Cryptic Space Crossword

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Across
  1. 3. - A shooting star (6)
  2. 8. - The hottest planet in the solar system (5)
  3. 9. - A planet made mostly of hydrogen and helium (3,5)
  4. 11. - The coldest planet in the solar system (6)
  5. 12. - An imaginary line that determines day and night on Earth (4)
  6. 15. - An elliptical path undertaken by large objects in space (5)
  7. 16. - A term used to describe Pluto and Ceres (5,6)
  8. 18. - A group of stars that tells a story (13)
Down
  1. 1. - A chunk of rock ripped apart from the Earth that controls the tide (4)
  2. 2. - Clouds of dust and gas that are also know as "star nurseries" (7)
  3. 4. - A term used to describe a type of satellite born from an exploding star (7)
  4. 5. - A planet named after a God of War (4)
  5. 6. - The first element fused in the core of a star (8)
  6. 7. - The largest planet in the solar system (7)
  7. 9. - The force that gives rise to nuclear fusion within the core of a star (7)
  8. 10. - The closest galaxy to the Milky Way (9)
  9. 13. - A period of time determined by the Earth's axis (6)
  10. 14. - A remains of a meteor (9)
  11. 17. - The only known galaxy with life (5,3)