Astronomy final exam question: Earth in Kuiper belt?

Astronomy 210 Final Exam, fall semester 2010
Cuesta College, San Luis Obispo, CA

[20 points.] In an open letter to the 27th International Astronomical Union General Assembly, an amateur astronomer advocated against Pluto's planet demotion:

"If Earth were in Pluto's orbit, according to [the IAU classification scheme], it would not be considered a planet either."
--Laurel E. Kornfeld, http://laurele.livejournal.com/10429.html, August 6, 2009.

Decide whether this statement is correct or incorrect, and how you know this. Explain using the International Astronomical Union classification scheme.

Solution and grading rubric:

• p = 20/20:
  Correct. Pluto only satisfies the first two of the three requirements (orbits the sun, has a rounded shape, cleared/dominates its orbit) to be a planet. The online comment could either be considered incorrect or correct, depending on whether Earth would be able to dominate or not dominate Kuiper belt objects near its new orbit (the degree of which is beyond the scope of this course).
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Discussion of IAU requirements, or why online comment is incorrect/correct is not clear.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Problematic discussion of IAU requirements and why online comment is incorrect/correct.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Discussion of criteria only tangentially related to the IAU requirements.
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit. Discussion of criteria unrelated to the IAU requirements.
  y = 2/20: Irrelevant discussion/effectively blank.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:
Section 70158
p: 30 students
r: 7 students
t: 2 student
v: 2 students
x: 1 student
y: 0 students
z: 1 student

A sample "p" response (from student 3272), discussing how the statement is incorrect:

A sample "p" response (from student 2369), discussing instead how the statement is correct:

A sample "p" response (from student 1234), recognizing that the statement may be correct or incorrect, but more information is needed to determine the extent of Earth's influence on Kuiper belt objects: