Shatered, you may want to ask your chemtrail overlords to confirm this bit of science, but here goes;
Planes fly by using combustion (either piston engines or jet turbines, not sure about warp drives so you may have to check with your over lords on this too).
But these combustion type engines take a hydrocarbon fuel and oxygen, combust it and make CO2, water and energy. Now alot of water in the sky clings together and forms something we like to call clouds. Naturaly they are big wide and fluffy. The water comming out of an engine going 350 mph on the other hand will look thin. If there is nothing else up there these water molecules are happy enough to stick around with their mates. And unless turbulance comes to make them move on, the will stay there for a very long time. This is what people that like to wear aluminium hats like to call chemtrails.
So if it is a conspiracy, then you had better blame your over lords for bringing fire and teaching combustion to us poor ape creatures in the first place.
Cheers and happy spotting