The audiobook for this film released early in Europe. I can reveal all kinds of spoilers. I will simply say for now that there will not only be the invading aliens in this film from the first one and the reason they came in the first place was to acquire fuel for their ships. Having read ID4: Silent Zone this is strange because it shows the aliens eating instead deep into the mantle. If anyone wants to know more PM me.
Seems likely an awfully inefficient way to fuel ships, exterminating lifeforms on the surface... besides, the official novelization of the movie made pretty clear they were coming for resources and taking the planet as their own.