"Our planet is Earth."

Translation: Nasza planeta to Ziemia.

[hwodzak]

Can't it be "ziemia jest naszą planetą"?

[Jellei]

That's "Earth is our planet", so you start with the knowledge about "the Earth".

Here we have a sentence that says what is the planet we live on called.

[IE99]

Why use naszą planetą here?

[Emwue]

That's how copula works in Polish – if you are using „być”, you have to put predicate in Instrumental, so „Ziemia jest naszą planetą”.

More about the topic

[grettaelizabeth]

I wonder why not: "Nasza planeta jest Ziemią" - how do you decide which half is the predicate?

[Jellei]

Maybe just because it's a proper noun. Your sentence is like saying that "our planet" is "an Earth".

To show it even more clearly, "Paweł jest lekarzem" (Paul is a doctor) and not "Lekarz jest Pawłem" (The doctor is a Paul).

[grettaelizabeth]

That's helpful, thank you!