When Is a Planet Not a Planet?

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Across
  1. 3. Not on the plane, not flat
  2. 5. Elon Musk wants us to go there
  3. 7. Shape of most planets' orbits
  4. 8. The telescope that took pictures on p411
  5. 9. Pluto's moon
  6. 11. The only planet with known biology
  7. 15. The disc around our sun before planets were formed
  8. 17. Hottest planet in our solar system
  9. 19. Discovered Pluto's moon
  10. 20. The coldest planet
  11. 21. Nebula 1500 ly away
  12. 22. Jupiter, Saturn, Uranus* & Neptune*
  13. 25. First planet from the Sun
  14. 26. Scientists studying the stars & planets
  15. 29. Was once considered a planet
  16. 30. The solid outer layer of a rocky planet
Down
  1. 1. Imaginary flat disc that shows where the planets and other objects travel around the sun
  2. 2. Space cloud of dust, ice, gas and rocks
  3. 4. The line from one side to the other through the center of a circle
  4. 6. Nebula pictured on title page
  5. 10. The 8th planet
  6. 12. The "A" in IAU
  7. 13. The center or middle of an object or planet
  8. 14. Has the widest rings in our solar system
  9. 16. Made of rocks and dirt
  10. 18. Energy visible from our star
  11. 22. Force that keeps objects orbiting
  12. 23. An important star
  13. 24. What Pluto's core is probably made of
  14. 27. Shape of Pluto's orbit
  15. 28. The largest planet in our solar system