Three hidden Green Stars open the Planet of Trials. They are part of the regular 120-star collection, not an extra set of 120 like the sequel’s Green Stars.
Battlerock — Luigi Under the Saucer
Rescue Luigi in Ghostly Galaxy and wait for his Battlerock letter. Replay Battlerock Barrage and check underneath the ending saucer. Lead a Bullet Bill around the underside so it breaks the cage trapping Luigi, then speak to him for the Green Star.
Buoy Base — The Secret of Buoy Base
Dive below the starting area and find the pipe enclosed in a cage. Draw a Torpedo Ted into the cage to break it, then take the pipe to the hidden exterior area. Lead a Bullet Bill to the final star cage, keeping clear until it strikes the target.
Dusty Dune — Treasure of the Pyramid
Choose Blasting through the Sand and feed the hidden Hungry Luma the required 20 Star Bits. Enter the pyramid it creates. Activate the moving sand structure and collect all five Silver Stars from its passages before leaving with the resulting Green Star.
Reach the Planet of Trials
When all three are collected, use the green Launch Star at the Comet Observatory. Three green Lumas offer the rolling, bubble and Ray-surfing trials. Each trial supplies one Power Star towards the 120-star total.