Here is the full list of all 41 ALIENS INVADED OUR PLANET achievements.
Kill at least one Alien
Kill 100 Alien
Pick 50 PowerUp
Defeat First Boss
Defeat Mr. One Eye
Defeat Blind Slime
Defeat Third Eye
Defeat Piece Of Garbage
Defeat Petrol Man
Defeat Mech Brain
Defeat Invisible Hand
Defeat Flying Thing
Defeat Psychopath
Shoot 500 Bullets
Throw 50 Dynamite
Drink 25 Health Potion
Drink 25 Armor Potion
Jump 250 Times
Spend 100K Points
Use Weapon Automat
Use Potion Automat
Use Magic Ball
Make x50 Combo
End The Day Under 25 HP
Play Until The 10th Day
Play Until The 20th Day
Play Until The 30th Day
Play Until The 40th Day
Play Until The 49th Day
Play as Cowboy 01
Play as Cowboy 02
Play as Cowboy 03
Play as Cowboy 04
Play as Santa
Play as Skeleton
Play as Prisoner
Play as Crusader
Play as Cowgirl
Unlock and Carry All Weapons
Complete 5 day in Impossible Difficulty
Help a Friend Up