Comparing planets

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Across
  1. 2. A scientist who studies objects in space
  2. 6. Outer planets made mostly of gases
  3. 9. The movement of one object around a center or another object
  4. 10. The layer of gas surrounding a planet that is held in place by gravity
  5. 11. Large and small rocks or metallic masses orbiting the Sun.
  6. 13. The galaxy in which Earth and the Sun are
Down
  1. 1. The shape of the illuminated portion of the Moon as seen by an observer, usually on Earth
  2. 3. In comparison to something else
  3. 4. The study of celestial objects, space, and the universe as a whole
  4. 5. A large celestial body that revolves around a star in a solar system
  5. 7. A celestial body of ice, dust, and rock with an elongated and elliptical orbit
  6. 8. A large grouping of stars in space
  7. 12. A curved path followed by a satellite as it revolves around an object