20161014

Astronomy quiz question: IAU classification of Phoebe, orbiting Saturn

Astronomy 210 Quiz 4, fall 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.
The following description of Phoebe's history in the solar system was made in a news release from NASA's Cassini spacecraft mission[*]:
Cassini images suggest Phoebe originated in the far-off Kuiper belt, the region of ancient, icy, rocky bodies beyond Neptune's orbit. Data show Phoebe was spherical and hot early in its history, and has denser rock-rich material concentrated near its center. Its average density is about the same as Pluto, another object in the Kuiper belt. Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet.
According to the IAU qualifications, after being captured by Saturn's gravity, Phoebe is now:
(A) a moon.
(B) solar system debris.
(C) a dwarf planet.
(D) a planet.
(E) (None of the above choices.)

[*] nasa.gov/mission_pages/cassini/whycassini/cassini20120426.html.

Correct answer (highlight to unhide): (A)

Now that it orbits Saturn, Phoebe no longer directly orbits the sun (failing qualification I), which immediately classifies it as a moon. (Phoebe's spherical shape means that it satisfies qualification II, and it definitely does not clear its orbit around the sun (as it orbits Saturn), so it fails qualification III.)

Section 70158
Exam code: quiz04s5ie
(A) : 25 students
(B) : 1 students
(C) : 7 students
(D) : 0 students
(E) : 0 students

Success level: 75% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.70

Section 70160
Exam code: quiz04n4uG
(A) : 19 students
(B) : 3 students
(C) : 3 students
(D) : 0 student
(E) : 1 student
(No response: 1 student)

Success level: 72% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.55

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