Whenever The Binding of Isaac receives a new DLC, it gains a huge amount of content added for players to discover, and with the release of Repentance, this is still very much the case. It has added a ton of new items, bosses, characters, and achievements to play with and in true rogue-lite fashion, unlock, and some of these are more esoteric than others.
One particular addition is the new Planetarium rooms. These are unlocked by acquiring three different astrology-related items over the course of a single run, like the astrological sign items or the Crystal Ball. These rooms contain extremely powerful items based on heavenly bodies that can vastly alter a given run, and are yet another thing players must learn about the game.
11 Venus
While still very powerful, Venus may be one of the more underwhelming Planetarium items that a player can encounter. By no means a disappointment, Venus grants Isaac an extra heart container and gives him an aura that charms any enemy that gets close to him.
Where Venus may be less useful is that the charm wears off very quickly and some characters like The Lost and ??? cannot benefit from the extra health.
10 Mercurius
Speed is one of the more important stats in The Binding of Isaac. It makes almost too much sense that the planet named for Mercury, the fastest god of the Roman pantheon, would grant a very sizeable speed boost.
While the speed boost is usually much appreciated by players, Mercurius also gives them a permanent effect similar to the Door Stop trinket, which keeps all doors open when passed through.
9 Saturnus
The ringed planet gets a suitably ring-themed item here in The Binding of Isaac. Upon pickup, a circle will surround the player that activates once Isaac enters a new room. The ring fills up with seven tears that orbit the player that can damage enemies and will remain for a good period of time.
Like similar items such as Maw of the Void and Monstrance, Saturnus can be used to deal damage to enemies at odd angles and at a decent distance. This makes the item perfect for frail characters.
8 Mars
Mars is an item that can change how a player plays the game for the current run, and for that, it's pretty cool. This item lets Isaac perform a quick dash by double-tapping the movement key or wiggling the analog stick in the same direction twice in quick succession.
This dash makes Isaac invulnerable and is more controllable than the dash granted by the Aries item. This is a very useful tool for getting out of sticky situations, as many players will know the sinking feeling of panicking into a corner surrounded by foes.
7 Pluto
Befitting the tiny rock-that-used-to-be-a-planet, Pluto as an item makes Isaac very small. So small in fact, that enemy projectiles can pass directly overhead, leaving the poor child completely unharmed.
This item also grants a moderate increase to Isaac's rate of fire. This means he cries more, and this may be a reflection of the real-life Pluto's sad demotion.
6 Jupiter
Jupiter is a... unique item. Upon pickup, Isaac loses a decent amount of speed and gains two heart containers — seems like a fair trade to some. However, Jupiter's main draw comes from its effect when Isaac stands still.
Players will suddenly experience an assault on their senses as they realize Jupiter causes them to build up speed while stationary and uh... expel some built-up gasses that trade the momentary boost of speed for a passive poison effect. Look, the game was never going to win awards for being classy.
5 Terra
Named for our very own planet, Terra replaces Isaac's tears with rocks that serve both as an offensive boost and as a neat utility upgrade. In fights, these tears deal variable damage which can be up to double Isaac's damage stat, which with luck can shorten encounters considerably.
As a utility item though, Isaac's tears now destroy obstacles open up closed doors. Everything that bombs can do, his tears can now do for free. He can even use his rocky tears to break the Angel Statues required to collect the Key Pieces to face off against Mega Satan!
4 Luna
This item is a neat passive boon that doesn't affect the player directly but affects the floors that they move through over the course of the game. Luna adds two new Secret Rooms to each floor and places beams of moonlight in them that give Isaac a temporary boost to his rate of fire.
This item synergizes perfectly with I Can See Forever pills and items that let the player know exactly where Secret Rooms lie, like X-Ray Vision. Veteran players will know and love the power of Secret Rooms and will jump for joy whenever Luna shows up in a Planetarium.
3 Sol
Like Luna, Sol is an item that provides a bit of mapping utility to Isaac. Initially, it just reveals the location of the Boss Room on the floor, but it has a secondary effect that kicks in once the boss is defeated. Upon victory, Isaac is fully healed, the rest of the floor is revealed, and he gains a buff to both his damage and luck until he moves to the next stage.
This item greatly incentivizes making a direct line to the boss and works incredibly well with characters who can rack up a great number of red heart containers.
2 Neptunus
The planet named for the Roman god of the sea is appropriately water-themed. When not firing tears, Isaac's mouth will fill up with water over time. These will release in one large gush that can deal a large amount of damage if aimed correctly.
This item is great for use against enemies who like to hide away off-screen for whatever reason, as the player can use that downtime to save up tears and release them all at once when the enemy becomes visible again.
1 Uranus
Uranus turns Isaac's tears into icicles that slow enemies while they still live, and freeze them upon death. Added in Repentance, the freeze effect is a very powerful offensive tool that can be used to rack up damage quickly.
Frozen enemies can be walked into to cause them to slide across the floor and explode into a fan of shards, which can damage and freeze further enemies. The potential havoc that a player can wreak with this item makes it a very attractive pick indeed.