The Good Egg Galaxy is one of the first worlds in Super Mario Galaxy. Not much is known about this galaxy but it is known that both the Dino Piranha and King Kaliente will be fought here.
Planets
Pill Planet
The planet is shaped like a giant, transparent pill. It contains many obstacles and items, mainly Coins and Item Crystals.
Atop the towering planet is a Boost Star that Mario can break out of the Pill Planet's glassy surface with. It has been shown that Mario will have to walk on both the floor and ceiling to go through it.
Magma Planet
This is where the boss battle against King Kaliente takes place. It consists of a hot lava pool with a shiny tar-like ring of ground around it, and the rest of the planet is just simple hot rock. The planet is quite big compared to others like the Tropic Planet, since it needs to have a decently large size for the fight to be held. Underneath the Magma Planet is a rocky area with yellow rings on it like a target.
Yoshi Egg Planet
A very small planet. The planet resembles a Yoshi Egg, and was first seen in a video at GDC 2007. It is currently unknown if anything is inside the egg, or if it is just a reference to the Yoshi clan. There is a Luma moving on top of the egg, as well as a Goomba. It is found in the Good Egg Galaxy, and may be the "Main Planet" since it is, after all, an Egg Planet.
Geo Planet
The Geo Planet is a planet. It is a small planet where the Dino Piranha boss is battled. The Geo Planet is made of gravel and rock with hints of grass sticking out, much like a simple ball of earth mixed up together. There are Item Crystals on it with a few Coins in them.
Boulder Planet
It is a double-sphere rocky planet, like two Asteroid Planets merged together. It only consists of about three large boulders that Mario must dodge, as seen in the screenshot. It was first seen in the Star World (Galaxy), but an copy of the planet (albeit in a different colour) was seen in the Good Egg Galaxy.
Known Levels
Dino Piranha
A Snack of Cosmic Proportions
A Snack of Cosmic Proportions is the second level of the Good Egg Galaxy, and once completed you'll be rewarded with a star. Your goal in this level is to feed a particular Luma on the Yoshi Egg planet so that it'll transform, leading you to the final area.
The Luma mentioned above will require 100 star bits. Star bits can be found everywhere in this level, and there are seemingly an infinite amount of them. The final platform here is a large, star shaped planetoid with an open center. Your objective here is to collect all of the blue star shards, thus revealing five Pull Stars which can be used to collect the star in the middle.
The designers created this mission so that if you don't have enough star bits when it's time to feed the Luma, you'll be allowed to wrap around a set amount of planets as many times as you want until you get the appropriate amount.