The Good Egg Galaxy is one of the first worlds in Super Mario Galaxy. It's a galaxy that, unlike many of the others in the game, doesn't have a set theme such as ice or fire. Rather, it mainly consists of multiple small planetoids and a variety of different themes that usually contradicts that of the previous planet you explored. Dino Piranha and King Kaliente will be fought here, with the former being a boss battle two times.
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. The planet can be found in the mission known as A Snack of Cosmic Proportions, where a hungry Luma will await a feeding.
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.
Levels
Dino Piranha
The Dino Piranha level is the first of the missions in the Good Egg Galaxy. As its name implies, you'll fight the Dino Piranha here at the end, though before doing so you'll have to traverse the galaxy. Once you land on the piranha's planet, you'll be forced to attack its tail a few times before coming in possession of the level's star.
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.
King Kaliente's Battle Fleet
During the beginning of this level you'll have to go through the orange pipe on the underside of the first planet in order to get to King Kaliente, the second and arguably more challenging boss of the Good Egg Galaxy. Before the fight, the Toad in this level will explain how if you spin next to a coconut, the item will be launched toward your enemies. This is mainly training you for the final fight with the boss.
Dino Piranha Speed Run
Purple Coin Omelet
Luigi on the Roof
Sometime after Mario saves Luigi from the Ghostly Galaxy, Luigi will go out to the Good Egg Galaxy to search for a Power Star. Shortly after, the Mailtoad informs Mario that Luigi is lost and shows Mario a picture of Luigi's location. Mario needs to go to the first level of the Good Egg Galaxy and needs to get Luigi on top of the roof of the house. This is done by going to the bottom side of the first planet and then jumping in the yellow Warp Pipe and then into the next yellow Warp Pipe. Luigi will give Mario his Power Star.