Free Practice Question
RC · Reading Comprehension
655-705
Short Passage - Science
Reading Passage
In 2006, the International Astronomical Union changed the definition of planet. Controversially, the new definition disqualifies Pluto as a planet. Such a change is not unprecedented. The first asteroids discovered were initially classified as planets. But astronomers soon realized that asteroids are much smaller than the other objects captured by their definition and so numerous that it is impractical to consider them all planets. So planets were redefined to be bodies larger than asteroids.Similarly, astronomers now know that Pluto belongs to a vast population of bodies—several much larger than the largest asteroid—called Kuiper belt objects (KBOs). Eris, a KBO discovered in 2005, is larger than Pluto. If Pluto were a planet, then Eris would also be one, along with many other large KBOs. The number of official planets again threatened to become unwieldy. So, as before, the term planet needed redefinition.
The new definition of planet effectively requires a planet to have more mass than all other bodies in its orbital zone combined (roughly speaking, two bodies share an orbital zone if their orbits cross). Pluto, Eris, and the asteroids do not. But each planet in our solar system has at least 5,000 times the combined mass of the other bodies in its orbital zone. Thus, the definition both restricts the number of planets and captures an important natural division between planets and all other bodies in our solar system.
The new definition of planet effectively requires a planet to have more mass than all other bodies in its orbital zone combined (roughly speaking, two bodies share an orbital zone if their orbits cross). Pluto, Eris, and the asteroids do not. But each planet in our solar system has at least 5,000 times the combined mass of the other bodies in its orbital zone. Thus, the definition both restricts the number of planets and captures an important natural division between planets and all other bodies in our solar system.
In the context of the passage as a whole, the main function of the first paragraph is to
Answer Choices
Correct answer marked belowpresent a historic precedent for the recent change in the definition of planet
Correct
B
clarify why the first asteroids discovered were initially classified as planets
C
argue that asteroids should not be classified as planets
D
describe certain details of the new definition of planet
E
explain why Pluto does not qualify as a planet
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What it tests
Your ability to understand a passage, its structure, and what the author is really saying. At the 655-705 level, accuracy and speed both matter.
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Answering from memory or outside knowledge instead of strictly from the passage.
Details
Difficulty
655-705
Type
RC
Category