discovered today, it could have been called a Kuiper Belt Object instead.

    As mentioned, there are other gray areas like in the planet-star dividing line and in the comet-asteroid dividing line.
  7. Oct 12, 2004 #6
    So how about these exosolar planets astronomers are finding around other stars? I know some of them (according to accepted calculations) are thought to be much larger than Jupiter. How can we determine that these aren't brown dwarfs, since their masses are very similar?
  8. Oct 12, 2004 #7
    Good question. One of the methods is this:
    "Brown dwarfs are rare around Solar-Type stars. At most 1% of Solar-type stars has a brown dwarf within 5 AU. Thus, planets are distinguished from brown dwarfs by virtue of their relative occurrence."


    This dearth of brown dwarfs at short distances of their star is called the "Brown dwarf desert"
    I'm sure that are other factors helping to determine if the thing that are you viewing is a planet or a brown dwarf
    Last edited: Oct 12, 2004
  9. Oct 12, 2004 #8
    There is, believe it or not, a "political side" to this planet question. People have for a long time been told it is a planet and therefore object to any change of status. School children would become confused. Didn't Donald Duck have a dog called "Pluto?" http://www.redrosetexas.com/catalog/mickey_minnie_donald_duck_pluto_3071767.htm [Broken]

    Do you want that dog to be slighted and thought less of because of its name? Similarly for planatary exhibits that feature the grand planet Pluto. Just imagine the let down--world wide!
    Last edited by a moderator: May 1, 2017
  10. Oct 13, 2004 #9


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    Not as bad as telling them that the universe doesn't revolve around the Earth.
  11. Oct 19, 2004 #10
    "political side" Pluto

    I believe in the 1990’s there was an international effort to define Planets in two categories - Rocky (typically interior orbits) and GAS Giants (typically outer orbits)
    Leaving Pluto as just another “Ice Ball” in an odd orbit out where Comets come from.

    However, Pluto was the one and only planet discovered by someone in the “New World” 1930 by American astronomer Clyde W. Tombaugh.
    Mickey Mouse’s dog did not show up till 1931 - maybe named for the Planet.

    Maybe it shouldn’t be a Planet, but with Charon orbiting it don’t ya just want to keep it a Planet. But I’m from the “New World” so leave it to the internationals. And the Europeans said we could still count it as number 9 !!
  12. Oct 20, 2004 #11
    Though that the IAU declared in 1999 that Pluto is a planet, I don't think so. THe overall characteristics of Pluto really don't "fit" in the planetary family, so it's very probable that was formed in a different way that its eight neighbours. Like this page says
    http://www.astro.rug.nl/~mwester/aos/aose.html [Broken]

    "However Pluto probably wasn't always a planet. Current models indicate that Pluto was formed as a KBO, became a satellite of Neptune, collided with Triton and then ended up in it's current orbit and became one of the nine planets"
    Last edited by a moderator: May 1, 2017
  13. Oct 21, 2004 #12
    Some moons are larger than planets, so it's not size that separates moon and planet. It's a moon if it orbits a planet, it's a planet if it orbits a star. And I suppose that, yes, for it to be a planet it has to be large enough to be round. Moons don't necessarily have to be round...that's a grayer area.
  14. Oct 22, 2004 #13


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    Welcome to Physics Forums, RandallB.

    Hard to say "typically". We have yet to find any terrestrial extrasolar planets (too hard to detect with current technology).

    I'm pretty sure I recall hearing that the Disney dog was named after the planet.
  15. Oct 22, 2004 #14
    Thanks for the welcome in.

    I remember the comment that the expected rocky planets to be in inner orbits - but not any justification for it. I agree the sample of our one system is thin for a conclusion. Maybe they had other reasons to expect it.

    I'd agree that for formation model it not likly pluto was a part of it.
    But we could count the astroid belt as #9 that didn't work out.
    makes ya wonder why it didn't and the others did.

  16. Oct 23, 2004 #15


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    Only planets are permitted to nominate other celestial bodies for the title 'planet'. Gravitational might makes right. If it orbits you, it is a moon, not a planet. If it is less massive than any moon in the system, the planet union will not recognize it. A political matter, to be sure.
  17. Oct 23, 2004 #16
    Phobos said:
    I read from a book that when Galileo flew past the asteroid Ida, the pictures it sent back revealed that it was orbited by a small moon.m So is Ida a planet or is it still a asteroid?
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