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Is Jupiter a planet>

  1. Apr 10, 2007 #1
    We debated on it, scientest were divided oner Pluto is a planet. I say I could be a special type. People are thinking it's an escaped moon or a Keiper belt object. I just don't see since Ceres an asteroid is being considered as a planet and it's far smaller.

    Then there's Jupiter. Aside from it's small size I just don't see how it coul d be a planet. Scientests recently determined Jupiter has no solid surface. It's just gas and maybe highly pressurized liquid at the core. This definetly doesn't constitute a planet since it has no rocky surface to land on. It's basically a giant bubble of gay and liquid. If you or your vehicale could tolerate the pressures you could go straight through Jupiter and not hve anything stop you. Just fly through the core and out the other side liek nothing was there. Tis is hard to link with all the other planets in my book. A planet has to have a surface to land on. Then there's Saturn the other gas supergiant. If it has no surface to wwalk or land on is it scientific and logial to call it a planet?WHat about Uranus and Meptune..........are they all gas bubbles?

    I find it funny there's all this fuss about a tiny rock but none ocer the giant bubble. If having no solid center cdoesn't disqualify you from being a planet then The Sun couldbe one if it orbited a star. Infact the say Jupiter would have been a star if it were just a few times larger.

    If you think about it it's all atmosphere and ocean. If they'd know this before they probrably wouldn't have called it a planet. SOme planets have/had no atmosphere so having an atmosphere definetly isn't a requirment.
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  3. Apr 10, 2007 #2


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    Having a solid surface is not a requirement for being a planet. At present, the IAU defines a planet as " A body that orbits the Sun, is large enough for its own gravity to make it round, and has 'cleared its neighbourhood' of smaller objects". Composition of the body is not considered.

    Ceres fits in a new designation of "dwarf planet" which is different from "planet".

    Actually, the usage of the name "planet" has changed since its inception. In its original language it means "wanderer" and refered to any permanent object in the sky that moved with respect to the stars. Originally, the Sun and the Moon were planets.
  4. Apr 10, 2007 #3
    Using that system, Neptune would not qualify as a planet. Neptune does not have enough gravity to clear Pluto out of its orbit. There is also the issue of exactly how clear an orbit must be. Earth hasn't cleared all of the debris out of its orbit either. The system is inconsistent.

    Personally, I think that any body meeting the following criteria whould be deemed a planet:
    This would exclude Jupiter, Saturn etc. It was also include the Earth's moon, many of Jupiters moons, many of Saturn's moons etc.
  5. Apr 10, 2007 #4

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    That phrase has a mathematical definition: The ratio of the mass of the body to the masses of other objects that potentially can collide with the body must exceed 10 (or 100). The exact value isn't critical because there is a three order of magnitude gap between Mars (the planet with the smallest planet discriminator) and Pluto. Neptune's mass is 8600 times that of Pluto. Neptune has cleared its orbit.

    Moons of a planet do not count as potential colliders as gravity will keep them from colliding. Moons of a planet do not count as planets, either. They are moons. (Specifically, a body is a moon of another body if the center of mass of the pair lies inside the larger body.)

    That this definition excludes as planets the most massive bodies that orbit the sun but allows moons of these bodies to be called planets speaks volumes against that definition.
  6. Apr 10, 2007 #5
    Weather it can clear objects out of it's orbit should not be criteria.

    So if Ceres is a drawrf planet isn't Pluto?
  7. Apr 10, 2007 #6


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    Yes, Pluto is now designated as a Dwarf Planet.
  8. Apr 10, 2007 #7
    If it's a dwarf plane it's still a planet.
  9. Apr 10, 2007 #8

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    Why not? This is your opinion. The IAU is of a different mind.

    The IAU clearly defined planet and dwarf planet as two distinct categories.
  10. Apr 11, 2007 #9
    Can anyone here say if it is possible for overlapping orbits to be near 100% stable? Say, if Pluto's year was exactly 3 times that of Neptune's.
    Many years ago when the Earth was considered the center of the universe, the Sun was considred a planet. Today, the Sun is almost thought of as the center of the Universe. I'm not saying that is wrong, but I think we should be less Solar-centric. I think we should be more concerned about what the body IS, than with what the body DOES.
    Last edited: Apr 11, 2007
  11. Apr 11, 2007 #10

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    Pluto's year is, on average, exactly 1.5 times that of Neptune. Pluto is locked in a 3:2 resonance with Neptune that keeps Pluto from being a collision threat to Neptune.

    No one in the IAU thinks the Sun is the center of the Universe. They do think that the Sun is very close to the center of the Solar System.

    The definition of "planet" reflects your thinking to some extent. A planet is something that orbits the sun (something it DOES) and has "cleared" its orbit of other objects (something it has DONE). The IAU always constrains "planet" to denote a body that IS roughly spherical.
  12. Apr 11, 2007 #11


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    Actually, its a celestial body that orbits the sun. Technically, this excludes Earth, as Earth is not a celestial body. They should just drop the adjetive.

    I agree. The strange thing about the definition of a planet is that takes into account both physical properties, orbital properties and what it has accomplished (clearing of its orbit). This is very unusual in the definition of anything. A tennis ball is a tennis ball, no matter where it is, whether it in play, or sitting in a box in someone's garage. But I can think of a few examples: pedestrian, motorist, etc. But even here, they are both subsets of humans. Its strange that there is no major catagory that distinguishes between objects like Mars and Jupiter, but that there is one that distinguishes between Mars and Titan.

    I'd prefer definitions that took into account only physical properties. In a binary star system, we don't disqualify the smaller star from being a star simply because it orbits a larger star.

    Asteroids and comets are distinguished from each other by their physical properties. An asteroid in a comet-like orbit is still an asteroid.

    Why should planets be any different? Why should Ganymede and Titan be excluded simply because they orbit a larger object?

    I like the OP's point. Mars, with its solid surface has more in common with Titan than it has with Jupiter. [Jupiter,Saturn, Uranus & Neptune] are simply a different breed of object than [Mercury, Venus, Earth, Luna, Mars, Titan, Ganymede, Europa...]. Why shouldn't the definition reflect this?

    Of course making a definition that reflected this would be straying far from the original definition of a planet as something in the sky that moves against the background stars. Mars and Jupiter aren't so different in this case. But neither are Vesta and Uranus, both objects that can barely be seen naked-eye by keen observers, and we find it necessary to distinguish between them.

    Maybe its time to abandon "planet" altogether as a scientific term. Restore it to its definition of an object that wanders with respect to the background stars. This will make it a useful term to amateur astronomers and the general public. Then we could come up with some new terms that are scientifically useful, based upon an object's physical properties. Just off the top of my head:

    World: An object with a solid surface that has enough gravity to pull itself into a sphere. (perhaps massive enough to have differentiated into layers)
    Asteroid: no change
    Meteoroid: A small asteroid with an arbitrary cut-off point (like a pebble is a small rock)
    Comet: no change
    Moon: a world, asteroid, or comet that orbits a larger world.
    Gas Giant: Objects like Jupiter, Saturn, Uranus, & Neptune (but we have to come up with a better name than Gas Giant)
    Brown Dwarf: object massive enough to fuse deuterium at some point in their existance.
    Star: object massive enough to fuse hydrogen into helium.

    And if we want to describe an object based on its motion, we could have subsets as well. Europa is a satellite world, just as a pedestrian is a walking person.
  13. Apr 11, 2007 #12


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    I agree (though you seem to go back on this in your next part...) The word "planet" predates science and never carried any useful scientific meaning before, so why bother now? So what if schoolchildren are learning something without scientific importance - your first astronomy class is as much a history class as it is a science class. Heck, if we want to be more scientific about it, we should stop using the concept of constellations; there aren't any hunters, warriors, queens, or bears up there!

    Pluto was discovered in 1930. Up until very recently, it wasn't known that there were more objects out there like it, but it was supected that there were (though it wasn't realized that there would be so many). Since the word "planet" carries no scientific weight anyway, there is no reason not to just close the category with Pluto and teach the word as an historical categorization. You can then just add the next headers in the intro astronomy book: "Asteroids", "Comets", "Other Objects". The moons of Jupier can be taught in the section about Jupiter and/or in the section about our moon. Since these "Other Objects" have all been found in a very short time, but a long time after Pluto, it makes logical sense to me to give them their own category.
    Last edited: Apr 11, 2007
  14. Apr 14, 2007 #13
    I'm not a physicist, so how does gas form a ball rather than a cloud without a solid core to begin with? Would Jupiter have to have started a an asteroid or something similar?
    The moons orbiting Jupiter have a definate diameter, and orbital speed, so from that could the gravity of Jupiter be determined? And from that, it's solid mass?
    On 1994 July 16-22, over twenty fragments of comet Shoemaker-Levy 9 collided with the planet Jupiter, what did the impact show about the makeup of the planet? Was debris ejected, or was gas sucked into a hole following the passage of an object?
  15. Apr 15, 2007 #14


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    That's a good collection of questions. Here's my guesses:

    Jupiter could just be a large cloud that collapsed upon itself. Too much mass in too little volume, with small relative velocities, just couldn't hold up.

    But maybe it does require an asteroid (or more likely a planetesimal) nucleus. Many theorize that Jupiter does have a rocky core.

    From the velocities of Jupiter's satellites (including the man-made Galilleo spacecraft), Jupiter's mass is known with great precision. Through observations, its size and hence volume, is well-known. If you know mass to great precision, and if you know volume to great precision, then you know density to great precision. From density, you can theorize what Jupiter is made of.

    SL-9... Explosions with fireballs larger than Earth were observed. Jupiter was left with what looked like black-eyes for months following the collisions. But I doubt anything escaped Jupiter.
  16. Apr 18, 2007 #15
    Terrestrial planets and Jovian planets. :)
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