This is very similar to an article posted on kuro5hin in 2006 called “So… what is a planet again?”. Unfortunately kuro5hin is gone and I can’t find any mirror of that article :(
It used the simpler definition:
- Planet: rocky/round and with significant atmosphere
- Moon: rocky/round without significant atmosphere
- Gas giant: self explanatory
So titan would be a “planet of saturn”, earth would be “a planet of the sun”, mercury would be “a moon of the sun”, etc. I think this is pretty elegant and nice.