NOTE: If you want only POE plus why (E) is correct, scroll down. POE is labeled.

warrior1991 , answer E is the missing puzzle piece. It would be almost impossible to "pre-think" the answer.

As I read this prompt, I thought, "Too much information.
Too many possible logical relationships.
What the h*ll does a moon's being captured by Neptune's gravity have to do with anything?"

Let the answer choices sort out
information, logical relationships, and which part of this prompt matters.

Only this part ends up mattering: "Pluto formed in orbit around Neptune."
(Pluto was formed while it orbited around Neptune.)

• Straight logic: (E) works perfectly.

E) For a celestial body to be a true planet it must have formed in orbit around the sun exclusively.

If X is a true planet, then X formed in orbit around the sun and only the sun.

Pluto did not form in orbit around the sun.
Pluto formed in orbit around Neptune.
Because Pluto did not form in orbit around the sun exclusively,
Pluto cannot be a true planet.
That last statement is the conclusion.

This assumption led to that conclusion.
Answer E is correct.

• Breaking down the prompt: (E) gets us to the conclusion.

Note: we can infer a factual statement, a premise, from part of this conclusion.

Conclusion: Pluto is not a planet

Fact #1:
At the least, a true planet has an atmosphere and is bigger than an asteroid.
→ Pluto has an atmosphere and is bigger than an asteroid but is not a planet.
Something is missing or something extra is illegal.
We infer Fact #1 from "Although Pluto has X and Y [and therefore has at least part of what is required to be 'a planet']..."

Fact #2: Pluto formed in orbit around Neptune

Fact #3: Pluto was ejected from Neptune's orbit [when something happened to a moon]

In terms of "what ingredients equal a PLANET,"
what is missing? Or what extra thing is illegal?

(E) For a celestial body to be a true planet it must have formed in orbit around the sun exclusively.

Ah. The missing piece.
Three ingredients (at least) constitute a planet.
-- The body (1) has an atmosphere, (2) is bigger than an asteroid, and (3) is formed in orbit around the sun exclusively.
Pluto has the first two ingredients.

Ingredient #3? No.
Pluto formed in orbit around Neptune.
Pluto did not form in orbit around the sun.
Pluto lacks a key ingredient: formation in orbit around the sun exclusively.
Therefore, Pluto is not a true planet.

Conclusion proven. (E) is correct.

• POE

(A) No celestial body can simultaneously be a moon and a planet.
Irrelevant. Prompt says nothing about whether Pluto is a moon.

(B) Not all celestial bodies that have an atmosphere and orbit the sun are true planets.
Irrelevant OR assumes what it is trying to prove (that the conclusion is true).

-- IRRELEVANT: nothing in the prompt says that Pluto now orbits the sun.
Pluto could have been ejected from Neptune's orbit
and sucked into the orbit of Uranus, for all we know.
I would stop here and eliminate B. Many aspirants will not be comfortable with that move.

-- Assumes the truth of the conclusion rather than proves the truth of the conclusion:
The conclusion to be proven is that Pluto is not a planet.

Do not assume that Pluto is NOT a planet.

Pluto has an atmosphere. Assume that Pluto orbits the sun. (Again, I'm not willing to go this far.)
We do not know whether Pluto is or is not a planet.

IF the conclusion is true, then maybe we can utter statement (B).
IF Pluto is not a planet, then we know
that at least one celestial body has an atmosphere, orbits the sun, and is not a planet.

But an assumption cannot assume that the conclusion is true.
An assumption helps prove that the conclusion is true.

This option does hint that the sun might be important.

(C) If Pluto had not been ejected from its orbit around Neptune, Pluto would not have its current orbit around the sun and would still be a moon.
Off topic.
We have no idea whether Pluto was a moon. Ignore the rest of the answer because
"Pluto was a moon" is not a given fact.

(D) The size of a celestial body in orbit around the sun is not relevant to determining whether or not it is a true planet.
Counterfactual or irrelevant.
-- Counterfactual: size IS relevant.
We can infer from the FACTUAL phrase "although Pluto has an atmosphere and is bigger than an asteroid"
that a true planet is bigger than an asteroid.
If something had not gone wrong with Pluto, it would indeed be a planet.
Size DOES matter. (D) is counterfactual and does not help to prove the conclusion.

-- irrelevant.
Okay. Size is not relevant. That fact does not prove that Pluto is NOT a planet.
If size is not relevant, then whatever size Pluto is does not matter.
"Size is not relevant" does NOT prove that Pluto is not a planet.

(E) For a celestial body to be a true planet it must have formed in orbit around the sun exclusively.
By POE, we are left with E.

This statement works. Its content is exactly what we are missing.

ASSUMPTION THAT IS TRUE:
If a celestial body is a true planet, then it formed in orbit around the sun and only the sun.
Pluto formed in orbit around Neptune.
Pluto did not form in orbit around the sun.
Pluto cannot be a true planet because it did NOT form in orbit around the sun.

[Notice: we still do not know whether Pluto orbits the sun. I like this answer better already.]

True planets must have been formed in orbit around the sun exclusively.
Pluto, which formed in orbit around Neptune, is not a true planet.
That's the conclusion.

Option E is the correct answer.

Hope that helps.
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Although Pluto has an atmosphere and is much larger than any asteroid, Pluto is not a true planet. Pluto formed in orbit around the planet Neptune and was then ejected from orbit around Neptune when Triton, Neptune's largest moon, was captured by Neptune's gravity.

The conclusion of the argument follows logically if which one of the following is assumed?

(A) No celestial body can simultaneously be a moon and a planet. -The passage doesn't state that Pluto was a moon
(B) Not all celestial bodies that have an atmosphere and orbit the sun are true planets. -This is just a fact statement.
(C) If Pluto had not been ejected from its orbit around Neptune, Pluto would not have its current orbit around the sun and would still be a moon. -This is a hypothetical situation, not talked about in the passage.
(D) The size of a celestial body in orbit around the sun is not relevant to determining whether or not it is a true planet. -This is opposite of the premise, which compares the size of Pluto to that of an asteroid.
(E) For a celestial body to be a true planet it must have formed in orbit around the sun exclusively. -Correct. If Pluto wasn't Neptune's part, it would have been a true planet
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Pluto has an atmosphere and is much larger than any asteroid (some characteristics of a planet)
Pluto formed in orbit around the planet Neptune and was then ejected (supports Pluto is not a planet)

Conclusion: Pluto is not a true planet.

What will help us establish that Pluto is not a true planet.

We are looking for an assumption (a missing necessary premise) which will help us establish the conclusion. Which of the options should we add to our argument so that we can conclude that Pluto is indeed not a planet?

(A) No celestial body can simultaneously be a moon and a planet.

Pluto is not a moon now. This doesn't tell us why it cannot be a planet.

(B) Not all celestial bodies that have an atmosphere and orbit the sun are true planets.

Doesn't tell us why Pluto cannot be a planet. What characteristics ensure that it is not a planet.

(C) If Pluto had not been ejected from its orbit around Neptune, Pluto would not have its current orbit around the sun and would still be a moon.

Even if this were true, Pluto did get ejected from its orbit and is not a moon now. How can we say that it cannot be a planet now?

(D) The size of a celestial body in orbit around the sun is not relevant to determining whether or not it is a true planet.

The question is what determines whether a celestial body is a true planet.

(E) For a celestial body to be a true planet it must have formed in orbit around the sun exclusively.

Perfect. If we are given that to be a true planet, the body must have formed in orbit around the sun, then we can say for sure that Pluto is not a true planet. Let's add this to our argument and see if our conclusion can be drawn now.

Pluto has an atmosphere and is much larger than any asteroid.
Pluto formed in orbit around the planet Neptune and was then ejected.
For a celestial body to be a true planet it must have formed in orbit around the sun exclusively.

Conclusion: Pluto is not a true planet.

Correct. We can conclude our conclusion now.
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