Planet of Trials
From the Super Mario Wiki
The Planet of Trials is a small planet orbiting the Comet Observatory in Super Mario Galaxy. The planet can only be accessed once all three Green Stars have been recovered. Once they've been collected, the Green Lumas activate a green Launch Star which allows Mario or Luigi to travel to the planet. This Launch Star is located on a small hill next to the Garage, with the three Green Lumas positioned around it.
From this point on, the planet serves as a hub for the three Trial Galaxies: the Rolling Gizmo Galaxy, the Bubble Blast Galaxy, and the Loopdeeswoop Galaxy. After all 120 Power Stars have been collected with both Mario and Luigi, a fourth Green Luma appears on the planet and grants access to the Grand Finale Galaxy. There is also an orange Warp Pipe on the Planet of Trials that leads back to the Comet Observatory.
If the player tries to get to the Planet of Trials using the Red Star instead of using the green Launch Star, they will run into an invisible barrier that prevents them from doing so and get pulled back to the Observatory, even after they have collected all three of the Green Stars. This is most likely to prevent Mario or Luigi from reaching the planet without collecting the three Green Stars.
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