This manga is... genuinely fascinating. Bit of a mind screw but in a good way.
Pondering the question, and assuming an accurate translation, it was "what is your WORLD made of", not "what is your PLANET made of."
If the question was planet, the answer could be a number of things - different elements, highly prevalent molecules like water, etc. Or going slightly more obtuse, cosmic dust.
The question was world though, which makes me think the alien wants to know about lived experiences. It makes sense to ask that, since they are trying to decide if the species is worth saving.
So I think the answer, rather than just people, should be something like "family," albeit in more scientific terms.