When certain stars and planets are lined up, they form shapes in the sky. In the game, players try to capture specific planets in different systems.
A set of Constellation cards, kept separate from the regular deck.
Each player draws a Constellation card at the start of the game. Cards may be played face-up, or secretly.
Each player has a Constellation card that outlines which planets in various planetary systems they must occupy in order to win the game. In six player game, for example, the card will show which foreign colonies you must occupy, and which home colony you must retain. In games with only four or five players, the card will show the home colony and a number of foreign colonies needed (and the remaining number of foreign colonies can be on any foreign planets.
This has a subtle (and sometimes not-so-subtle) effect on game play. Obviously, when one player is attacking a colony they need, other players may not be so anxious to ally on the offensive side if it is not a colony they also need. Uninvolved players will ally with the Defensive Main Player more often than in regular games (if there is such thing as a "regular" game).
Players will also have to defend certain home colonies more fervently, or actually make attacks against their home system in order to regain a colony (something that rarely occurs in games with our group).
The cards themselves will have many duplicate bases (depending on the number of players), so that joint attacks are still potentially beneficial for more than one party. The set of cards can continue to grow, depending on whether or not players wish to include moons in their win objectives, as well as other expansions.
In order to account for many different scenarios, I have constructed different Constellation "decks" for different numbers of players in a game. The actual formations of the Constellation are, of course irrelevant, but feel free to connect the planets and decide what shape they make.
Custom Constellations cards on Artscow.