To do so, use any method that ignores item pools ( Chaos, Eden, etc.).

Planetarium items can be found before Planetariums have been unlocked in-game.

In other words, Isaac must have 3 items simultaneously.

If Isaac loses passive items (ie: using the D4) they will not count towards the unlock.Holding both of them with Schoolbag doesn't count either.

The Crystal Ball and Deck of Cards will not count towards the unlock if they are left behind after picking them up, they must be actively held.Entering Treasure Rooms in the Mirrored World of Downpour II / Dross II will not affect the generation chance.The combined percentage from descending floors and entering Treasure Rooms will never go below 0%.However, if additional Treasure Rooms are entered, such as those created by Forget Me Now or Red Key, then the 20% penalty for entering a Treasure Room will be taken without receiving the 20% bonus for descending a floor. In other words, every Treasure Room skipped will add 20% to the generation chance. +100% if a Treasure Room has been skipped This chance can be modified in various ways: The base chance for a Planetarium to generate is 1%.

However, Planetariums can appear in the additional Mausoleum II / Gehenna II level that is accessed through A Strange Door.

Telescope Lens allows Planetariums to generate in Womb and Corpse.

Planetariums only appear in chapters 1, 2, and 3.

Telescope stellarium