The United Planets, much like the real-life United Nations, is a council formed of all major races in the galaxy that regularly convenes to make decisions. The amount of clout a race has in the council depends on their influence, which is partially a factor of population, but which is also affected by planetary improvements and race bonuses (among other factors). The United Planets tab of the Foreign Relations panel shows just how the galactic voting power is distributed, how many weeks remain until the next meeting convenes, and a list of propositions that have been passed.
The United Planets does not usually have a great effect on the game, but some of the propositions — for example, the transfer of unique researched technologies — can be extremely destabilizing if passed. Some of the propositions can also significantly and permanently change gameplay from that point onwards, for example if the Neutral Grounds proposition passes (and it almost always does) then warships are relocated away from hostile territory upon a declaration of war, making cowardly sneak attacks in the heart of another empire's territory less likely. Those propositions which are not permanent will usually state an end date, after which they expire and can be voted on again.
The player does have the option of leaving the United Planets, but only at the expense of losing all trade revenue. If the player leaves the United Planets council, there is no way to rejoin.
The United Planets can, however, be used to your advantage. By building up your technologies and then trading them with other races for their influence points, you can easily take control of the senate by having the majority of the votes, thereby ensuring that the motions are always passed in your favour.
No Minor race participates in the UP.
List of United Planets issues