The energy obtained by collecting items after comets are destroyed can be used to restore the planet as well as purchase upgrades. Upgrading the green level of the planet will also make more powerful comets appear. Manage your energy and balance the upgrades. Once the planet has been fully restored, you will begin a new restoration cycle with the green level dropping back to zero. However all remaining energy will be brought over. This makes it possible to quickly restore the new planet if enough energy was saved to purchase the more important upgrades immediately.