There could be tens of billions of planets

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Across
  1. 3. movement of a planet around a star
  2. 5. the sun and the planets which go around it
  3. 6. the power that makes something fall to the ground
Down
  1. 1. the mixture of gases around the planet
  2. 2. space and everything that exists in it
  3. 4. extremely large group of stars and planets