Planetarium

For a list of other rooms featured in Antibirth, check out Chapters.

The Planetarium is a new unlock-able 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 possessing three astrology-related items simultaneously. 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. The Treasure Room must not be entered at all to be considered skipped. This chance is further increased by an unknown percentage if Isaac has Magic 8 Ball or Crystal Ball. If Isaac enters a Planetarium, the chance will be set to 0% and can't be increased again

These rooms also have an individual chance to spawn in the purple-tinted areas of the Mausoleum, without Isaac previously skipping any Treasure Rooms.

Planetariums are similar to Treasure Rooms, but only contain special astronomy-themed items that are only available through Planetariums. They may also contain Signs of the Zodiac.

If a Planetarium spawns on one of the alternative floors where Treasure Rooms would normally have 2 items to choose from, the Planetarium may still only have one item with no other choice available.

If Isaac has Rosary Bead there is a chance for planetarium item to transform into The Bible.

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.

Zodiac collectibles from the base game are placed in Planetarium item pools:

Trivia

The Planetarium concept started out as a mod request on Reddit.^[1]
All of the Planetarium items resemble celestial bodies from the Solar System: Sol and Luna resemble the Sun and the Moon respectively, while the rest of the items stand for the planets from Mercury to Pluto.
- Jupiter and Neptune are the only planets in the Solar System not represented by any items of this kind.
  - Oddly, Pluto is not considered a planet by the scientific community since 2006 but does have a Planetarium item named after it.
All of the items' names are written in Latin (though some of these Latin names are shared with modern languages):
- "Sol" and "Luna" are both Latin for "Sun" and "Moon".
- "Mercurius", "Saturnus", and "Terra" are Latin for "Mercury", "Saturn", and "Earth".
  - The cut "Neptune" item is referred to as "Neptunus" in the concept art, the Latin name for the planet.
- The names of Venus, Mars, Uranus and Pluto are in English.
The tarot card, The Stars, prioritizes a Planetarium over a Treasure Room. This is probably because a Planetarium is more related to the stars.