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Mercury is 15 329 km in circumference. Pluto is 7 232 km in circumference. Callisto is 15 144 km in circumference.

So why is it that Mercury is considered a planet and Pluto was once considered a planet, but Callisto is not considered a planet?

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    $\begingroup$ I would suggest you to ask this type of stuff on astronomy stack exchange . Because this site is solely for physics purposes. $\endgroup$ – David Oct 25 '20 at 3:31
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    $\begingroup$ Because someone set up a definition. The question has been beaten to death on Astronomy SE. See also astronomy.stackexchange.com/q/27673 $\endgroup$ – ProfRob Oct 25 '20 at 8:17
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    $\begingroup$ @RobJeffries It has been beaten to death on Physics.SE as well. Personally, I would rather see all questions along the lines of "why isn't X a planet?" closed as off-topic, both here and at Astronomy.SE, precisely because these questions have been beaten to death, and also because diehards who for emotional reasons do not like the IAU definition will not accept an answer that is contrary to their diehard emotional position. (And that would include Alan Stern, for whom I have lost a lot of respect.) $\endgroup$ – David Hammen Oct 25 '20 at 15:04
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    $\begingroup$ Why is this question opinion-based? Every question in this post can and has been answered with facts. If anything, it should be closed as duplicate... $\endgroup$ – Jonas Nov 17 '20 at 16:48
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    $\begingroup$ The OP seems to think that the definition of a planet depends on size. It does not, as stated in the answer. But the OP never said that the definition of a planet should depend on size. This question is not opinion-based. I have suggested an edit to the question to have a more relevant title and to make the body clearer. $\endgroup$ – Brian Drake Nov 29 '20 at 13:20
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Let's take a look at how a planet is defined. According to IAU, it is

a celestial body which:

  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
  3. has "cleared the neighborhood" around its orbit.

(IAU definition of planet on Wikipedia and the original Press Release by the IAU)

As you can see, the size of the object is not relevant, although it somehow relates to the object's mass in 2.

Why is Mercury considered a planet at 15329km in circumference when Callisto is not at 15144km?

As we saw before, the size doesn't matter. Callisto does not fulfill the first requirement to be a planet - it is not in orbit around the sun, but is a satellite of Jupiter. This means that it isn't a planet, but a moon.

Pluto is 7232 km in circumference... If it was once a planet, why not Callisto?

Pluto was considered to be a planet because it was a relatively large body orbiting the sun - unlike Callisto, which was orbiting another planet. However, in 2006, Pluto was deprived the status of planet when the IAU set up the definition above. Pluto is now considered a dwarf planet which fulfills every requirement except 3. and is not a satellite of another planet.

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