Gateway Galaxy

The Gateway Galaxy is one of the many galaxies from Super Mario Galaxy for the Wii. Princess Rosalina calls this galaxy the "Gateway to the Starry Sky". It serves as the opening level for the game. This is the first Galaxy in the game, and after Mario clears the first level in the Galaxy, he won't be able to return until he unlocks the Gate, later on in the game.

Main Planet
This is the first planet (other than the Mushroom Planet) Mario explores. It is where Mario first wakes up after being blasted of Princess Peach's castle. Here he meets two yellow lumas and the apricot luma that will be at Mario's(or Luigi's) side until the end of the game. Mario also meets Rosalina for the first time. It has three holes(two of which connect), two connecting pipes, several bungee plants, two pools, two cottages, and a castle. Later in the game Mario must collect 100 purple coins to earn the flying suit.

Black Hole Planet
This this is a planet made of dirt and stone. It is being eaten away by a black hole so Mario must be careful. There are five yellow star chips that Mario needs to procced. The planet is also being bombarded by meteors.

Metal Planets
These are two planets that look very similar. Both with spires that can send vibrations across the planets. there are several goombas on these planets including a grand goomba wich is the only one in the game. Inside the second one there is a reactor that is draining the energy from a grand star. Mario must free this star to continue.

Grand Star Rescue
When Mario awakens on this planet, a Space Bunny will challenge him to a game of hide-and-seek. The player will have to follow the bunny until he reaches his two friends, who will run and hide. One is found in a crater near the tree stump and a small lake, another in the grass near the Warp Pipe, and the last in a Warp Pipe. The player will have to catch them all, and a new structure on the planet will appear. Mario will meet with Rosalina and gain the Star Spin ability. Mario needs to Star Spin the nearby Launch Star to reach the next planet, which has five Star Chips scattered about. To go to the next planet, Mario will have to grab the Star Chips to make a Launch Star and blast Mario to the next planet. Here, a Luma in a cage tells Mario that one of the Goombas on the planet has the key. The player will have to defeat the Goombas on the planet until one gives up a key. The Luma then transforms into a Sling Star, which slings Mario to the next planet. On this planet, Mario must defeat the Giant Goomba near the top of the planet to free the other Luma and enter the Warp Pipe. The inside of the planet is filled with yellow switches and moving electrical walls. A big machine stands in the middle of the room; a Grand Star is inside it. Mario needs to step on all the yellow switches to turn them blue, which will shut the machine down and reveal the Grand Star.

Gateway's Purple Coins
This Power Star is only accessible after the Garden has been opened. When Mario enters the galaxy, Rosalina will meet. A Red Luma notices how much Rosalina trusts Mario, so he gives Mario the challenge of collecting 100 Purple Coins scattered around the planet to earn his trust as well. There are seventy coins on the planet and thirty in the air. Mario must use the new Red Star power-up that gives him the ability of flight to reach the thirty in the air. Completion of this mission will award Mario with the game's sole Red Power Star. One collected, it allows for use of the Red Star power-up in the Comet Observatory.

Trivia

 * A planet in the Deep Dark Galaxy resembles the first planet of the Gateway Galaxy. It can be seen from a planet full of Electrogoombas. The planet is also reachable, but is much smaller. There are three Goombas, and a cork can be unscrewed using the star spin to shrink the planet and disappear, also revealing many coins.