Think about these questions as you watch the brief video:
- How many planets were listed in the solar system at the middle of the 19th century? Why did the number of planets go down to eight by the early 20th century?
- How do advances in technology affect our understanding of the solar system?
- Why does scientific classification change over time?
- What are the three criteria in the IAU definition of a planet? What is the IAU definition of a dwarf planet?
- Why do some astronomers disagree with the IAU definitions?