Why is Earth the only place we've found life?

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Across
  1. 6. the universe contains about 100 billion of these
  2. 8. how much of Earth is covered in ocean
  3. 10. the planet orbiting Earth
  4. 13. a liquid essential for all life
  5. 15. the form that water needs to be in order to support life
  6. 16. the layer of air that surrounds a planet
Down
  1. 1. an ocean will boil if a planet is too ________
  2. 2. a planet has more of this when it is bigger
  3. 3. it contains 100 billion galaxies
  4. 4. when a liquid turns into a solid
  5. 5. an ocean will freeze if a planet is too ________
  6. 7. a planet that can support life
  7. 9. the star around which the Earth orbits
  8. 11. the only planet we know of that has life
  9. 12. when a solid turns into a liquid
  10. 14. when a liquid turns into gas