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Re: Should Pluto Get Its Planethood Back?

by wjw (Priest)
on Mar 10, 2017 at 19:09 UTC ( #1184214=note: print w/replies, xml ) Need Help??

in reply to Should Pluto Get Its Planethood Back?


We constantly redefine our world based on new information. Things get re-classified. Hell, I am now classified as 'old'! :-)

Is Pluto a planet? Does it qualify? For an object to be a planet, it needs to meet these three requirements defined by the IAU:

  • It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet.
  • It needs to have enough gravity to pull itself into a spherical shape – Pluto…check
  • It needs to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule breaker. According to this, Pluto is not a planet.

...the majority is always wrong, and always the last to know about it...

A solution is nothing more than a clearly stated problem...

  • Comment on Re: Should Pluto Get Its Planethood Back?

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Re^2: Should Pluto Get Its Planethood Back?
by ExReg (Priest) on Mar 11, 2017 at 20:45 UTC

    The definition of planet the IAU came up with in 2006 was shortsighted. It was crafted to make sure that only bodies that formed from the accretion disk are planets, hence the third definition part. (By the third part, Neptune is not a planet since it has not finished clearing Pluto out of its path.)

    Their whole definition falls apart when applying it to the almost weekly announcement of new planets orbiting other stars (which also violates the first part.) Do we know if those new planets have cleared their paths? Do we know if they are all in their ecliptic planes? We don't even know if they are spherical (although I don't know how they couldn't be for the size we have been finding.)

    Why should it matter how a body got into orbit around a star? If we have a huge round ball orbiting, it is a planet. Doesn't matter if it is in the ecliptic or 90 degrees off. Doesn't matter if it was born there or left on the doorstep from a close stellar encounter. Should we do the same for moons? Jupiter has four moons and umpteen dwarf moons? Mars has no moons?

    I would rather say we have dozens of planets orbiting the sun than the current artificial distinction. Modify the definition above to replace sun with star in the first part and get rid of the third part completely. Then just live with the idea that there are still planets orbiting the sun waiting to be discovered.

Re^2: Should Pluto Get Its Planethood Back?
by Arunbear (Prior) on Mar 11, 2017 at 12:47 UTC
    The page I linked to earlier mentions that
    In the new paper, which the scientists plan to put forward to the IAU, the current definition is described as "technically flawed" - it only recognises planets as objects that are orbiting our Sun and not other stars. It also says the need for zone-clearing means "no planet in our Solar System" meets the criteria for planets.

    The authors argue the new definition is "geophysical" and based on the physics of the world itself.

    So e.g. if the positions of Earth and Pluto were swapped, the Earth would no longer be a planet according to the IAU's definition.

    The paper they refer to is short and readable.
Re^2: Should Pluto Get Its Planethood Back?
by chacham (Prior) on Mar 11, 2017 at 18:12 UTC

    We constantly redefine our world based on new information.

    But what's the point?

    When classification for the purpose of helping or understanding, reclassification helps. But what does reclassifying what a planet is help? Classification for the sake of classification doesn't seem to be very popular.

      But what does reclassifying what a planet is help?

      Reclassification becomes necessary when our thinking (or models, if you will) on a certain subject cannot jive or be sustained given the new findings. (Re)classification is going on all of the time. There is already substantial resistance to it since doing so means rerunning potentially thousands of experiments and rewriting a very many papers/textbooks on anything involving it. Imagine how many things had to be rewritten/done when Einstein's theories were verified!

      Let me be clear that there is no classification for its own sake going on. The sheer amount of work and potential embarrassment involved if your findings are unsound is enough to keep most people in the scientific community from doing virtually anything trivial.

      Celebrate Intellectual Diversity

Re^2: Should Pluto Get Its Planethood Back?
by pryrt (Monsignor) on Mar 31, 2017 at 22:52 UTC
Re^2: Should Pluto Get Its Planethood Back?
by Anonymous Monk on Mar 24, 2017 at 18:35 UTC
    You forgot one... a planet's orbit should be distinct from other planets' orbits... in other words, two planets' orbits should not cross paths... This is true for the other 8, but Pluto cuts across Neptune's orbit, which is a big no-no.

      in other words, two planets' orbits should not cross paths

      And why is that Pluto crosses Neptune's path? Why not the other way around? It's not like Neptune hasn't done this before.

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