Determining the Definition of “Planet”

1. It only pertains to the local solar system. It was never designed with other solar systems in mind.
2. It does not specify that stars (and brown dwarfs) are disqualified from being planets. Everybody agrees that stellar objects which are large enough to initiate nuclear fusion and emit light should be classified as stars, rather than planets. A corollary is that it’s not possible for a planet to be bigger than a certain size. If a planet is too big, then it will become a brown dwarf or a star, and thus not a planet.
3. A good definition should be more explicit about what counts as a satellite vs a planet, and whether there can be overlap between the two. The IAU’s definition for “planet” takes the definition for “satellite” for granted, in the clause of the definition’s lead sentence. This is partially because the IAU definition never considered planetary masses which are similar enough in mass to orbit each other.
4. It would’ve been better if it stated in a separate paragraph which planetary bodies should be considered planets, dwarf planets, satellites, etc. By stating those clarifications as footnotes instead, it makes the resolution appear more prescriptive, rather than descriptive, as it should be. Such important clarifications should be made paragraphs anyway because they contain such important information. Another problem with stating this information in a footnote is that it tempts people to read the footnote before reading the definition. People should read the definition first instead, since it should (ideally) be clear which planetary bodies count as planets, dwarf planets, etc, after reading everything.
5. A good definition should explicitly consider the mass and diameter of planetary bodies. In this solar system, there are moons which are bigger than planets, but they aren’t considered planets. For example, Ganymede and Titan are both more massive and larger than the diameter of the planet Mercury, yet they are considered moons, rather than planets. It may be appropriate to have some generic word which can refer to these large moons, even if they technically aren’t planets.
6. Since Pluto sometimes orbits Neptune rather than the Sun, I feel that it should be noted that situations like this should probably disqualify objects like Pluto from fitting a standard definition of “Planet”. Why wasn’t this fact mentioned in Resolution B6?
7. The meaning of “cleared the neighborhood around its orbit” is not explicitly defined inside the resolution (although it may have been defined inside other preceding resolutions and/or a different part of the resolutions document, which I have not seen). In any case, it’s very important that “cleared the neighborhood around its orbit” should be clearly defined and undisputedly unambiguous within the document. This is especially because it’s the hardest criterion to understand out of the three criteria that the IAU proposed. “Cleared the neighborhood around its orbit” was the key criterion which excluded Pluto from being classified as a planet.

I propose that the official definition of a planet should be: A celestial body that is:

1. Less massive than a star or brown dwarf, such than it cannot initiate nuclear fusion.
2. Has sufficient mass for its self-gravity to overcome rigid body forces, so that it assumes a hydrostatic equilibrium (nearly round) shape.
3. Has cleared the neighborhood around its orbit.
   1. In celestial mechanics, “clearing the neighbourhood” (or dynamical dominance) around a celestial body’s orbit describes the body becoming gravitationally dominant such that there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence.
   2. A planet is the largest and most massive object in its orbit.
   3. If a planetary body is both large and massive enough to ensure that the orbital center of mass is contained within the planet’s volume, then that planetary body is indisputably the “planet” of its orbit.
   4. If there is another massive object(s) in a planetary system which is/are large enough to make the orbital center of mass go outside the planets, then the biggest objects in the planetary system should be considered to be “co-planets” of each other.
   5. All the objects which weren’t massive enough to make the orbital center of mass go outside the planets should be classified as “satellites”.
   6. A planet is not gravitationally bound to anything larger than itself, other than its star(s).
4. In orbit around a star or co-orbiting star system.

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By this definition, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune would all be considered to be planets of the Sun’s solar system. Pluto would be excluded from being classified as a planet under this definition, since:

• There are other celestial bodies of comparable size besides Pluto within its neighborhood, like Eres/Xenes, among others, i.e. it fails criterion #3a.
• Its moon Charon is large enough to make the barycenter go outside of Pluto’s diameter, i.e. it fails criterion #3d. which would make it a co-planet of Charon.
• It also occasionally orbits Neptune, i.e. it fails criterion #3f.
• Pluto and Charon would both be classified as co-dwarf planets of each other.

Ceres would be classified as a dwarf planet, rather than a planet under this definition, since there are many other asteroids of comparable sizes within Ceres’s neighborhood, i.e. it fails criterion #3a.

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Dwarf planets are usually less massive than planets, but whether they can clear the orbit sometimes depends on their distance away from their star. So, a world would not clear its orbit and thus be dwarf planet if it’s far enough away from its star, but would clear its orbit and thus be classed as a planet if it’s close enough to its star.

Planets, moons, round moons, and dwarf planets could each be considered “worlds”. A moon that has sufficient mass for its self-gravity to overcome rigid body forces, so that it assumes a hydrostatic equilibrium (nearly round) shape would be classified as a “round moon”. A round moon that orbits a planet is obviously a moon. Round moons are massive enough to be round, whereas moons in general may not be massive enough to be round, in standard English parlance. A moon that is not massive enough to be round should be called a “non-round moon” or a “minor moon”.

Under this classification system, Ganymede and Titan would both be classified as moons, rather than planets, despite being more massive and larger than the diameter of the planet Mercury. Nevertheless, I believe that their classification as moons is warranted, since they clearly fail to satisfy criterion #3 and each of its explicit subcriteria for being a planet.