Astronomy clicker question: what's wrong with the IAU?

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.] What is problematic about the IAU (International Astronomical Union) classification scheme for planets and dwarf planets?
(A) It does not define how planets around other stars should be classified.
(B) It is not yet possible to measure the roundness of many distant, small bodies.
(C) It is too vague on how much a planet should "clear out" its orbit.
(D) The classification of a planet or dwarf planet depends on its neighbors, not on its own attributes.

Correct answer: (none, opinion only, but see comments below).

Student responses
Section 4136
(A) : 3 students
(B) : 4 students
(C) : 19 students
(D) : 6 students

Section 5076
(A) : 2 students
(B) : 0 students
(C) : 9 students
(D) : 7 students

This question prompts discussion on some of the objections to the recent International Astronomical Union (IAU) categorization scheme adopted at their August 2006 meeting in Prague.

(A) = The categorization scheme only applies to planets, dwarf planets, etc. around the Sun, and does not deign to classify the recently discovered "super-Jupiters" orbiting around other stars.

(B) = Many dwarf planet candidates are in limbo, without more information about their actual shape. The asteroids Pallas and Vesta, and as many as a hundred other Kuiper belt objects ("Santa" (2003 EL₆₁), the "Easterbunny" (2005 FY₉, et al.) may or may not be classified as dwarf planets for many years to come.

(C) = This is a real point of contention, as the IAU states that a planet must "clear out" its orbit. Students bring up the fact that the Earth definitely has not cleared out its orbit, as the dinosaurs can attest! The various Earth-crossing asteroid groups (the Apollos being the most prominent) indicate that the Earth has not "cleared out" its orbit, and thus under the letter of the law, the Earth is a dwarf planet, and not a planet! However, the spirit of the law suggests that since the Earth "dominates" its orbit, it clearly should not be demoted to dwarf planet status. Seems like the IAU needs some lawyers on retainer to help clean up their wording next time around.

(D) = This is also a real concern, as the "planetness" of an object is not an inherent property, it depends on the properties of its neighbors (whether they have been cleared out or not). Mercury is a good example, as it is in an empty part of the solar system. But placing it in the asteroid belt would mean that it is a dwarf planet, as it would not have enough gravitational influence to dominate, much less clear out its neighbors.

So stay tuned, as this topic is still very active and contentious...