Astronomy clicker question: why is Pluto not a planet?

Astronomy 10, Spring Semester 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal M2.5

Students were asked the following clicker question (Classroom Performance System, einstruction.com) at the beginning of their learning cycle:

[0.3 points.] Why was Pluto demoted from planet to dwarf planet?
(A) It does not orbit the Sun.
(B) It did not have enough formation heat to completely melt, and enough gravity to round itself into a spherical shape.
(C) It did not clear out its orbit.
(D) (All of the above choices (A)-(C).)

Correct answer: (C).

Student responses
Section 4136
(A) : 1 student
(B) : 10 students
(C) : 10 students
(D) : 11 students

Section 5076
(A) : 2 students
(B) : 8 students
(C) : 6 students
(D) : 5 students

At the International Astronomical Union (IAU) August 2006 meeting in Prague, the solar system was categorized by three criteria, as shown at right. To be classified as a planet, Pluto must separately fulfill all three criterion--it orbits the Sun (and not a planet), it is sufficiently large enough to have retained formation heat and enough gravity to reshape itself into a sphere (and not some lumpy potato shape), and enough influence over its domain to clear its orbit (which it has not, being one of as many as 10 billion other Kuiper belt objects located 30-50 AU from the Sun, just beyond the orbit of Neptune).

The Astronomy Education Review (http://aer.noao.edu) has several articles of interest:

Fraknoi, A. 2006, "Teaching What a Planet Is: A Roundtable on the Educational Implications of the New Definition of a Planet," Astronomy Education Review, 5(2).
An overview of the IAU categorization scheme.

LoPresto, M. C. 2006, "A First Glimpse of Student Attitudes about Pluto's 'Demotion,'" Astronomy Education Review, 5(2).
Further discussion on student views of Pluto's planetary demise.