Space age research shows that the Solar System’s members are all cousins. They started with the same ingredients in the same nebula, and underwent related processes.
Their mass, and how hot they got inside, govern which processes each object underwent, making them the way they are today. So my graph plots mass versus how hot they got, arraying characteristics that are all talking about the same things.
Some of the old categories are distinguishable, and some are not. Comets (retaining original ices) all plot left of “the water’s edge”. Meteorites are all the small things at the bottom. Stars shine at top right.
But “planets” includes some objects that are physically like brown-dwarf almost-stars, other objects that are like the 7 big moons, and one object scarcely distinguishable from comets. “Asteroids” now have known borderline-cases with comets, meteoroids, and moons; planetologists have long suspected that small moons are captured asteroids and comets, and not original equipment.
Tectonics, subduction, and volcanism only occur on a few differentiated objects. These processes require a rigid (solid, cooled) surface, overlying a warm, fluid interior. On my graph, these conditions occupy a small zone: the smaller planets, and larger moons and asteroids.
Everything above that zone (more massive) has stayed fluid through the present, so they have no crust on which to show tectonics, subduction, or volcanism.
Everything below that zone (less massive) is so small it lost heat almost as fast as it gained heat, and probably never melted, differentiated, and formed a solid crust over a liquid mantle.
Everything left of that zone (colder) never melted and differentiated, so there was no warm fluid to drive tectonics, subduction, or volcanism.
And no object lies right of that zone because anything that hot is so massive that it plots higher on the graph.
excerpt from What Your Astronomy Textbook Won't Tell You, pp 68-70.
