The Pewter Moon is the moon of planet Poptropica. It is inhabited, albeit sparsely, by Pewts and nothing else. All of the charted surface is rocky and barren aside from the occasional Pewt building. The only point of interest for most visitors is an Astrozone station specifically designed as a stopping point on the way to somewhere more exciting.
The Pewter Moon was first made known to the Poptropicans by an astronomer named Mordred, who simultaneously discovered that it was inhabited. Very few of them ever visit it however.