Major Planets are Populated worlds that have full voting membership in your empire. They produce EP (from taxes) in addition to any Personnel and Supplies. Planetary defense systems may be put into place. You may also build construction facilities on or in orbit of Major Planets.
Each Major planet (and capital) has one Small Scientific Outpost Station. It may be funded like any other. Major planets (in general) produce 25EP, 25 Crew, and 25 Marines each turn. Homeworlds produce 50EP, 100 Crew, and 25 Marines. Secondary homeworlds are all different, are considered Major planets for all purposes, and produce whatever is on the chart.
Populated worlds may be converted into Major Planets. They must have spent at least five turns as a Populated world before conversion.5 It costs 500EP and 2 turns to build the government (and Shipyard) facilities required.