Or on demoting the Gas Giants, reinstating Pluto, adding major moons
Okay, you can see the astronomer’s point of view.
But from “a truly human point of view”, most of us are interested in places that can be imagined as “worlds, theaters for human life”, even if the living conditions are much, much less comfortable than we are used to, even if we are talking about spartan, and possibly very temporary outposts or one time exploratory visits. Even if, we may add, we are talking about proxy human visits through the eyes of robot rovers, robot aircraft, robot balloons, etc., that can transport us to these alien landscapes, as opposed to distant views from orbit.
Worlds are places humans can conceivably experience first hand.
So let’s exclude worlds without a hard surface, or world’s on which the surface atmospheric pressure exceeds 100 times what we are used to on Earth.
That standard excludes Jupiter, Saturn, Uranus, Neptune – and is craftily drafted to allow Venus to make the cut. Aerostats hovering just below the Veneran cloud deck at tolerable atmospheric pressure levels, may provide future tourists a real glimpse of Venus’ overheated landscapes.
Our definition includes any world with enough mass to force itself into a spheroidal shape. Even Earth, slightly flattened at the poles, is not a true sphere!
That gives us the following list of worlds, grouped by distance from the Sun:
3-4 Earth, Luna
6-8 Ceres, Pallas, Vesta
9-12 Io, Europa, Ganymede, Callisto
13-19 Mimas, Enceladys, Tethys, Dione, Rhea, Titan, Iapetus
20-24 Miranda, Ariel, Umbriel, Titania, Oberon
26-27 Pluto, Charon
and other “Plutonians”
“Planet” – a word that can include Jupiter and Mercury in the same category, is a bit contrived. It includes only objects which orbit the sun directly. But what difference does that make? Luna and Europa orbit the Sun just as surely, admittedly while also orbiting Earth and Jupiter respectively. Phenomenologically, there is no difference. Luna and Europa and other major satellites enjoy sunrises and sunsets just as do Earth and Jupiter.
To the public, to the imagination of the would be explorer, traveler, tourist, trader, diplomat, and on and on, Ganymede is a world, Jupiter is not. Titan is a world, Saturn is not, and so on.
So let the astronomers gloat in their victory over the public. Their victory is hollow. The emperor, empress, prince, and princess (the gas giants) have no clothes. Yet they do fill an important role; they create communal gravity wells, making it possible to collocate mulitple moon-worlds in the same solar orbit, very handy for transportation to and fro!
Let’s keep the Solar System open. May the human viewpoint prevail!