The case for defining “major planet” and “minor planet” only based on mass.

I don’t have a firm opinion on exactly where to draw the line but going by mass seems like the best option since that is closest to the [IAU definition]( Here is one suggestion that would align with current classifications in the solar system.

Major planet: Any object that isn’t a black hole with mass of at least 2\*10^(23) Kg but less than 2.4 \* 10^(28) Kg

Minor planet: Any object that isn’t a black hole with mass of at least 5\*10^(20) Kg but less than 2 \* 10^(23) Kg

The “orbiting the sun” criteria is nonsense since it would mean exoplanets wouldn’t be planets, that’s very much nonsensical.

The “clear it’s orbit” criteria is arbitrary and depends on the orbit and distance from the sun, earth would stop being a planet if it would orbit sufficiently far from the sun.

The “hydrostatic equilibrium” criteria is problematic since that depends on the composition of the planet, rock is a lot harder to deform than water. Also how would that work for small gas planets?


**Why some satellites should be classified as planets**

Let’s say you have a binary gas-giant system orbiting a star, then which of them should merely be classified as a moon?

It’s probably best not to have the definition depend on position at all. A horse remains a horse even if we put it deep into the sea. A star remains a star even if it’s orbiting a larger star.

**Radius vs mass as the determining factor**

Radius is easier to measure but it’s further away from how we currently classify whether or not an object is a dwarf planet (having enough mass to achieve hydrostatic equilibrium).

It would be funny to set the minimum radius to 1200 Km, that would still exclude pluto while including [Ganymede]( [Titan]( [Callisto]( [Europa]( [Triton]( making the maximum number of people upset.

Note that regardless of whether or not you classify moons as planet you should probably use the following to determine what actually counts as a moon:

>It’s a binary system if the barycenter of the objects’ orbits is not within one of the objects. That’s the line between binary system and planetary system. In that case, neither object is a moon and they co-dominate their orbit, so they’d both be a part of a binary planet system.