It doesn’t, but moons don’t say much about what a world is like; there are terrestrial planets with and without them, other Kuiper belt objects with and maybe without them, and asteroids with and without them. The point is that the object itself is probably akin to Pluto, having formed in the same region and later captured by Neptune. To those of us interested in their nature, it’s the natural comparison.
Pluto was not suddenly dropped from a list, any more than mushrooms were suddenly expelled from the plant kingdom; its position was eventually changed after a long time learning more about things. I know you say you’re familiar with the reasons, but dismissing them as “technical specifications” makes it sound like you don’t really understand my perspective; so I’m hoping you’ll indulge me in repeating some things you know in order to explain myself.
Without making any attempt to classify them, here are all the largest objects in the solar system, save maybe a few that were discovered in the last few years; I’ve arranged them by mass in kilograms on a logarithmic scale:
30 – Sun
27 – Jupiter, Saturn
26 – Neptune, Uranus
25 – Earth, Venus
24 – Mars, Mercury
23 – Ganymede, Titan, Callisto, Io, Moon, Europa
22 – Triton, Eris, Pluto, Sedna, Haumea, Makemake, Titania
21 – Oberon, 2007 OR10, Quaoar, Rhea, Iapetus, 2002 TC302, Charon, Ariel, Umbriel, 2005 QU182, Dione, Ceres, 2007 UK126, Orcus, Ixion, Tethys, 2005 UQ513, Varuna
All of these are large enough to be round, in contrast to the 20 bracket which includes objects like Vesta and Pallas that aren’t. The sun is of course a star made of plasma; the next four are primarily gaseous, and the others are variously rocky or icy.
From Ganymede downward, many of these don’t orbit the sun directly, but instead circle larger objects. The others in this size range are all part of two belts containing a multitude of objects of various sizes, with generally eccentric and overlapping orbits. Both kinds probably formed there, but with some interchange; as I said, Triton probably started as one of the outer belt objects before Neptune captured it.
The only exceptions to these categories are the eight objects from Jupiter down to Mercury. These go around the sun directly but are not part of any belt, with orbits cleared of all but very small debris. And this is probably an important part of how the system formed; they are not all expected to have formed in place by any means, but not from belts or left-overs like the others.
If you care about knowing your place in space, this is what it looks like. I certainly care, and so I don’t consider the words we use to describe it mere semantics, but rather think it is important to choose them to paint this picture as clearly as possible. And to me the best way to do that is categories: an inner and outer belt, satellites, and eight larger objects that define regions on their own.
I do appreciate history, but that involves recognizing it as something separate from our current understanding and shifting over time. When Pluto was discovered, Disney thought to name a character after it. Holst preferred to stick with tradition and mythology, and left his planets suite with just the seven besides Earth. Even these were not all known to the ancients, to whom the god Uranus was the sky itself not an object in it. Before plutonium came cerium and palladium, named for new planets that proved asteroids. What reason is there besides nostalgia to enjoy all these centuries of learning, except for the last one?
I assure you I’m very excited for New Horizons. I’m excited because it’s the first time we’ve seen Pluto up close, and are bound to discover lots of things about it. And I’m even more excited because one of those discoveries is that it is part of a vast unexplored belt, and this will be the first close look we have at anything there. If you’re stuck thinking of it as the one final planet, a peculiar cousin of the eight places we’ve been, you’re missing out on this greater context.
I do embrace Pluto, for what it is and what we can learn about it, and I’m frustrated by the claim we can’t do that and embrace what we are learning about its actual place in the solar system. It amazes me every time somebody talks about the nine planets, as if that was a more inclusive take at what’s out there. It seems to me an obvious mistake, and one that short-changes Eris, Sedna, Haumea, and all the other new friends we could be making.
