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- Wed Mar 30, 2016 6:15 pm
#22637
Question #15: Strengthen—CE. The correct answer choice is (C).
The astronomer’s argument is structured as follows:
Rather than questioning the causal relationships that underlie this line of reasoning, you should recognize that this is an argument from analogy. The author uses the perceived similarity between the orbits of comets and those of distant planets—both are oval—as a basis to infer some further similarity between their causes: just like the oval orbits of comets were caused by close encounters with planets in our solar system, so were the oval orbits of planets orbiting distant stars:
Answer choice (A): While this statement logically makes sense, it has absolutely no bearing on the issue at stake. The argument only assumes that the gravitational forces of one planet can affect the orbit of another; the relative size of the planets is wholly irrelevant
Answer choice (B): This answer choice may seem attractive, because it appears to suggest that where the effect does not occur (the planets in the solar system generally do not have oval orbits), the cause does not occur (the orbits of these planets were not affected by a close encounter with another planet orbiting the sun). However, the conclusion is about the planets orbiting distant stars, not about the planets in our solar system. Indeed, the causal nature of the author’s conclusion is not the central weakness of her argument—the conclusion relies on a causal premise that is not under debate, and is sufficiently well-qualified. Consequently, strengthening the causal relationship in it will not be terribly productive. Instead, focus on strengthening the analogy between the planets orbiting distant stars and the comets orbiting the sun, ideally by showing that the two are similar in an important respect. Answer choice (B) fails to do that, and indeed can only strengthen the conclusion if we already assume that such a similarity exists.
Answer choice (C): This is the correct answer choice. If, in most cases in which planets have been discovered orbiting a star, more than one planet has been found orbiting the star, then it is possible that close encounters with such planets caused the distinctly oval orbits described in the stimulus. This answer choice establishes an important similarity between our solar system, in which comets can have close encounters with one (or more) of the planets orbiting the sun, and the planetary systems of distant stars (where the same can apparently happen). Indeed, the presence of more than one planet orbiting the distant stars is the central assumption in this argument: without it, the conclusion would make little sense.
Answer choice (D): This answer choice attempts to strengthen a premise, which is already a red flag. We know for a fact that many comets orbiting the sun have been thrown into oval orbits by close encounters with planets orbiting our sun. If most comets with such orbits experienced the same effect, that would lend additional support to the conclusion. Unfortunately, answer choice (D) only states that most comets were thrown into that orbit by a close encounter with some other object. That object need not necessarily be a planet: it could be a meteorite, or perhaps another comet.
Answer choice (E): This is the Opposite answer, as it weakens the conclusion. If no other object large enough to affect the distant planets’ orbits has been found orbiting their stars, then a close encounter with such an object (such as a planet) would be virtually impossible.
The astronomer’s argument is structured as follows:
Premise—Most planets orbiting distant stars are in oval orbits around those stars.The conclusion attempts to explain why planets orbiting distant stars have oval orbits: the oval shape of these orbits was probably caused by the same gravitational forces that caused the comets orbiting our sun to have oval orbits. Notice the qualified nature of this statement (“some of the planets … were probably thrown into those orbits”), suggesting that the causal relationship in the conclusion is far from absolute. Indeed, the author never claims that a close encounter with a planet always results in an oval orbit: the cause should not be interpreted as a sufficient condition for the effect. Accordingly, counterexamples whereby the cause occurs without the effect will have no bearing on the validity of the conclusion.
Premise—Comets orbiting our sun are in oval orbits too, due to their close encounters with planets orbiting our sun.
Conclusion—Some planets with oval orbits around distant stars were probably thrown into those orbits by close encounters with other planets orbiting the same stars.
Rather than questioning the causal relationships that underlie this line of reasoning, you should recognize that this is an argument from analogy. The author uses the perceived similarity between the orbits of comets and those of distant planets—both are oval—as a basis to infer some further similarity between their causes: just like the oval orbits of comets were caused by close encounters with planets in our solar system, so were the oval orbits of planets orbiting distant stars:
Premise: Close encounters with planets orbiting the sun (cause)To weaken this argument from analogy, you would need to show that there is a material difference between our solar system and the planetary systems of distant stars. What if distant stars, unlike the sun, only have one planet orbiting them? If that were true, then the close encounters with other planets orbiting the same stars would be impossible. To strengthen the argument (which is what the question stem is asking you to do), you need to show a material similarity between the solar system and the planetary systems of distant stars.Oval orbits of comets orbiting the sun (effect)
Conclusion: Close encounters with planets orbiting stars (cause)
Answer choice (A): While this statement logically makes sense, it has absolutely no bearing on the issue at stake. The argument only assumes that the gravitational forces of one planet can affect the orbit of another; the relative size of the planets is wholly irrelevant
Answer choice (B): This answer choice may seem attractive, because it appears to suggest that where the effect does not occur (the planets in the solar system generally do not have oval orbits), the cause does not occur (the orbits of these planets were not affected by a close encounter with another planet orbiting the sun). However, the conclusion is about the planets orbiting distant stars, not about the planets in our solar system. Indeed, the causal nature of the author’s conclusion is not the central weakness of her argument—the conclusion relies on a causal premise that is not under debate, and is sufficiently well-qualified. Consequently, strengthening the causal relationship in it will not be terribly productive. Instead, focus on strengthening the analogy between the planets orbiting distant stars and the comets orbiting the sun, ideally by showing that the two are similar in an important respect. Answer choice (B) fails to do that, and indeed can only strengthen the conclusion if we already assume that such a similarity exists.
Answer choice (C): This is the correct answer choice. If, in most cases in which planets have been discovered orbiting a star, more than one planet has been found orbiting the star, then it is possible that close encounters with such planets caused the distinctly oval orbits described in the stimulus. This answer choice establishes an important similarity between our solar system, in which comets can have close encounters with one (or more) of the planets orbiting the sun, and the planetary systems of distant stars (where the same can apparently happen). Indeed, the presence of more than one planet orbiting the distant stars is the central assumption in this argument: without it, the conclusion would make little sense.
Answer choice (D): This answer choice attempts to strengthen a premise, which is already a red flag. We know for a fact that many comets orbiting the sun have been thrown into oval orbits by close encounters with planets orbiting our sun. If most comets with such orbits experienced the same effect, that would lend additional support to the conclusion. Unfortunately, answer choice (D) only states that most comets were thrown into that orbit by a close encounter with some other object. That object need not necessarily be a planet: it could be a meteorite, or perhaps another comet.
Answer choice (E): This is the Opposite answer, as it weakens the conclusion. If no other object large enough to affect the distant planets’ orbits has been found orbiting their stars, then a close encounter with such an object (such as a planet) would be virtually impossible.
