Astronomy midterm question: new two-rule planet classification rules

Astronomy 210 Midterm 2, fall semester 2015
Cuesta College, San Luis Obispo, CA

An astronomer at the Southwest Research Institute in Boulder, CO proposed an alternate scheme[*] for defining a planet, paraphrased here as:

Rule 1: Must be small enough that it is not a star.
Rule 2: Must also be large enough that it formed itself into a spherical shape.

Discuss one example of something in our solar system that is currently not a planet according to the International Astronomical Union that would now be considered a planet under this new two-rule scheme. Explain using both the International Astronomical Union classification scheme, and this new two-rule scheme.

[*] Mark W. Buie, "Definition of a Planet," boulder.swri.edu/~buie/pluto/planetdefn.html.

Solution and grading rubric:

• p:
  Correct. Selects a round object such as a dwarf planet (such a Ceres, or Pluto) or the moon that is not a planet (due to not dominating its orbit around the sun; or because it does not orbit the sun directly), and shows that these objects would be considered a planet under the new two-rule scheme, as they would be small enough to not be a star, and also be rounded in the shape.
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors.
• 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: midterm02sP3c
p: 18 students
r: 4 students
t: 11 students
v: 0 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 3695) discussing how a dwarf planet such as Pluto or Ceres would then be considered a planet:
A sample "p" response (from student 1022) discussing how the moon would then be considered a planet: