The Senate is made up of 16 representatives from the Earth and its colonies. It is made up of 6 representatives from Earth, 4 from Mars, and 2 from the Earth’s Moon, Demeter, and Poseidon. The Senators are voted on by the general population.
Beta Ceres will be given 2 seats in the Senate after it has been an established colony for ten years. But due to the length of the trip back, the colony will not have a senator for 90 years after departure.