Learn Italian: Qualsiasi pianeta esplorino, si domandano se il caos duri per l'eternità. - Whichever planet they explore, they wonder if chaos lasts for eternity.

This is the core of the current lesson! The phrase se il caos duri is an indirect question. Because the main verb si domandano (they wonder) expresses uncertainty, it triggers the subjunctive mood for the verb in the se (if/whether) clause. Here, duri is the present subjunctive of durare.

Could I use the indicative dura instead of the subjunctive duri?

In everyday spoken Italian, you will actually hear people use the indicative here quite a lot. However, at a B2 level and in written or careful speech, the subjunctive (duri) is expected. It is grammatically correct and properly highlights the doubt of the indirect question.

Why is the first verb, esplorino, also in the subjunctive?

The word qualsiasi (any/whichever) triggers the subjunctive when it sets up an indefinite or hypothetical condition. When saying qualsiasi pianeta esplorino (whichever planet they explore), you aren't talking about a specific planet, so the subjunctive is used to show that indefinite nature. esplorino is the present subjunctive of esplorare.

How does the verb si domandano work?

It comes from the reflexive verb domandarsi, which means to wonder, or literally, to ask oneself. The si is the reflexive pronoun for themselves, making it a very common way to introduce an indirect question.