Science

Science is the extent of a robust research and development industry in your planets. It is a Planetary Resource that can only be provided by annexing planets.

Mechanics
Science is a straight multiplier to your investment in research, so the amount of tech points (points towards the next tech) are decided by:

TECH POINTS = SCIENCE * RESEARCH (each round)

Science of planets in rebellion or planets that were captured in the previous round is often inaccessible.

See also...
See Planetary Resources for a table of all planets and how much science they provide.