Here is the full list of all 21 Planets Under Attack achievements.
Complete half of the Humanity Inc.campaign.
Get 20 stars in the Humanity Inc.campaign.
Win in each basic game mode: Elimination, Capture, Domination, King of the Hill.
Purge all Swarm planets before completing the mission.
Get 10 stars in the Humanity Inc.campaign.
Earn all stars in a single mission in the Humanity Inc.campaign.
Get 5 stars in the Humanity Inc.campaign.
Capture a planet when you have no planets left in any mode except for offline multiplayer.
Successfully defend a planet with just a few citizens on it (except for offline multiplayer).
Complete all missions in the Humanity Inc. campaign.
Get 60 stars in the Humanity Inc.campaign.
Get 30 stars in the Humanity Inc.campaign.
Get 40 stars in the Humanity Inc.campaign.
Get 50 stars in the Humanity Inc.campaign.
Reach level 20 for Human race.
Reach level 10 for Robot race.
Win a 4 player free for all match against 3 hard AI.
Destroy one million enemy population. Yes, 1 000 000.
Complete 30 online multiplayer matches.
Trick Destrobos' death planet into attacking its own ally during the boss battle.
Win an online team multiplayer match.