August 28, 2006

 A Local View on Pluto’s Demotion

 “Forget the expression, ‘My Very Educated Mother Just Served Us Nine Pizzas’,” says Stephanie Gove, planetarium director at Waubonsie Valley High School. The familiar mnemonic no longer provides an accurate memory aid for the names and order of planets because Pluto has been demoted from planet ranks. The idea that Pluto is not a planet has been debated since its discovery in 1930. This debate centered on the question, “What is a planet?”

Recently the International Astronomical Unit (IAU), the official scientific body that names celestial bodies, demoted Pluto to the rank of “dwarf planet.”  The IAU listed three criteria for planet status.  Two of them are familiar:  Orbits the sun and assumes a nearly round shape.  Pluto does not fit the third point that a body must gravitationally clear the neighborhood around its orbit.  Pluto crosses the orbit of Neptune. “Perhaps under this definition Neptune is not a planet and neither is the Earth,” stated Gove.  “Both have objects that cross their orbits.”

The IAU created the category “dwarf planets” for objects like Pluto that fit the first two criteria of the definition but not the third. Two other objects have become dwarf planets according to the new definition. Pluto will be joined by the asteroid Ceres, which was considered a planet in the 1800s, and 2003 UB313 (nicknamed “Xena”). On a side note, Charon, Pluto’s largest moon, will not get any special label.

“After a lifetime of learning and teaching that Pluto is the ninth planet in our solar system, the new definition is a big adjustment,” stated Gove.  “As a science educator, I am not surprised by the change. The field of science constantly evolves, and this is just another example of how.”

Pluto’s demotion does not mean that it will no longer be taught in schools. “Pluto is still there. We just have a better way of defining it,” reminded Gove.  For teachers, lesson plans will be changed, textbooks will be amended, and a new mnemonic will need to be created.

 For more information contact:  Stephanie Gove at stephanie_gove@ipsd.org