So in this sentence 'our planet' is in instrumental case and 'earth' in nominative case.
Can it be stated reversing the cases? Like 'nasza planeta jest Ziemią? Or is that not allowed with certain types of nouns?
Semantically yes, but the connection is different:
- „nasza planeta jest Ziemią” – our planet is of type Earth; I would even go as far as actually putting 'an' before Earth here if translating to English
- „naszą planetą jest Ziemia” – Ziemia is of type 'our planet'; Here it's firmly 'the Earth'
So it's not really the same… Yet it does carry very similar meaning. It's also seriously hard to explain in English, maybe someone else will have a better idea how to explain it.
So, assuming SVO, we would really write "Ziemia jest naszą planetą"/"Earth is our planet".
I guess, but such simple copula is actually not the best place to make assumptions about word order in Polish – first of all, we generally try to put the 'most new' information last, IIRC, so that might affect where the subject appears and… For me actually, the [y in instrumental] [być] [x in nominative] word order sounds better anyway, so I would choose „naszą planetą jest Ziemia” generally – though I'm 100% sure that's actually my personal preference(ie. Idiolect) and not a general rule. ;)
I made an attempt to write something about the problem you mentioned here, part 1,5.
Even if that could work for "an Earth" as Emwue said, because it can be argued that's some type of a planet, that wouldn't work for "Mars" for example. Mars is a planet, a planet is not "Mars".