Siege a Planet

I spent several hours watching DasTactic (who is great) on Youtube.  In his last episode he had a Siege ship in his fleet and when he approached a Drengin world, it said he did not have enough POWER, or something like that despite having the Siege ship.  He was confused as was I.  Based on so many other games, I assumed that if you had any amount of 'Siege Strength' you could siege, but it would take a long time.  Can anyone clarify this when they have time.  I am going to buy GalCiv4 this afternoon and I would like to have an understanding of that part of the game.  Thank you in advance.