Re: Call me simple
It seems to me that the current classification system for planets does not give enough detail as to what a planet is and would benefit from the addition of information about size and makeup at the very least.
This. 10,000 times this. There are some basic classifications that are informally used.
Brown Dwarf Star: not actually a star, this is a really large gas giant that outputs substantial heat but does not sustain fusion of hydrogen. Brown dwarfs can be extremely heavy and may well undergo hyrdrogen fusion in smallish amounts (in addition to the massive amounts of fission they are undergoing) but never quite "ignite" into burning balls of plasma in the sky. Can range from similar or smaller radius do superjovian to nearly the size of a red dwarf. May actually have a habitable zone, but many questions about radiation belts and magnetospherics remain.
Superjovian: large enough to output minor radiation and potentially notable heat to its inner moons but not large enough to be classified as a "brown dwarf star". Heat is primarily fission-based, megnetodynamic (depending on stellar flux) gravitational and remnants of its accretion. Can be absolutely huge planets, but don't start getting much bigger than Jupiter until they've gotten to be about 80 Jupiters in mass. Gravitational interaction with most moons will cause internal heating.
Jovian: Gas giants. They range from large but "fluffy" and not very dense planets (like Saturn) to several time Jupiter's mass. They they do not emit any noticeable radiation or heat past their roche limit, however, they are not massive enough for their megetosphere to deflect stellar radation completely outside their likely moon orbits. Thus most moons will pass through a belt of deadly radiation similar to the Van Allen belts here at Earth. Gravitational interaction with larger moon may cause internal heating of moons.
Neptunian: These planets have a roche limit that is far enough away from the planet that many planets this size will be capable of supporting ring systems. Planets are large enough to capture smaller terrestrial planets as moons. Gravity is high enough to hang on to hydrogen in the atmosphere. Expect ammonia in the atmosphere as well as water.
Superterrestrial: Planets larger than Earth but unable to hang on to hydrogen unless it is bound into a heavier molecule like water or ammonia. May have ring systems but highly unlikely. Unlikely to have captured moons. Can potentially be habitable. Very thin atmosphere compared to Neptunians.
Terrestrial: Planets just large enough to hang on to an atmosphere. Like all planets larger than it, silicates and metals form the planet's core/mantle/crust. Very thin atmosphere. Marginally habitable in that it is continually bleeding away lighter gases into space. These planets require life to be able to continually recycle molecules in order to have an atmosphere that contains lighter molecules that are critical to complex life.
*****Special case terrestrial: Metal Planet. Metal Planets can form very near a star where metals are hyper-concentrated in the core of a large gas giant as part of the regular formation process. The gas giant's atmosphere is then blown off (typically by the sun expanding to engulf the planet as a red giant for a billion years or so) and the metallic core is left behind, typically orbiting a dwarf star.
Subterrestrial: rocky planets that are too small to hold on to much of an atmosphere at all. Likely to cool after only a few billion years and not have much of a magnetosphere. May briefly be habitable. May briefly sustain a hydrosphere.
Dwarf: smaller than a subterrestrial. Not likely to ever have a hydrosphere. Not likely to ever have much of an atmosphere. (Minor outgassings and capturing of an inch or two of solar wind aside.) Dwarfs are separated from subterrestrials mostly because of density. They may be nearly as large as a subterrestrial, but they contain a lot more ices. Some dwarf planets at the fringes of a system may be mostly or all ices.
The big issues are drawing firm lines between the classifications. While in use by many astronomers, formal definitions require drawing arbitrary lines and this causes much consternation and debate.
Plus ca change...