This Article Contains information directly from the Pikmin Games. This article is based off of an Enemy/Treasure/Area/Cave/Pikmin/Character from the Pikmin series of Games.
The Stellar Orb is simply a lightbulb. It was found on the last (9) sublevel of the Subterranean Complex, and was held by a powerful Man-at-Legs. When you retrieve this treasure, you unlock the Solar System, a device that lights up even the darkest shadows inside caves.
Being in the Explorer's Friend Series, it is part of the Exploration Kit.