Kenneth Chang

The IAU screwed up their planet definition 13 years ago today, but we've learned that dwarf planets like Pluto are full-fledged planets, &, numbering over 120, the most common type of planet, ahead of giants & terrestrials. @AlanStern @DrPhiltill @carolynporco @plutokiller

Is the moon a planet? If yes, you have a self-consistent argument that “planet” just means “round thing.” If no, you agree that the dynamics of the orbit are part of the definition.

Phil's got the historical argument. Coming from the modern planetary science community, yes, moons are planets. I usually call them "planetary bodies" but sometimes I slip & call them planets and don't feel technically incorrect.

Phil's historical document mentions major planets, secondary planets and minor planets. You're lumping major planets and secondary planets but leaving out minor planets as "planets." None of this includes the notion of "round." Also ignores the history of Ceres.

2/ A preprint of that paper is available for free download, here: https://arxiv.org/pdf/1805.04115.pdf …

3/ The paper shows that the community forged consensus to not treat asteroids as planets based on their small size, not because of their orbit sharing. No specific size limit was set, but Ceres was argued to be large enough to be a planet. (Vesta to a lesser degree.)

4/ So even though the community had decided in the 1960s that small bodies are not planets, we never forged a broad consensus on what the exact size limit is. We are *NOW* arguing (since ~2004?) that geological rounding is the size limit that is most taxonomically useful.