How do I unlock the bonus moon levels?

On the level select screen of Splitter Critters, there are five visible moons. According to App Unwrapper, each moon has three unlockable bonus levels. However, when I select a moon on the menu, I'm not able to view any of the bonus levels.

How do I unlock the bonus moon levels in Splitter Critters?

Five moons on level select screen


Solution 1:

The bonus moons will unlock after you've completed all the main planets.