20160324

Astronomy quiz question: meeting only second IAU planetary qualification

Astronomy 210 Quiz 4, spring semester 2016
Cuesta College, San Luis Obispo, CA

Refer below to the minimal qualifications established by the International Astronomical Union for a planet:
I. Orbits the sun.
II. Shape "rounded-out" by gravity.
III. Cleared/dominates orbit around sun.
Which solar system object(s) only meets qualification II, but does not meet qualifications I and III?
(A) Earth's moon.
(B) Mercury.
(C) Makemake, a dwarf planet.
(D) Eugenia, a small asteroid.
(E) (More than one of the above choices.)
(F) (All of the above choices.)
(G) (None of the above choices.)

[*] en.wikipedia.org/wiki/588_Achilles.

Correct answer (highlight to unhide): (A)

The moon orbits Earth (and does not directly orbit the sun, failing qualification I); is "rounded out" in shape (meeting qualification II); and does not dominate its orbit around the sun (failing qualification III), as it is gravitationally bound to orbiting Earth as Earth orbits the sun.

Mercury orbits the sun (meets I); is "rounded out" in shape (meets II); and does not share its orbit around the sun (meets III).

Makemake orbits the sun (meets I); is "rounded out" in shape (meets II); but does not dominate its orbit around the sun, sharing it with other Kuiper Belt objects (fails III).

Eugenia orbits the sun (meets I); its shape is not "rounded out" (fails II); and does not dominate its orbit around the sun, sharing it with other asteroids (fails III).

Thus the moon is the only object listed here that only meets qualification II, but does not meet qualifications I and III.

Section 30674
Exam code: quiz04nG4N
(A) : 11 students
(B) : 0 students
(C) : 4 students
(D) : 1 student
(E) : 1 student
(F) : 0 students
(G) : 2 students

Success level: 60% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.80