Planet Cards level up their associated poker hand type by increasing the base value of chips and multiplier. There are 12 different cards, but three of them - Eris, Ceres, and Planet X - are secret and not initially available in the shop. There are no unlock conditions for Planet Cards, except for the three secret planet cards, which can only be found in the shop after playing their associated poker hand type, and only for the rest of that run.
Discovering all 12 Planet Cards will unlock the AstronomerJoker.
If a planet card is in the shop, or in your consumable slots, it cannot also be in that shop's pack.
This can be used to drammatically increase the odds of finding the planet card in a pack you're looking for. This can be done by intentionally holding unwanted planets, or rolling the shop until an unwanted planet appears, then opening a pack. With 2 planets in your consumable slots, and no secret hands unlocked, a pack with 5 planet cards will go from a 5/9 chance of having the desired planet to a 5/7 chance.
Ceres is a dwarf planet located in the asteroid belt between Mars and Jupiter. The asteroids surrounding it on its card reflect this.
Eris is a dwarf planet located far beyond Pluto, about 3 times further from the Sun than Pluto is. Its only known moon, Dysnomia, can be seen on its card.
On mouseover, all planet cards are classed as "Planet", except for Pluto, Ceres, and Eris, which are all classed as "Dwarf Planet"; and Planet X, which is classed as "Planet?".
Even though Black Hole is not a planet card in and of itself, it can still be found in Celestial Packs on extremely rare occasions and is also stylized like a traditional planet card.
A "Royal Flush" does not have a Planet Card associated with it, because it levels up alongside a "Straight Flush".
Eris, 
Ceres, and 
Planet X - are secret and not initially available in the shop. There are no unlock conditions for Planet Cards, except for the three secret planet cards, which can only be found in the shop after playing their associated poker hand type, and only for the rest of that run.
Astronomer Joker.
Nebula Deck starts with the 
