The Stellar Orb is a treasure found in Pikmin 2 that is actually a lightbulb. It can be found on the last sublevel of the Subterranean Complex, and is held by a powerful Man-at-Legs. When Olimar retrieves this treasure, he unlocks the Solar System component of the Exploration Kit.
Solar System
The Solar System is a device that will illuminate any sort of darkness inside a cave's sublevels. This way, it is possible to see things that are very far away when underground. The name does not refer to where we live, but really means a system that produces solar or light energy.
Notes
Olimar's Journal
"Stellar Orb/journal"
Sales Pitch
"Stellar Orb/sale"