I do see your points. But how would you seperate planets from non-planets by internal structure -- I assume differentiated vs homogeneous? If that's what you mean, that may not be too far off from the size scheme, since most spheroids do end up differentiating in one way or another (yes, I know there are some exceptions). Alternatively, you could group them into different classes based on interior...but again, this wouldn't be far from groups based on location.

When it comes down to the wire, it's primarily opinion. Your idea works well, though I still think size is better simply because it's easier to gauge and allows more flexibility with less confusion.

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