Space

2. A power that keeps planets orbiting the sun
5. A gravitationally bound system of starts, stellar remnants, interstellar gas, dust and dark matter
9. A satellite orbiting the Sun, may go really far away from the Sun and return after hundreds or thousands of year. Some contain water ice.
12. The biggest planet of the Solar system
14. Last name of the man who did the first spacewalk
15. Eighth planet from the sun
16. A natural satellite orbiting Earth
17. The center of the Solar system
19. A star system consisting of two stars orbiting around their common barycenter
20. Third planet from the Sun

1. The biggest galaxy of the Local group
3. The smallest planet of the Solar system
4. The planet with least density on the Solar system
6. A circumstellar disc between planets Mars and Jupiter that contains asteroids, minor planets and debris
7. The fourth biggest moon in the solar system. Has a lot of volcanoes.
8. Number of the Apollo flight that landed to the Moon
10. Second biggest galaxy of the Local group
11. Number of moons orbiting planet Mars
13. Second planet from the sun
18. The biggest moon orbiting Saturn