I've seen this occurrence a few times- for example, 9, right now. When the attacker stops attacking before he has 100% control of the planet, but there are no enemy planets it's linked to, so no enemy can ever attack or defend it. I feel that making the attacker come back and finish it (which in some cases you can't- see, in fact, 9, which is unattackable for Cybran even though it's 95% Cybran 5% Seraphim) is generally a waste of the attacker's time.
I suggest that, if a planet is >50% one faction, and, all the planets it is linked to are >50% that faction, then that faction should gain an automatic attack on that planet. An automatic attack charges just like an attack made by a player, but doesn't require a player to be active to maintain it. The automatic attack is ended when the trigger conditions no longer exist or the planet reaches 100%. This way, the system can handle some automated mop-up actions that aren't interesting to the players without making it imbalanced.