Step 1: Define the minimum dominant mass (MDM) of a solar system

The mass of the smallest rounded celestial body in orbit around a star (or stellar remnant) that has cleared the neighborhood of [or is dynamically dominant in] its orbit using Jean-Luc Margot’s planetary discriminant (where ? ? 1).

For example, for our solar system Mercury is the MDM; in the solar system known as Kepler-37, the MDM is Kepler-37b (which has a diameter slightly greater than Earth’s moon). See the following link for a more technical explanation of Margot’s practical planetary discriminant:

http://mel.ess.ucla.edu/jlm/publications/Margot15.aj.PlanetDefinition.pdf.

Step 2: Define the term planet

A celestial body that…

(1) orbits one or more stars or stellar remnants;
(2) is a gravitationally dominant member of its solar system, defined as follows:
(a) has cleared the neighborhood of [or is dynamically dominant in] its orbit (e.g., Margot’s ? ? 1)
OR (Note: Skip 2b if 2a is already fulfilled, for example, to expedite exoplanet classification.)
(b) has a mass >= the MDM of its solar system;
(3) has a mass below 13 Jupiter masses, a nominal value close to the limiting mass for thermonuclear fusion of deuterium.

With this definition, as long as Earth and Jupiter orbit the sun directly, they will always remain planets regardless of their hypothetical location in the solar system (e.g., a “remote” Jupiter that orbits in the Oort Cloud or a “remote” Earth” that orbits at 100 AUs from the the sun). And with this definition, you don’t set an arbitrary cut-off point for planethood for all other solar systems at Mercury but instead use a contextual cut-off point for planethood unique to each solar system (as in the case of Kepler-37).

Everything less than the MDM will be a dwarf planet or small solar system body (SSSB, or sub-planetary mass object) so there won’t be any more Jupiter-like planets potentially mislabeled as “dwarf planets” because of their given location within a solar system. Rogue planets and large rounded satellites will remain separate categories under the classification of planetary mass objects (PMOs).

In summary, planets are by all rights the dominant players of any given solar system (after their parent star(s) of course). Dwarf planets, large rounded satellites, rogue planets, and SSSBs are, for various reasons, not dominant players of solar systems.

The way the physics works, Neptune controls Pluto’s orbit & period, causing it to speed up or slow down slightly when it wanders too far away from the 3:2 ratio. This ability to control the other objects in its neighborhood qualifies Neptune as a planet. (It doesn’t have to “eliminate” all objects around it.)

This same principle applies to the “Shepherd Moons” of Saturn that keep some of the tiny rings formed. Over time, they seem to play tag with each other.

This is a common thing with large orbiting bodies, so there are other planetary resonances, and most of the moons of Jupiter & Saturn also have resonances, etc.

Thank you, Brainstorms. That certainly answers the question. And I don’t want to belabor this, but can you tell me if there’s any scientific reason why there are 3 Neptune orbits vs. 2 Pluto orbits, precisely? By “precisely,” I mean it certainly can’t simply be a coincidence. I wouldn’t be surprised if there’s no well-defined answer to this.

This resonance “passes the test” regarding clearing an orbit (otherwise no planet would be a planet). Bodies with objects orbiting around Lagrange points also pass the test.

http://www.orbitsimulator.com/gravity/articles/pluto.html