You must pay any mandatory additional costs (such as sacrificing a creature) and you may pay any optional additional costs (such as kicker) for the card you cast.

You cannot choose to pay the mana cost or any alternate costs for the card if you choose to cast it, and if it has in its mana cost, you must choose 0 as the value of .

Once you choose to cast the top card of your library, Planetarium of Wan Shi Tong's ability won't trigger again that turn.

Planetarium of Wan Shi Tong's triggered ability allows you to cast the top card of your library during the resolution of the ability. It doesn't allow you to wait and cast the card later.