I think stack's version is good. Maybe get in that the planets are small and close to the earth? If our goal is to be less vague, here is a more direct version:
Q. The planets appear to be visually round. If the planets are round, why isn't the earth?
A. The earth is unlike the planets in a number of ways. The planets are small bodies which circle the sun, while the earth is a terrestrial plane which sits just below the celestial bodies and is so much bigger than everything else. The question is like asking why basketballs are round, but not the basketball court. The basketball court is a fundamentally different kind of entity than the small balls which may bounce upon its surface. To this point, the earth is not a planet.
For the more direct version, however it is phrased, should get in:
- The earth is big
- The planets are small and close to the earth
- The basketball analogy
- Perhaps not directly stating by us that the planets are round, which I saw was a concern by some. In the version of the Q. above, it is the reader who believes that the planets are visually round.
- Does not imply a heliocentric model or traditional solar system
- Finishes with disassociating the planets with the earth
"The planets are small bodies which circle the sun"
I think you kinda lose the thread right there. It implies that the planets are smaller than earth (which may be the case in FET, idk, I haven't seen that - I've seen moon/sun sizes, but not planets) B/c most people will simply default to, "Ah, no, some are, but Jupiter and Saturn sure aren't." In essence, the earth's size seems sort of neither here nor there.
"The planets are small and close to the earth"
Same as above, but specific to "close to earth", I haven't seen any distances for planets - Same caveat as above, I may have missed it. All in all, distance doesn't seem to be as compelling as position/orientation. At least for an FAQ answer. Big difference when you dive in deeper.
"Perhaps not directly stating by us that the planets are round..."
You're on your own on this one. This leads to a whole different thing. The reader is definitely coming from that position (existent planet rotundity) and then you have to sashee to "The planets are not round..." If that's an FET thing, good on you. But I wouldn't want to be you.
"Does not imply a heliocentric model or traditional solar system"
See Boydster's point above. I agree, just wasn't sure how to do that.
"Finishes with disassociating the planets with the earth"
Yes, the crux of the biscuit.
But all in all, Tom, I think your proposed answer is sound, just that "The planets are
small bodies which circle the sun, while the earth is a terrestrial plane which sits just below the celestial bodies and is
so much bigger than everything else." doesn't really support "The earth is unlike the planets in a number of ways.". I think I'm hung up on the "small" and "bigger" part as being put forth as the definitive decider. But, again, if that's FET, I am no one to argue.