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
Zodiac collectibles from the base game are placed in Planetarium item pools:
 * Aries
 * Taurus
 * Gemini
 * Cancer
 * Leo
 * Virgo
 * Libra
 * Scorpio
 * Sagittarius
 * Capricorn
 * Aquarius
 * Pisces

Trivia

 * The Planetarium concept started out as a mod request on Reddit.
 * 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.