The Planetarium is a new unlockable room type introduced in Antibirth that has a small chance to appear after certain criteria are met. Like treasure rooms, they require a key to enter.
Planetariums are unlocked by acquiring three astrology-related items in a single run. These items include the Signs of the Zodiac, Tiny Planet, Magic 8 Ball, and Crystal Ball.
The chance for a Planetarium to appear starts at 1% and will increase by +20% whenever a Treasure Room is skipped (i.e. not taking an item from the room). This chance is further increased by an unknown percentage if Isaac has Magic 8 Ball or Crystal Ball.
Planetariums are similar to treasure rooms, but only contain special astronomy-themed items that are only available through Planetariums.
If a planetarium spawns on one of the alternative floors where treasure rooms would normally have 2 items to choose from, the Planetarium will still have only one item with no alternative choice available.
Planetarium Items
| Name | Icon | Description |
|---|---|---|
| Sol |  | Reveals the location of the Boss room. Upon defeating a boss, activates XIX - The Sun, fully recharges Isaac's active item, and grants bonus damage and luck for the rest of the floor. |
| Luna |  | Adds one extra Secret Room and one Super Secret Room on each floor. |
| Mercurius |  | Doors stay open after entering a room, even with enemies in it, allowing Isaac to leave any time. Increases speed. |
| Venus |  | Adds one Red heart. Enemies near Isaac are Charmed, causing them to attack other enemies. |
| Terra |  | Isaac's tears become rocks that have increased knockback and deal variable damage, and have a high chance to destroy obstacles. |
| Mars |  | Charge in a direction by double-tapping the movement key, becoming invulnerable and damaging enemies. There's a short cooldown before Isaac can dash again. |
| Saturnus |  | Isaac is surrounded by an aura which has a chance to absorb incoming enemy tears. It begins with 7 orbiting tears when entering a new room. |
| Uranus |  | Isaac shoots ice tears, which freeze enemies they kill. Frozen enemies slide away and explode into ice shards when touched. |
| Pluto |  | Significantly shrinks Isaac, allowing him to squeeze between objects and making some enemy shots pass over him. Increases tears and speed, but decreases range. |