Aliens are the various "avatars" of Cosmic Encounter players. Each alien grants its player a unique power that allows the player to distort one of the rules of the game to the player's advantage. At the start of a game, each player is given an through various selection methods, most commonly by shuffling the alien sheets or flares, giving each player two and letting them choose one. If a player has fewer than three of their home planets colonized, they lose the use of their power until they get enough colonies back.
In the current edition, each alien has an "alert level" of green, yellow or red, represented by colored lamps in the corners of the card. The alert levels do not necessarily indicate how powerful the alien is but rather how complex the alien is to use and how much complexity the alien's power will add to the game, with green aliens being the simplest and red aliens being the most complex. It is officially recommended that new players play a few games with green aliens only before mixing in yellow aliens, and only include red aliens when they are comfortable with yellow aliens.
In many editions, each alien also has a corresponding flare card that enhances its own powers or gives other players a small amount of its power.
Official variants for using aliens include:
- Hidden Powers: Players start the game with their alien sheets face down and their powers are not applied. A player may reveal their alien sheet at any time to start using its power, at which point it remains face up and is used for the rest of the game.
- Multiple Powers: Players receive two (or more) alien sheets at the start of the game, and use all of their powers when appropriate.