Astronomy midterm question: plausible AMNH classification of Ceres and asteroids?

Astronomy 210 Midterm 2, spring semester 2017
Cuesta College, San Luis Obispo, CA

An astronomer at the American Museum of Natural History proposed an alternate scheme for defining planets and non-planets[*]:

A planet is (1) a body that has swept up or scattered most of the material from its orbit around the sun, and (2) has an orbit that can never collide with another planet. A non-planet is (1) a body that has not swept up or scattered most of the material from its orbit around the sun, and (2) has an orbit that can collide with either a planet or another non-planet.

Discuss how Ceres could be considered a planet under these new rules, but the asteroids would not. Explain your answer using these new rules, and characteristics of Ceres and of the asteroids.

[*] Steven Soter, "What is a Planet?" The Astronomical Journal, vol. 132, pp. 2513-2519 (August 16, 2006), arxiv.org/pdf/astro-ph/0608359.pdf. (As discussed in this article, however, Ceres would still not be considered a planet with this new scheme.)

Solution and grading rubric:

• p:
  Ceres is a dwarf planet that is rounded in shape and is much larger than the remainder of the asteroids, which are much smaller and are irregular in shape. Since Ceres is much larger, it can be argued that (1) if it "swept up most of the material from its orbit," and (2) since it can only collide with asteroids in its orbit, and thus can be considered a planet under these two rules. Since the asteroids are much smaller, it can be argued that (1) they did not sweep up most of the material from their orbits, and (2) are in orbits that can collide with each other or with Ceres, classifying them as non-planets.
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. At least discusses three of the four points above.
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Explicitly discusses the AMNH rules, but does not apply them correctly/consistently/completely, typically only Ceres or only asteroids, or only the first or second criteria to both.
• 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 AMNH rules.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Discussion unrelated to the AMNH rules.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Section 30674
Exam code: midterm02nDcc
p: 13 students
r: 2 students
t: 5 students
v: 1 student
x: 0 students
y: 0 students
z: 0 students

Section 30676
Exam code: midterm02sL0w
p: 28 students
r: 7 students
t: 5 students
v: 3 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1515):