Astronomy midterm question: Eris and Ceres

Astronomy 10 Midterm 2, Spring Semester 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal M2.5

[Version 1]

[3.0 points.] Which one of the following choices best explains why Eris (formerly called "Xena") is categorized as a dwarf planet instead of a Kuiper belt object under the new International Astronomical Union rules?
(A) Eris has a spherical shape.
(B) Eris cleared its orbit of other Kuiper belt objects.
(C) Eris has a satellite orbiting around itself.
(D) Eris has a tilted orbit around the Sun.
(E) Eris has a different composition than the other Kuiper belt objects.

Correct answer: (A)
Eris is a member of the Kuiper belt, but due to its spherical shape, it is considered a dwarf planet rather than solar system debris (of which Kuiper belt objects are included).

Student responses
Section 4136
(A) : 10 students
(B) : 10 students
(C) : 9 students
(D) : 3 students
(E) : 3 students

[Version 2]
[3.0 points.] Which one of the following choices best explains why Ceres is categorized as a dwarf planet instead of an asteroid under the new International Astronomical Union rules?
(A) Ceres has a spherical shape.
(B) Ceres did not clear its orbit of other asteroids.
(C) Ceres has satellites orbiting around itself.
(D) Ceres has a tilted orbit around the Sun.
(E) Ceres has a different composition than the other asteroids.

Correct answer: (A)
Ceres is a member of the asteroid belt, but due to its spherical shape, it is considered a dwarf planet rather than solar system debris (of which asteroids are included). Note that (B) would be the correct answer as to why Ceres is considered a dwarf planet instead of a planet.

Student responses
Section 5076
(A) : 6 students
(B) : 9 students
(C) : 6 students
(D) : 1 student
(E) : 3 students