(update) This has been answered, thanks to all!

Question: How do the Viking ships contribute to defense? Should they be placed in orbit?

Answered in chat by Dragoneye99
"you leave the vikings on your planet but if you have a hangr you may have to turn FAS off. During the battle they count as a ground unit and have 3x their normal attack power"

Answered in chat by planetkiller
"If the vikings are placed in high orbit, they defent in high orbit. If they are on the planet, they defend the planet. ;) "

Answered in chat by ocraft
"If Vikings are on planet they defend in defense mode (x3 attack, no debris and battle experience to attacker, more lost Vikings will recover). If they are in defense on high orbit, they work in fleet mode (nominal attack, debris and battle experience)."

Answered in chat by Vicky
"1 - On planet means that you do not move the ships in any way. That is, vikings defend the planet when NOT FLYING in any way.
2 - On orbit means that the ships are flying and are stationary at some coordinates. This is seen on the flight screen where on the specific flight is shows a round black sign with a white cross like crosshair.
3 - When a fleet is on orbit at coordinates IT IS NOT related to any planet (even the underlying one) IN ANY WAY. Someone may attack the underlying planet and you will not realize it.
"