World 9 in New Super Mario Bros. Wii is a rainbow colored star in space. It is technically the last world in the game, but is not needed to finish the game. The world is a bonus after finishing World 8, where 8 different levels can be unlocked, one for each world where all the Star Coins are collected. In the stage select menu, it appears in the middle of the circle created by the other worlds, above the rest.
9-1: Level 9-1 is unlocked when all the Star Coins from World 1 are collected. Level 9-1 puts you in the clouds where you will encounter enemies like the Koopas and Piranha Plants where you must jump on twirling blocks and avoid fire balls to beat the level.
9-2: Level 9-2 is unlocked when all the Star Coins from World 2 are Collected. Level 9-2 puts you at the coast where you are constantly having to avoid a Porcu-Puffer while using platforms that move either up or down when stepped on.
9-3: Level 9-3 is unlocked when all the Star Coins from World 3 are collected. Level 9-3 puts you back in the clouds where you will encounter Koopas and Bullet Bills while twirling to repel enveloping clouds that are hiding secrets.
9-4: Level 9-4 is unlocked when all the Star Coins from World 4 are collected. Level 9-4 puts you in the desert where the screen is scrolling to your right making the level very diffecult. You will encounter enemies like Bob-ombs and fire ball spitting Piranha Plants as you use platforms similiar to the ones from 9-2 to complete the level.