Did Mercury clear its neighborhood? For a body to qualify as a planet according to the IAU definition it must have &quot;cleared its neighborhood&quot;. 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.
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 &quot;How&quot; 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.