Planetarium



The Planetarium is a new unlockable room type added in The Binding of Isaac: Repentance. Planetariums are similar to Treasure Rooms, in that they both require a key to enter, but purely contain special astronomy-themed items that are only available through Planetariums. Their chance for appearing increases with each Treasure Room skipped. They are unlocked by acquiring three different astrology-related items in a single run.

Generation Chance
Planetariums are very low priority for the game when it generates a level and places rooms, above only Bedrooms, and as such the chances listed below are for an attempt to place the room; there are reasons one may not be generated even if otherwise guaranteed.

The base chance for a Planetarium starts at 1% and will increase by +20% whenever a Treasure Room is skipped. The Treasure Room must not be entered at all to be considered skipped. This chance is further increased by 9% with Telescope Lens, by 15% with, and by 15% with. The Crystal Ball bonus is 100% if a Treasure Room has been skipped. It is possible to encounter Planetarium multiple times in a single run. If Isaac enters a Planetarium, the chance will be set to 1% and can be increased only with a Telescope Lens, by 15%.

The Treasure Room bonus is calculated separately before item bonuses and can be reduced to a negative value when more Treasure Rooms have been entered than floors generated, such as when Treasure Rooms in the Mirrored World of II /  II are entered (including the Treasure Room required for ).

Note that the bonuses are additive (not multiplicative) and can reach 100%. If the Planetarium cannot be placed or is not entered, the bonus will remain intact for a later level if any capable of hosting a Planetarium are left.

Planetariums cannot normally be encountered after II, though Telescope Lens allows them to appear in  and. Do note that Planetariums CAN spawn in the additional II /  II level that is accessed through the special door.

General Strategy
Given the new Ascent path takes you back through previous floors, it is viable to skip item rooms on the way down to improve your chances of the Planetarium spawning and then getting the items from the rooms you skipped on the way back up.

Anything that rerolls the player's inventory (such as Dice Rooms,, or Tainted Eden) can increase the value of the trade-off as a rerolled Planetarium item is much more likely to be useful than a rerolled Treasure Room item.

Items such as or  can be used to check the contents of a Treasure Room without negatively affecting the chances for a Planetarium to spawn.

Statistics
Without Telescope Lens, Planetarium rooms can't be found in chapter 4 and after. Since the chance to find a Planetarium depends on the number of Treasure Rooms skipped, skipping earlier ones allows for a higher cumulative chance of encountering one.

The table below presents the chances if the first Treasure Rooms are skipped. For example, if the first two are passed on, there is an 84% chance to find a Planetarium between I and  I.

Seeds
2PYG 827H (Planetarium adjacent to spawn) 4GCR 9KYG (Planetarium adjacent to spawn)

0WVE F74D (Planetarium adjacent to spawn) 0FYV B9EG (Planetarium adjacent to spawn) NEHY 83K3 (Planetarium adjacent to spawn) R1J1 QPDL (Planetarium adjacent to spawn) KKYJ A07S (Planetarium adjacent to spawn) NMMF V0RH (Planetarium adjacent to spawn) TN6C 894H (Planetarium adjacent to spawn) PFFQ QX3L (Planetarium adjacent to spawn) 6D0N 0B62 (Planetarium adjacent to spawn) XB3C GJHM (Planetarium adjacent to spawn)