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For a body to qualify as a planet according to the IAU definition it must have "cleared its neighborhood". What evidence is there Mercury indeed cleared its neighborhood? Perhaps it migrated there afterwards, when the neighborhood had already been cleared. Does the Grand Tack hypothesis impact our definition of the inner planets as planets?

https://en.m.wikipedia.org/wiki/IAU_definition_of_planet

See Grand Tack Hypothesis.

The present definition of a planet is vulnerable as it seems connected to a model of formation of the solar system. The answer below states that in practice an operational definition used that I believe is adequate.

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  • $\begingroup$ Can you explain that the "Grand Tack Hypothesis" is? $\endgroup$
    – WarpPrime
    Nov 15, 2020 at 13:17
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    $\begingroup$ If we take the 2006 definition literally (which seemingly noone does) no planet 'cleared its neighbourhood'. en.wikipedia.org/wiki/List_of_Mercury-crossing_minor_planets $\endgroup$
    – John
    Nov 15, 2020 at 13:59
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    $\begingroup$ That's because the IAU are scientists not lawyers. $\endgroup$
    – James K
    Nov 15, 2020 at 16:38
  • $\begingroup$ @JamesK Scientists could have easily come up with a definition that would exclude Pluto, include 8 planets, and not state anything different or contradictory. $\endgroup$
    – John
    Nov 15, 2020 at 17:02
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    $\begingroup$ I know. They did. $\endgroup$
    – James K
    Nov 15, 2020 at 17:23

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One calls Mercury a planet because it doesn't share its orbit with any other bodies of comparative size. The fact that it is in its orbit means that that orbit hasn't been cleared by another body (for if it had been cleared by another planet, then Mercury would have been cleared too.)

In practice one doesn't look at "How" an orbit is cleared, only if there is one body that dominates the orbit, or if there are many with no one body dominating. So Mercury is clearly much larger than any other bodies in roughly 88 day orbits. But, (for example) Ceres does not dominate its orbit, as it shares its orbit with Vesta, Pallas and Hygeia and many other asteroids.

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    $\begingroup$ Indeed, but even 'objects of comparable size' would be vague. Ceres is the largest main belt object, and by far the most massive (mass is here more important than size) at 1/3 of the main belt's mass. However, you could argue that Jupiter dominates Ceres' orbit, just like Neptune dominates Pluto's. Still, Eris (the most massive known object beyond Neptune) would have to be a planet even if we stretch the 2006 definition. $\endgroup$
    – John
    Nov 15, 2020 at 13:56
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    $\begingroup$ @my2cts perhaps it's semantics, but "comparable size" isn't how I would say it because with our 8 known planets, nothing even comes close. There's orders of magnitude difference by mass between the planet and the next largest object in the orbital region. A planet's orbit is defined by one massive object and nothing in the area that even comes close. Granted, the further away from the sun and the longer the orbital periods become, there might be some variation in that, but for the 8 known planets, it's one big object and everything else is tiny by comparison. $\endgroup$
    – userLTK
    Nov 15, 2020 at 16:04
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    $\begingroup$ @userLTK Except when you start to question why massive moons larger than Mercury (Titan and Ganymede) aren't considered planets just because they orbit another planet rather than a star. :-) $\endgroup$
    – John
    Nov 15, 2020 at 16:59
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    $\begingroup$ -1 because all the "we call" and "we don't look" language is unsupported with authoritative sources. In Stack Exchange we supply information, we do not generate it ourselves. We are not the authority here. We are merely a service provider. Currently there is no way to tell if this is authoritative and correct or purely guesswork because you don't cite any supporting sources. This is Stack Exchange not Quora. $\endgroup$
    – uhoh
    Nov 17, 2020 at 3:41
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    $\begingroup$ Is it possible to state why you believe that Clearing the neighbourhood does not apply to Mercury for purposes of classification. Since the question is "Did Mercury clear its neighborhood?" and that requires a boolean response, I think that the OP requires more. $\endgroup$
    – uhoh
    Nov 17, 2020 at 3:45

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