7. a system of stars orbiting a common center of gravity a) solar system b) universe c) galaxy d) biosphere 8. the group of eight planets and other matter that orbits the sun. a) solar system b) universe c) galaxy d) biosphere 9. everything in outer space including stars, planets, and galaxies a) solar system b) universe c) galaxy d) biosphere 10. a planet that sustains life in its biosphere a) mars b) venus c) earth d) jupiter