alt Hacker News> The updated IAU definition is essentially reverse engineered to fit how they wanted to classify the planets.
I wholeheartedly agree with this take. I have something of a personal hatred of poorly executed retconning thus I present the following proposal.
For the sake of simplicity we should use even powers of 10 with earth as the primary reference point. Note that the earth is 10^24, mars is 10^23, and the earth's moon is 10^22 (pluto is also 10^22 and less massive than the earth's moon). It seems "obvious" then that 23-25 are planets, 22 or less are relegated to dwarfs, and 26 and over are giants (jupiter is a 27 and the sun a 30 for reference).
By that logic, if we want to consider pluto a planet then the earth's moon also has to qualify. I don't really care one way or the other but that just doesn't seem sensible to me.
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It's easy to exclude the moon, like the IAU does, by requiring that planets orbit the Sun. (Although you might want to avoid the IAU's parochialism, which excludes exoplanets, and say "a star" instead.)
> By that logic, if we want to consider pluto a planet then the earth's moon also has to qualify. I don't really care one way or the other but that just doesn't seem sensible to me.
Ganymede is larger than Mercury, so a definition that doesn't specify orbiting a star would have to include some moons, or exclude Mercury.
This exposes another problem with this kind of definition - it seems to make more sense to define a body in terms of its intrinsic properties rather than whatever location it happens to be in.
For example, a rogue planet is not a planet by the IAU definition. You'd have to call it something like a free-floating planetary-mass object. ("Free-floating" is not the right term either, but I don't have the research funds to come up with a better one.)
> I have something of a personal hatred of poorly executed retconning
Totally agree. Wanna join me outside George Lucas' house to protest the Kessel Run?
Following this logic, a black hole with earth's mass would be a planet.