Astronomy final exam question: Ceres not a moon because of size?

Astronomy 210 Final Exam, fall semester 2011
Cuesta College, San Luis Obispo, CA

[20 points.] An astronomy question on an online discussion board(*) was asked and answered:

Atanvaryar: What do you know about Ceres and [d]warf [p]lanets?
DVOTA: Dwarf planets aren't big enough to be planets, but too big to be moons. [Because of this,] Ceres is a dwarf planet [instead of a moon].

Discuss whether or not if this answer is correct, and how you know this. Explain using the International Astronomical Union classification scheme.

*Source: http://answers.yahoo.com/question/index;_ylt=AsNsF7bYsJ7Lg_p.GeR7uMojzKIX;_ylv=3?qid=20100219202117AAPqLPe.

Solution and grading rubric:

• p = 20/20:
  Correct. Explicitly shows how moons and dwarf planets are mutually exclusive categories in the IAU classification scheme, or explains that Ceres could be a moon if it orbited a planet (thus it is not "too big" to be a moon), and/or that Ceres is smaller than some jovian moons.
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Discusses IAU requirements, but does not address why online comment is incorrect or unclear.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Problematic/incomplete discussion of IAU classification scheme, and/or why online comment is incorrect.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least demonstrates some understanding of IAU classification scheme.
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:Section 70158
Exam code: finalsf4R
p: 7 students
r: 10 students
t: 12 students
v: 3 students
x: 0 students
y: 0 students
z: 1 student

Grading distribution:Section 70160
Exam code: finalN3aR
p: 22 students
r: 0 students
t: 1 student
v: 1 student
x: 1 student
y: 1 student
z: 0 students

A sample "p" response (from student 2286):
Another sample "p" response (from student 3592):