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 keys 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 can only appear in chapters 1, 2, and 3 (Including their alternative floors).

Telescope Lens allows Planetariums to generate in Womb and Corpse.
However, Planetariums can appear in the additional Mausoleum II / Gehenna II level that is accessed through A Strange Door.

The base chance for a Planetarium to generate is 1%. This chance can be modified in various ways:

Condition	Chance
Descending a floor^[1]	+20%
Entering a Treasure Room	-20%
Having Telescope Lens	+24%
Having Crystal Ball	+15% +100% if a Treasure Room has been skipped
Having Magic 8 Ball	+15%
Having Sausage	+6.9%
Entering a Planetarium	Set to 1% chance and cannot be increased Set to 10% with Telescope Lens

In other words, every Treasure Room skipped will add 20% to the generation chance. However, if additional Treasure Rooms are entered, such as those created by Forget Me Now or Red Key, then the 20% penalty for entering a Treasure Room will be taken without receiving the 20% bonus for descending a floor.

The combined percentage from descending floors and entering Treasure Rooms will never go below 0%.
Entering Treasure Rooms in the Mirrored World of Downpour II / Dross II will not affect the generation chance.

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 count 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.).
If attempting to find a Planetarium to obtain one of its items, very careful consideration should be taken before picking up Rosary, as this will add The Bible to the Planetarium's item pool. Given the pool's small size, you will have a high chance to find it instead of a normal Planetarium item.
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.
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 (challenge #41), a Planetarium will contain only a trinket.
If Isaac uses Glowing Hourglass 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.
If the Planetarium layout selected was a closet-sized room, and there are no valid locations for it to spawn, then the Planetarium will not generate.

Items

Name	ID	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.	2
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 Fire Rate for the current floor and gives a Half Soul Heart.	2 3
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. Knocks away nearby enemies and briefly grants a shield after dashing.	2 0
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 2
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	+0.7 tears. Significantly shrinks Isaac, allowing him to squeeze between objects and makes some enemy shots pass over him.	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.	2
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	+1 Damage. 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. Heals 1 additional Red Heart. Enemies near Isaac become charmed.	2

General Strategy

Overall, the Planetarium items are average to above-average. None of them are bad, but none of them are particularly gamebreaking either. So, given that skipping a treasure room doesn't guarantee a Planetarium showing up, it usually isn't worth skipping Treasure Rooms without a good reason to do so unless you're doing speedruns. In speedruns the global strategy is to skip everything to go to the last floor. So in that case, Terra and Mercurius are the best items and overall skipping Treasure rooms is always worth since we can have a build with only Devil or Angel deals.

Given the 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. However, Planetariums can NOT generate during the Ascent, nor will skipped Planetariums "reappear".

Items such as Glowing Hourglass 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%

Gallery

The Planetarium Room
Unlocking the Planetarium
Achievement icon from unlocking the Planetarium

Trivia

Planetariums were introduced in the Antibirth mod, which forms the basis for most of Repentance's content. Their unlock method and generation mechanics remain the same in both versions.
- Their item pool in Antibirth included the Signs of the Zodiac alongside the Planetarium-exclusive items. This is not the case in Repentance.
- The original effects of most of the Planetarium-exclusive items have been unchanged from Antibirth with the item pool now having the introduction of two new items, Jupiter and Neptunus, in Repentance.