A rare Planetarium layout with two items exists.

If a Planetarium has been generated, XVII - The Stars will prioritize teleporting Isaac into it over the Treasure Room.

However, acquiring the regular Zodiac items via Bag of Crafting will grant the achievement.Acquiring the astrology-related items from the Bag of Crafting will not guarantee the Planetarium achievement.The item pool includes Magic 8 Ball and all Treasure Room items (which include most of the pertinent items) therefore, it can easily be unlocked. A simple way to unlock Planetariums is to acquire a D20 in Greed Mode and re-roll the coins to keep buying store items (this requires a battery to recharge the D20).Given the pool's small size, you will have a high chance to find it instead of a normal Planetarium item. If attempting to find a Planetarium to obtain one of its items, very careful consideration should be taken before picking up Rosary, as this will add The Bible to the Planetarium's item pool.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.

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