Planetarium

The Planetarium is an 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 always 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 Clean Bedrooms and Dirty 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%, increases by +20% whenever Isaac descends a floor and is reduced by -20% when a Treasure Room is entered -- in most situations, this can be thought as skipping a Treasure Room granting a +20% bonus. The Treasure Room must not be entered by any means to be considered skipped. This chance is further increased by 9% with Telescope Lens, by 15% with Magic 8 Ball, and by 15% with Crystal Ball. 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%.

As a consequence of the game not counting skipped Treasure Rooms directly, but the difference of the current stage number and entered Treasure Rooms, the base bonus can be reduced to a negative value when more Treasure Rooms have been entered than floors generated. These extra Treasure Rooms can be encountered in the Mirrored World of Downpour II / Dross II (the Treasure Room required for Knife Piece 1 counts normally), levels reset using Forget Me Now or a 5-pip Dice Room, and in any level if created by Red Key or similar effects. The chance cannot be reduced below 1% (in levels where Planetariums are possible) even with penalties, however.

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 Depths II, though Telescope Lens allows them to appear in Womb and Corpse. Do note that Planetariums CAN spawn in the additional Mausoleum II / Gehenna II level that is accessed through the special door.

Notes

• The Crystal Ball and Deck of Cards will not count towards the unlock if they are left behind after picking them up, they must be actively held. Holding both of them with Schoolbag doesn't either.
• If Isaac loses passive items (ie: using the D4) they will not count towards the unlock. In other words, Isaac must have 3 items simultaneously.
• Planetarium items can be found before Planetariums have been unlocked in-game. To do so, use any method that ignores item pools ( Chaos, Eden, etc.).
• A simple way to unlock Planetariums is to acquire a D20 in Greed Mode and re-roll the coins to keep buying store items (this requires a battery to recharge the D20). The item pool includes Magic 8 Ball and all Treasure Room items (which include most of the pertinent items); therefore, it can easily be unlocked.
• Acquiring the astrology-related items from the Bag of Crafting will not guarantee the Planetarium achievement.
  • However, acquiring the regular Zodiac items via Bag of Crafting will grant the achievement, and items such as Aquarius are relatively easy to craft.
• If a Planetarium has been generated, XVII - The Stars will prioritize teleporting Isaac into it over the Treasure Room.
• A rare Planetarium layout with two items exists. Only one can be taken.
• If a Planetarium appears on Downpour II, even if the door has been unlocked, the planetarium in the Mirrored World will be locked and will contain nothing.
• Planetariums will not be available on challenges that prevent Treasure Rooms from spawning.
  • During Pica Run, a Planetarium will contain only a trinket.
• Luna and Voodoo Head may make Planetariums harder to encounter, as the extra Super Secret Rooms and Curse Rooms are prioritized over Planetariums for placement, potentially leaving no room for one in the stage.
• If Isaac uses Glowing Hour Glass to exit a Planetarium upon first entering one, the Planetarium does not count as entered, and Planetarium chance is preserved for later floors, allowing Isaac to effectively reroll his Planetarium until a desired item is found.
• Doing a Victory Lap does not reset the entered Treasure Room count, making it very difficult to have a high chance for Planetarium generation.
• Using R Key resets the entered Treasure Room count.
• With Curse of the Labyrinth, skipping one item room will increase your chances by 20%. While skipping both will increase the chance by 40%.

Items

Name	ID	Icon	Quote	Description	Quality
Jupiter	5.100.594		You're a gas giant!	+2 red Heart containers, -0.3 speed. Isaac builds speed while standing still, and releases poison gas clouds when he moves afterwards.	3
Luna	5.100.589		The moon's blessing shines upon you	Adds an extra Secret Room and Super Secret Room to each floor. Secret Rooms contain a beam of light that increases tears for the current floor and gives half a soul heart.	2
Mars	5.100.593		Double tap dash	Dash by double-tapping the movement key, becoming invulnerable and damaging enemies. There's a short cooldown before Isaac can dash again.	2
Mercurius	5.100.590		Speed up + you feel elusive	+0.4 speed. Doors stay open after entering a room, even in rooms with enemies.	3
Neptunus	5.100.597		Open the floodgates	Isaac's mouth fills with tears over time while not firing, up to a cap. Holding the fire button will release all stored tears in rapid succession.	3
Pluto	5.100.598		Size down	Significantly shrinks Isaac, allowing him to squeeze between objects and makes some enemy shots pass over him. Also gives +0.7 tears.	3
Saturnus	5.100.595		Ring of tears	Upon entering a room, a circle of tears orbits Isaac. Enemy tears have a chance to be caught in orbit, turning them friendly.	1
Sol	5.100.588		Radiant victory	Reveals the location of the Boss room. Upon defeating a boss, activates XIX - The Sun, fully recharges Isaac's active item, grants bonus damage and luck for the rest of the floor, and removes curses.	2
Terra	5.100.592		Born to rock	Replaces tears with rocks that deal variable damage, have increased knockback, and can destroy obstacles.	3
Uranus	5.100.596		Ice tears	Isaac shoots ice tears, which slow enemies and freeze monsters they kill. Frozen enemies slide away and explode into ice shards when touched.	3
Venus	5.100.591		HP up + you feel pretty	Adds one red Heart container. Enemies near Isaac become charmed.	1

General Strategy

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

Anything that rerolls Isaac's inventory (such as Dice Rooms, D4, 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 Glowing Hour Glass or Dream Catcher 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 Basement I and Depths I.

Treasure Rooms skipped	0	1	2	3	4	5	6
Chapter 1-1	1.0%	x	x	x	x	x	x
Chapter 1-2/1A-1	2.0%	21.8%	x	x	x	x	x
Chapter 2-1/1A-2	3.0%	38.2%	53.9%	x	x	x	x
Chapter 2-2/2A-1	3.9%	51.2%	72.8%	82.0%	x	x	x
Chapter 3-1/2A-2	4.9%	61.4%	83.9%	93.0%	96.6%	x	x
Chapter 3-2/3A-1	5.9%	69.5%	90.5%	97.3%	99.4%	100%	x
Chapter 3A-2 or 4-1 with Telescope Lens	6.8%	75.9%	94.4%	98.9%	99.9%	100%	100%
Chapter 4-2 with Telescope Lens	7.7%	81.0%	96.7%	99.6%	99.98%	100%	100%

Seeds

PC 2PYG 827H (Planetarium adjacent to spawn)

PC 4GCR 9KYG (Planetarium adjacent to spawn)

PC F21B 10E1 (planetarium adjacent to spawn)

PC XATY WS7S (Planetarium adjacent to spawn)

PC 0WVE F74D (Planetarium adjacent to spawn)

PC 0FYV B9EG (Planetarium adjacent to spawn)

PC NEHY 83K3 (Planetarium adjacent to spawn)

PC R1J1 QPDL (Planetarium adjacent to spawn)

PC KKYJ A07S (Planetarium adjacent to spawn)

PC NMMF V0RH (Planetarium adjacent to spawn)

PC TN6C 894H (Planetarium adjacent to spawn)

PC PFFQ QX3L (Planetarium adjacent to spawn)

PC 6D0N 0B62 (Planetarium adjacent to spawn)

PC XB3C GJHM (Planetarium adjacent to spawn)YHE3 8GHE (hard mode)

Bugs

Bug!	Entering the Treasure Room in the Mirrored World to get Knife Piece 1 counts normally. As such, upon entering the next floor, the Planetarium chance will be lowered by 20% even below the minimum 1% chance since it considers Isaac to have entered 2 treasure rooms in 1 floor.

Gallery

• 

  The Planetarium Room

• 

  Unlocking the Planetarium

• 

  Achievement icon from unlocking the Planetarium

References

The Binding of Isaac: Rebirth
Achievements	Attributes	Bosses	Cards and Runes	Challenges	Chapters
Characters	Co-op	Items	Item pools	Monsters	Objects
Pickups	Pills	Rooms	Seeds	Transformations	Trinkets