ASTRONOMY FUNDAMENTALS - Chapter 6, Exercise 1 Solution
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Applying the IAU's Three Planet Criteria

PROBLEM
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A newly discovered object orbits the Sun and is round due to its own
self-gravity, but shares its orbital region with many other similarly
sized bodies it has not cleared away. Classify this object using the
IAU's three real criteria, and explain which one it fails.

SOLUTION
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Check the object against each of the three real IAU criteria in turn:

    Criterion 1 - "Orbits the Sun": SATISFIED
    The object is stated to orbit the Sun.

    Criterion 2 - "Hydrostatic equilibrium (round shape)": SATISFIED
    The object is round due to its own self-gravity.

    Criterion 3 - "Cleared the neighborhood around its orbit": FAILED
    The object shares its orbital region with many other similarly
    sized bodies it has not cleared away - this is the direct opposite
    of having cleared its neighborhood.

ANSWER: This object is a DWARF PLANET, not a full planet - it satisfies
the first two criteria but fails criterion 3, the "cleared its
neighborhood" requirement.

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WHY THIS WORKS AS AN ANSWER

This is exactly the real situation Pluto itself is in, as covered in
the chapter: an object can be round and genuinely orbit the Sun while
still failing to qualify as a full planet, purely because it shares its
orbital region with other comparable bodies rather than gravitationally
dominating and clearing that region. The IAU's own three criteria are
applied as an all-or-nothing test - failing even one of the three
(here, specifically the third) is sufficient to place an object in the
dwarf-planet category instead of the full-planet category, regardless
of how well it satisfies the other two.