Anyone who went to school before 2006 learned about nine planets in our solar system— Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Today we recognize only the first eight of these as planets. What happened to Pluto?

For a long time, as telescopes were developing, people considered anything that orbited the sun a planet. As telescopes got more powerful, astronomers discovered an area called the Kuiper belt containing thousands of objects in the area where Pluto is, some even bigger than Pluto.

Pluto, in addition to being smaller than some of Jupiter’s moons, has not cleared its orbit of debris. With this more detailed picture of our solar system, astronomers decided they should devise a better definition of a planet. With this new definition, Pluto joined a number of other objects in the new category “dwarf planet.” 

But now, some scientists are trying to re-institute Pluto as a planetclaiming Pluto qualifies because the definition of “planet” as used by the International Astronomical Union isn’t correct. 

With this sample lab from our upcoming R.E.A.L. Science Odyssey Astronomy 1, you will get to be the scientist that decides the fate of Pluto. How do the criteria the “Pro Planet Pluto” scientists use stack up against the criteria we present in our lab?
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A note about Pluto’s inclusion in Astronomy 1:
In 2006, Pluto was demoted from planetary status to “dwarf planet.” There are dozens, if not hundreds, of other dwarf planets in our solar system and we do not mention them when studying planets. However we have decided to leave Pluto (as a dwarf planet) in this course for a few reasons: Pluto is useful when teaching about orbits and when defining planet; the reclassification of Pluto was quite controversial and it may be reinstated one day, maybe along with the announcement of several other “new” planets; and (perhaps most importantly) Pluto has a soft spot in our hearts.

"Beyond Neptune Lab" from R.E.A.L. Science Odyssey Astronomy 1