Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Sedna must be a planet (at least for now) according to new definition?

  1. Aug 24, 2006 #1


    User Avatar


    I saw the definition of a planet -- basically anything differentiated that has cleared everything in the area and orbits the central star.

    Pluto doesn't count because it's in the middle of a belt.
    Ceres doesn't count because it's in the middle of a belt.

    Sedna -- well, Sedna's an interesting case. You see, its aphelion is INSIDE the Oort Cloud and its perihelion is OUTSIDE the Kuiper Belt.

    Even astronomers aren't sure how it got there. This probably means there isn't much out there.

    So we have a large object in a zone which appears (at least for the time being) to have nothing in it.

    This makes Sedna a planet -- at least temporarily. It's basically in the same position Pluto was in 1930 -- it's in an area where there are no other known bodies. Perhaps we'll find some more Sednas in the future, in which case Sedna can be downgraded. But for now -- it sure fits the definition! The definition says nothing about how eccentric the orbit is.

    What's wrong with this reasoning?

  2. jcsd
  3. Aug 24, 2006 #2
    I don't see any problem with the reasoning, which is the big problem with this definition...it is WAY too vague to be useful. If you were to give an alien these rules, plop them in our solar system, and have them tell us what should be classified as a planet and what should not, I'd bet you we'd have an answer very different from 8!
  4. Aug 24, 2006 #3
    Were going to need a subforum about the defiention of a planet now...
  5. Aug 24, 2006 #4


    User Avatar
    Science Advisor

    We don't really know if that area of space is cleared out. In fact, many astronomers view Sedna as the "frist" Oort Cloud object to be spotted. And even if the area is clear, Do we really believe that Sedna cleared it? I think it more likely that that this body was perterbed and "fell into" this empty region (if it is empty).
  6. Aug 25, 2006 #5


    User Avatar

    You also have to consider the following: Sedna's orbit is highly eccentric, so the rule about it clearing out all of the mass in its vicinity doesn't make sense. If an object is in a circular orbit, it's fairly obvious what its neighborhood is. But if it's eccentric, the idea of a neighborhood makes new sense. Yes, its orbit is well defined. But given astronomical time, the orbit will precess like the hour hand on a clock and influence entirely different areas of the Solar System. Rotate Jupiter's orbit by 90 degrees and you still get the same orbit, more or less. Rotate Sedna's by 90 degrees and the new orbit is completely different.

    For something this eccentric to truly "clear everything out", it would have to knock out EVERYTHING at ANY distance between perihelion and aphelion, which makes no sense.
  7. Sep 7, 2006 #6
    Also Pluto crosses Neptune's path. Neptune, thus, has not cleared all the debris out of its orbital path and should not be considered a planet either. I think there are lots of Earth orbit crossing asteroids as well, no?
  8. Sep 8, 2006 #7


    User Avatar
    Science Advisor

    For the fifth time or so: No it doesn't!
    Pluto's orbit is tilted compared to the ecliptican, and its orbit never crosses Neptune's. In fact the orbits are several AU appart at their closest points.
  9. Sep 8, 2006 #8


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Sedna is a planette.
    Last edited: Sep 8, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook