From the Super Mario Wiki
If the player recovers three special Green Stars, three Green Lumas will appear on the Comet Observatory, which will then activate a green Launch Star located near the Garage. By using it, Mario or Luigi is able to access the Planet of Trials, a small planet orbiting the Comet Observatory. There, three more Green Lumas will each transform into an additional green Launch Star when talked to. These Launch Stars will take Mario or Luigi to the three Trial Galaxies. Once the player has collected all 120 Power Stars in the game with both Mario and Luigi, a fourth Green Luma will appear on the reverse side of the Planet of Trials. This Green Luma will allow the player to go to the Grand Finale Galaxy when Mario or Luigi talks to it.
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