The Planet of Trials is a planet that orbits the Comet Observatory in Super Mario Galaxy. It can be accessed once all three Green Power Stars in the game have been collected. Once they've been collected, the Green Lumas activate a green Launch Star that can be used to access the planet. This Launch Star is located on a small hill next to the Garage and has the three Green Lumas surrounding it. From this point on, the planet serves as a hub for the three Trial Galaxies: the Bubble Blast Galaxy, the Loopdeeswoop Galaxy and the Rolling Gizmo Galaxy, which can be reached by talking to the corresponding Green Luma for each galaxy. After all 120 Power Stars have been collected from playing as both Mario and Luigi, a fourth Green Luma will appear on this planet and can be talked to access the Grand Finale Galaxy. There is an orange Warp Pipe here that can be used to come back to the Comet Observatory. It is to note that The Gate and the Planet of Trials are not considered worlds like the Domes are even though they have galaxies within them. Bowser's Galaxy Reactor is not located in any area on the Comet Observatory and can only be reached by collecting 60 Power Stars and six Grand Stars, then talking to Rosalina.
- If the player tries to use the Red Star (unlocked by completing the Purple Coin mission in the Gateway Galaxy, which can be reached by entering The Gate) and flies toward the Planet of Trials, they will hit an invisible barrier blocking the way to the planet. This was possibly placed here to prevent players from accessing the planet if all three Green Power Stars have not yet been collected.