Aliens are rare to be found, theres only 30% chance you can find them in the wild.
They can be either found with Notch's Crystal Sword or Herobrine's Blood Sword.
They wear leather armor and uses Glass as helmets.
They can be also be found in the villages to steal some brains and turn testificates into zombies.
There are also baby aliens, They have not been found. There are some biomes that has crop circles or space ships.
if you go inside, theres gonna be some Cages with testificates, and rooms with baby aliens.