Image Credit: NASA, ESA and G. Bacon (STScI)
A long, long, time ago in a place not so far away (astronomically speaking that is) the IAU stripped Pluto of its planetary status & created a new class called dwarf planets.
Although I was not upset with the classification, I was irritated by the ambiguous 3rd planetary rule which states:
A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
Note: Emphasis is my own
By this definition Jupiter is not a planet due to the Trojan asteroids trapped in Jovian giant’s Lagrange points (L3 & L4, respectively).
Instead of adhering to a rule that was created to prevent school kids from easily memorizing the planets within the Sol Star System (I kid, I kid!), why not adopt Physorg’s logical standard instead‽
- planet: is a celestial body that a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium – nearly round shape, b) has a differentiated interior as a result of its formation c) has insufficient mass to fuse hydrogen in its core, d) does not match the definition of a moon.
- minor planet: is a planet with a mass less than one Pluto mass and does not match the definition of a moon.
- inter-Stellar (minor) planet: is a (minor) planet that is not gravitationally bound to a stellar object.
- binary (minor) planet: is a celestial body that is orbiting another (minor) planet for which the system’s barycenter resides above the surface of both bodies.
Physorg’s rationale not only removes ambiguity but is also an “upgrade” from prior classifications as it preserves Ceres’s status (as a non-asteroid) while recognizing Pluto’s Planethood (Eris’s too!) without adding confusion.
Pluto is a unique amongst worlds orbiting our Sol Star system & despite it’s current demotion is beloved by millions of children living upon planet Earth.
Welcoming the wonderful world of Pluto back into the planetary fold will not only enrich the diversity of our star system, but allow our star system to reach the coveted metric number of recognized planets: 10