#17 - Although Pluto has an atmosphere and is much larger

[Administrator]

Complete Question Explanation

Justify the Conclusion. The correct answer choice is (E)

This is a challenging question, in part because the author used none of the usual premise or conclusion indicators to help us determine the structure of the argument. Nevertheless, you should realize that the second sentence supports the first, because it explains why Pluto should not be regarded as a true planet. The stimulus, when reworded, is structured as follows:

• Premise: Pluto formed in orbit around Neptune, and was later ejected from it.
  
  Conclusion: Pluto is not a true planet.

The question stem asks us to identify a statement that, if assumed, would allow the conclusion to be properly drawn. Despite the word “assumed” in the stem, this is a Justify question because our job is not to identify a statement upon which the argument depends, but rather to prove the conclusion by adding a piece of information to the premises. The sufficient condition indicator (“if”) in the question stem is a reminder that you must select an answer that is sufficient to prove the conclusion by using the Justify Formula:

• Premises + Answer choice = Conclusion

As with most Justify questions, there is a logical gap between the premises and the conclusion. The author argues that Pluto is not a true planet, but the premises provide no way of determining exactly what that is. The correct answer choice must define what a “true planet” is, and the criteria need to be so narrow as to exclude celestial bodies such as Pluto. For instance, if true planets were never formed in orbit around other planets, this would prove that Pluto is not a true planet.

Answer choice (A): This answer choice would be correct if we had evidence that Pluto were a moon. We do not. Although Pluto was a moon when it formed an orbit around the planet Neptune, it has since left Neptune’s orbit and we have no reason to believe that it is orbiting any planet today.

Answer choice (B): This answer choice does not prove the conclusion; instead, it is proven by it. If Pluto has an atmosphere but is not a true planet, then clearly not all celestial bodies that have an atmosphere are true planets. To answer a Justify question correctly, we need to establish that the conclusion is true, not assume that it is true.

Answer choice (C): A savvy test taker would eliminate this answer choice immediately, because it presents a speculation that cannot possibly prove the conclusion. The circumstances under which Pluto would have remained a moon have no bearing on whether or not it is a true planet today.

Answer choice (D): Our job is to establish the relevant criteria for determining what constitutes a true planet, not reject criteria that are irrelevant to it. Like incorrect answer choice (B), this answer choice answers a Must Be True rather than a Justify question. Indeed, if Pluto is not a true planet despite being much larger than any asteroid, then clearly the size of a celestial body is not (always) relevant to determining whether or not it is a true planet. Rather than helping to prove the conclusion, this answer choice – just like answer choice (B) – assumes that the conclusion is true.

Answer choice (E): This is the correct answer choice, because it introduces a criterion in the definition of a true planet that Pluto cannot possibly meet. If true planets must have formed in orbit around the sun exclusively, and Pluto clearly formed in orbit around Neptune instead, then Pluto cannot be a true planet (by the contrapositive):

• Premise: Pluto Formed in orbit around Neptune
  
  Justify Formula: True planet Formed in orbit exclusively around the sun
  
  Conclusion: Pluto True planet

This answer choice can easily fool those who are looking to “match” key elements of the stimulus to the answer choices. Such test-takers would immediately eliminate answer choice (E), presumably because the sun was never mentioned in the stimulus. That would be a mistake: just because the stimulus makes no mention of a certain key word does not always mean that the answer choice is out of scope or incorrect. This is especially true in Justify and Strengthen questions, which functionally require the evaluation of new information and its implications.

[smile22]

For this justify question, I chose answer B. E doesn't make make sense to me. The introduction to term "sun" caused me to think that this answer choice was irrelevant. I appreciate any clarification that you can provide.

[Emily Haney-Caron]

Hi Smile,

Though B seems to make sense as an answer choice, it is not an assumption on which the argument relies. We're really looking for something to explain how knowing that Pluto formed in orbit around Neptune tells us anything about whether it is a planet. Answer E does that - it lets us know that the fact that Pluto formed around a planet, rather than the sun, means it CAN'T be a planet.

Hope that helps!
Emily

[smile22]

Ahh i see. I'm mad that I missed that. That makes complete sense. Thank you!

[Basia W]

Good evening,

For this JTC conclusion question I was looking for something to link the new term "true planet" into the argument and the regularity of its orbit. However I omitted answer E because it brought in a completely new element of "the sun." Shouldn't we be concentrated on linking new elements in the argument together to sufficiently prove the conclusion rather than terms outside of the question stem?

Thank you!

Best,

Basia

[Emily Haney-Caron]

Hi Basia,

That Pluto did NOT form around the sun is implied when we are told it formed around Neptune. Therefore, E is very relevant - it doesn't really bring up a new topic, it just explains that information we already had can definitively tell us that Pluto cannot be a planet. Does that make sense?

[Basia W]

Hello,

yes it does!

I guess it's just a bit different then the sorts of JTC I'm used to seeing.

thank you for your explanation,

best,

Basia

[bli2016]

I think I understand the reasoning behind the right answer (the only information we are given besides the fact that Pluto is not a true planet is that it once orbited Neptune) but I was confused because it seemed like the stimulus was stating that Pluto no longer orbits Neptune. In that case, wouldn't it be possible for Pluto to be currently orbiting the Sun? (assuming we have no outside information about the solar system, of course) Some clarification here would be greatly appreciated. Thanks!

[Francis O'Rourke]

That's right! Pluto was formed in orbit around Neptune, but now orbits the Sun.

Choice (E) picks out this characteristic of Pluto when it states that for something to be a planet "it must have been formed in orbit around the sun [i.e. not around Neptune] exclusively." Since Pluto was formed around Neptune instead of the Sun, it can't be a planet.

[cascott15]

My problem with E is that the argument only talks about Neptune without discussing any principles to go along with it. What if Pluto formed around Earth instead and was subsequently ejected? According to this argument, it could still be a planet because it didn't form around/get ejected from the orbit of Neptune. Thus, the argument does not rely on it having formed around the sun exclusively, it relies on not being formed around Neptune.