The Definition of Planet

• The Definition:A planet (in our Solar System) is an object gravitationally bound to the Sun which has the same diameter as Pluto or a larger diameter.
• The Planets: Mercury, Venus, Earth and Luna (the Moon), Mars, Jupiter, Ganymede, Io, Callisto, Europa, Saturn, Titan, Uranus, Neptune, Triton, Pluto, Xena [as we know it should be called; why can't a planet be a warrior princess?]. I have checked this and I believe I have the correct list of bodies, and moreover that the list remains the same if "diameter" is replaced by "mass" (unless Xena is paradoxically lighter than Pluto, which I have not checked).
• Advantages of this Definition: It is readily checked by observation and it is likely to be stable. The modifications of textbooks that it entails are virtuous: it is good for students to know at least the Galilean satellites of Jupiter and the largest moons of the other outer planets. No object which is a planet under the old definition ceases to be a planet.
• Disadvantages of other definitions: The newest definition is open to challenge on the grounds that it is unclear that the classical planets have really all "cleared their orbits" in any precise sense. There are plenty of asteroids that cross the orbits of Earth and Mars, and Pluto crosses the orbit of Neptune. The other definition proposed in the IAU is perverse in making Charon a planet without making Luna (our Moon) a planet and likely to be unstable in that numerous asteroids and large Kuiper belt objects become debatable cases: the textbooks will constantly be adjusted.
• A question: Should it be "mass" rather than "diameter"? Mass may be more easily observable; does it give a different class of objects (there seems not to be an object with smaller diameter than Pluto which is more massive; it appears that Xena is heavier than Pluto).
• The Boring Alternative My sons pointed out (as I knew perfectly well, of course) that adding the additional clause that a planet should not be gravitationally bound to anything larger than itself other than the Sun neatly restricts us to the familiar planets and Xena. But I've always thought (ever since learning that Ganymede has a larger diameter than Mercury when I was a child) that the major satellites are worlds in their own right...