Pointless

For some strange reason my troop transport ship (loaded with 1000 men) doesn't let me invade a planet. I know there aren't any ships in orbit. I click the ship to move to the planet the same way I did while colonising but it simply refuses to invade. Its only my first game so I reckon im missing something quite obvious... can anyone help me out here?

Hey im new to this game and just playing the first campain game. I need to invade 2 planets but somehow I cant seem to move my colony/troop transport ships to invade. I've destroyed all ships in orbit so I think I just had to move my ship onto the planet to commence invasion but it simply won't. I'm most probably overlooking something very obvious. please help me out