8 Simple Steps to Define a Planet [from PandiaPress – Real Science Curriculum : Astronomy 1)

(you can also download this free resource from PandiaPress called “Beyond Neptune Lab #1” —–>)


  1. Look up a definition for the word “planet.” Discuss the definition. How specific is it?


  1. Did the definition of planet say anything about the size of the object going around the sun? Maybe it just said a large or big object. If I say “big,” does this really tell you size? Pluto is very small compared to the planets (read more about the status of Pluto as a dwarf planet at the bottom of this page). Should an object be a certain size to be called a planet? Mark YES or NO on the lab page.


  1. Between Mars and Jupiter there are a tremendous number of rocks floating in orbit around the sun. They are smaller than the planets and clustered into a group. These are called asteroids. Find them on the lab page. When an object is that small, it doesn’t have enough gravity to make it form a nice round shape like we think planets have. Should an object have to be round to be called a planet? Mark YES or NO on the lab page.


  1. All of the planets are on the same plane, or flat piece of space. Pluto is on a different plane. Look at the lab page to see this. Should an object have to travel in the same plane as all of the other planets to be called a planet? Mark YES or NO on the lab page.


  1. The asteroids travel around the sun together in a group of millions of other asteroids. Could we name them all and call them all planets? Why or why not? Is an object more like a planet if it travels alone? Mark YES or NO on the lab page.


  1. What other factors might be important in determining whether or not something in space is a planet? Does a planet have to have moons orbiting it? Mark YES or NO on the lab page.


  1. Write a new definition for the word planet to make it more clear.


  1. Using your new definition, determine which of these things are planets: Pluto? The asteroids? Any or all of the other eight bodies we currently consider planets? According to your new, more accurate definition, color in all of the planets in the diagram on the lab page.

[UPDATED 2020]
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 REAL 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?
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