Astronomy final exam question: Styx classified as a moon and/or solar system debris?

Astronomy 210 Final Exam, fall semester 2014
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was asked:

Pd: Pluto is orbited by Styx (smaller and irregular in shape). Why is Styx classified as a moon and not solar system debris? Or can it be both a moon and solar system debris?
t153: It can be (and is) both.

Discuss why this answer is incorrect, and how you know this. Explain using the International Astronomical Union classification scheme.

[*] answers.yahoo.com/question/index?qid=20141017172710AAEQDF4.

Solution and grading rubric:

• p:
  Correct. Of the three IAU requirements for a planet (orbits the sun directly, has a rounded shape, cleared/dominates its orbit) Styx does not meet the first question (orbits the sun directly), which immediately categorizes it as a moon. If Styx had met the first question (orbits the sun directly), its irregular shape would make it not satisfy the second question (has a rounded shape), and would then categorize it as solar system debris if it was not orbiting Pluto. Thus it is not possible in the IAU classification scheme to be both a moon and solar system debris.
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Discussion of either IAU classification scheme, or why online comment is incorrect is not clear.
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Problematic discussion of both IAU classification scheme and why online comment is incorrect.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Discussion only tangentially related to the IAU classification scheme.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Discussion unrelated to the IAU classification scheme.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Section 70158
Exam code: finalst0P
p: 18 students
r: 4 students
t: 11 students
v: 0 students
x: 1 student
y: 0 students
z: 0 students

Section 70160
Exam code: finaln3rF
p: 13 students
r: 10 student
t: 5 students
v: 1 student
x: 0 students
y: 2 students
z: 0 students

A sample "p" response (from student 7433):
A sample "y" response (from student 9426):