Calc'd and written by Jon K. Hart.

I originally placed Pluto's moon Charon (Sharon?)--as well as Luna--among the planets... for three reasons. Two good, one not so good.

There is a continuum of sizes among the orbiting bodies of the solar system. We distinguish planets from planetisimals (also called asteroids --tho technically incorrect) by a convenient gap between their sizes. Now Pluto has confounded that. It's still a good bit more massive than known 'tisimals, but not by the large margin we had before. Worse, it's a whole lot smaller than Luna: only one-sixth its mass!
. . How can we still call Pluto a planet? If it were found today, it would not be. They called it a planet only because they were looking for one in that area at that time, and couldn't tell its size. It was found only by accident, while looking for something else. It was so hard to find that there could be many more like it, and not necessarily farther away. (Tho the next found was farther.)
. . Must we label it at all, anyway? Yes, because we understand things better in categories. (Not a bad thing when/if there are big differences between categories.) We make a useful distinction between "gas giant" planets and "terrestrials" because there's an enormous gap in size between Uranus and Earth. (Note relative volume, and relative mass.)
. . Well, we may go along with the nomination because of inertia, but there's hardly a gap in sizes between Planet and 'tisimal any more, so it's a gray area, so let's just play with it.
. . There comes a time --when new data comes in-- to back up, drop your previous viewpoint, and look again. Come to think of it, in the formerly convenient gap between Luna --the smallest planet-- and Ceres-- the largest 'tisimal... lies Pluto, and it is only barely closer to Luna's size! There are six moons bigger than Pluto! There are gaps, all right, but one occurs on either side of Pluto's size! Worse, those two gaps are small. What do we call it?
. . And there may be more bodies around its size to classify. There's plenty of room out there. And not all the 'tisimals are between Mars and Jupiter.
. . Now, where does that leave Charon? A moon? A co-planet? Well.... You notice that one of the calc lines in my spreadsheet [write for an attachment in a message] figures the location of the center of gravity (barycenter), based on center-to-center distance and mass. This is one of those "two good reasons" I spoke of. But it's the lesser one.
. . One criterion for a "planet-pair" that seems reasonable is: the center of gravity of the pair occurs outside (above) the surface of either. It used to be that we thought that the Earth-moon pair was the most closely matched in size, tho it misses this criterion of "pairdom".
. . Pluto/Charon has taken the most-closely-matched distinction now. But if Charon is one of a planet-pair, is it a planet too?

I'm going to back out of the hypothesis a little. There are several 'tisimals (Pallas, Hector, Victoria, Herculina...) that have "moons". Many of these pairs must be close enough in size to fulfill my first criterion: a center of gravity external to both bodies. Obviously, that alone does not elevate them to planet status. Another criterion might be whether the body is larger than a real moon --the largest, i.e. Ganymede. But that disqualifies Mercury too!

[A diversion: are Pluto and Charon remnants of a collision? Will photos show matching faces, where the two broke apart? (Re-rounded by gravity, of course.) Or jumbled surfaces like Miranda's? (It was broken apart and fell back together.)

Is Charon's gravity small enough so that nearby pieces may have hit lightly enough that they'd be on the surface? I estimate Pluto's escape velocity at around one kps, still enough to smash falling things to vapor, because escape velocity would equal the minimum hit-speed. But perhaps besides Charon's light gravity (maybe .6kps; I need that formula!) it had a temporary atmosphere of ices vaporized by the collision. That would break the piece's fall a little more, as would an angled fall. If so, (I'm tryin' hard here...) it might skip like a stone on water.)

A sight: a jagged boulder of ice that soft-landed that way in a shallow pond of melted ices. The ices soon froze as the planet cooled, then the boulder frosted over as the atmosphere froze back out. In such extremely low gravity, the frost might be super-scale, like the fivefold facets of water-snowflakes enlarged a hundred or a thousand times. And what pattern does a frost of nitrogen gas take? A flake of nitrogen snow?

These would be acceptable reasons for a trip there by an early probe, but not just because it's the last "planet"... because it's not one. Or two. A Galileo-type moon-hopping mission to Neptune would probably be more rewarding.

On to reason number two for classifying Luna as a planet. The concept of the Tug-of-War value comes from Asimov's fascinating (if old: 1975) "Asimov on Astronomy" book. A must-have. The Tug ratio describes and evaluates the relative pull of the sun and a planet on that planet's moon. (The formula is Newton's, not Kepler's.) Note Luna. This is reason number two for giving this "moon" a "planet" definition. This is the more valid criterion. It applies only to Luna; the first applies only to Charon.
. . Another "however": If Pluto and Charon were at Earth's distance from the sun --here we go again-- Charon would be pulled almost three times as much by the sun (.3486) as Pluto would. By that criterion, Charon would have planet status. Are we back where we started? Nah. Still too small. It's just a point of interest: the Tug-ratio--and thereby the "planet" nomination--depends on an accident of location, too: distance from the sun. Also: if Earth/Luna was at Mars distance, the tug-ratio drops to 1.056! That's just over the 1.0 line. Then we'd have to call it a moon!
. . Asimov (as usual) put it well. Luna's "orbit is everywhere concave to the sun". Besides, it "does not revolve in the plane of Earth's equator, as would be expected..." but in the ecliptic, as "would be expected of a planet."
. . Yet we'll probably always call it a moon, just as we say that the sun is rising, not that the Earth is turning toward it, which is the reality.

That .46 ratio means that Luna revolves primarily around Sol, and only secondarily around Earth! Seriously; it is a planet! There are nine. (So far, perhaps.) However, the nine are: Mercury, Venus, Earth, Luna, Mars, Jupiter, Saturn, Uranus, and Neptune.

Earth and Luna's average orbital line is identical. They form an essentially two-dimensional, two-line braid along the path around the sun, first one planet leading, then the other. But it's not a swap in the same way as Saturn's Dione and Helene, which leave each other's gravity well, and circle another primary body: Saturn. Earth and Luna both circle their primary body --Sol-- at all times.

Oh, the third reason that Pluto and Charon are among the planets --the other good reason? Convenience. Pluto has only the one moon (we think), so Charon is easier found in charts if listed beside Pluto. And Pluto is there mostly out of intellectual momentum.

From a fresh-look perspective, Pluto could as easily be put with the 'tisimals. Or with Chiron in the "odd bunch". Besides, without Pluto/Charon (and Chiron), the 'tisimals are a clear category. However, many moons are called moons and not 'tisimals only because of their location in orbit around a planet. Indeed, many of Jupiter's outer moons are thought to be captured 'tisimals. Conversely, we don't call Ganymede a planet, but merely because of its location in orbit around Jupiter. Were it not there, we could fairly call it a planet. The same for several other moons. Is this fair? But there it is, and being where it is, that close to Jupiter, it is not past the tug-of-war criterion --so "moon" it is.

The same does not apply to Luna. Do you find it difficult to call Luna a planet? Why? It is not mainly a satellite of the Earth, but of the sun.
. . After all, the tiny third star of the Alpha Centauri system --our closest neighbor-- is so far out that it revolves around both the central suns! Is that more or less fascinating or improbable than our twin planet? What would the view be like from a planet of that star? What a dance would be done by a moon of that planet! And what bet would you have made against the orbit-swapping dance done by Saturn's Helene and Dione? Are there planets or even stars that swap orbits, or planets that swing in figure eights around a star-pair?
. . Charon still bugs me. It's not a moon, but a co-"planetisimal", because it's big enough (relatively) to satisfy that other criterion: that it and Pluto both orbit an external point between them. So I refer to them together: Pluto/Charon. (Sol and Jupiter orbit a point external to the body of Sol!)
. . Perhaps we could revive the term "planetoid" for those (presently three) known small bodies that orbit the sun entirely outside the orbit of Saturn. These would be of icy composition, as distinct from the rocky, ferrous, or carbonaceous Planetisimals.

It's been thought that Chiron is rocky! Tho by late '90, a comet-like outgassing was found, 80k Km either side. These are not mutually exclusive conditions. It made perihelion in 1996, so maybe they learned more then --I never heard. Tracing it back, it came within 16 million K of Saturn in the year 1660. Phoebe's mean distance from Saturn is almost 13 million; and it's the only moon in that system with an appreciable inclination: 150 degrees! Could something have hit it?! Or them? Or could the gravity wells of a close call have pulled them into new orbits? Did Phoebe "slingshot" it out?

Maybe my best motive for briefly calling Charon a planet was that it was just so much more interesting! And I'm leaving Luna in the list of planets for a good rational reason --and that makes it even more interesting, Horatio!

* Actually, we're the third planet from the sun only half the time. Half the time we're the fourth!