Planetarium is an unlockable room in The Binding of Isaac, added in the Repentance DLC.

The Planetarium requires a key to open. It contains a single item from the Planetarium item pool and, rarely, some cards.

Planetariums may only appear up to Chapter 3. They can also appear in Mausoleum/Gehenna II accessed via A Strange Door.

The base chance for a Planetarium to appear is 1%, and it can be modified in the following ways:
• +20% → going to the next floor.
• -20% → entering a Treasure Room.
• +24% → holding Telescope Lens.
• +15% → holding Crystal Ball, or 100% if a Treasure Room has been skipped.
• +15% → holding Magic 8 Ball.
• +6.9% → holding Sausage.
• Set to 1% → entering a Planetarium, or set to 10% if holding Telescope Lens.

The chance for a Planetarium to appear may not go below 0%.

Entering a Treasure Room inside the Mirrored World does not affect the Planetarium chance.

If the floor is affected by Curse of the Labyrinth!, it will grant a double bonus to the Planetarium chance when going down to the next floor.

This page shows the latest information as of the new Repentance+ (Online Co-op) DLC.
To see information about this room for the previous Repentance version, check this page instead.

Collect 3 items with the stars item tag in a single run.

The Planetarium is associated with the Planetarium item pool. Items that spawn in this room will be selected from the associated item pool.

• Check out the room description on the external Isaac Wiki.