Q13 - Astronomers have long thought that the irregularity in

[dukeag]

Why D and not C?

[fmuirhea]

(C) is a little too specific - who says it has to be a planet that's exerting the extra gravitational pull? Perhaps some other celestial body is responsible; the stimulus does not state or suggest that planets are the only things that can exert gravitational pull. Furthermore, how can we say that it's undiscovered or beyond the orbit of Pluto? Sometimes vague, boring answers are your best bet on inference questions.

The stimulus presents a common LSAT pattern and expects you to fill in the missing piece:

"We used to think [some idea], but then we learned [new evidence], so now we think that [previous idea] is/might be incorrect."

Or, a slight variation:

"Some people think [some idea], but they haven't considered [some evidence], so these people are/might be wrong."

(You can look back to Q3 in this section, "A new medication for migraine..." to see another example of this pattern.)

The stimulus here conforms to the first pattern:

"Astronomers used to think that [Neptune's irregular orbit was fully explained by Pluto's gravitational pull], but then they learned that [Pluto isn't big enough to exert as much gravitational pull as they once thought]..."

Noticing this pattern and the missing final piece, my prephrase would be, "Neptune's irregular orbit is not fully explained by Pluto's gravitational pull." That exact phrasing doesn't turn up in the answer choices, but (D) is certainly the best paraphrase.

[ohthatpatrick]

Awesome explanation, as always, fmuirhea. You should consider teaching LSAT, if you’re not already.

I’ll just add to that explanation that if we were picking (C), we would ALSO think (D) is true.

We would only pick (C) if we thought we needed something BESIDES the gravitational pull of Pluto to account for Neptune’s irregular orbit.

Well if we think that, then (D) is definitely something we agree with!

Now and then, you’ll be down to two answer and you’ll see that one of them is actually embedded in, or a logical implication of, the other.

i.e. you’ll be down to (C) and (D) and realize, “Well if (C) is true, then (D) is ALSO true”. In those cases, you can never pick that first one that implies the second. You have to pick the more limited, boring, weaker second one that doesn’t necessarily imply the first.

=== other answers ===

(A) Can’t support that Neptune is larger. What changed our impression of the Neptune / Pluto situation was that Pluto is smaller (or at least less massive) than we previously believed.

(B) Can’t support “more irregular”. The idea is that that Neptune has a consistent but irregular orbit. We use to explain its irregularity by Pluto’s influence, but now we realize there must be more to the story. But Neptune’s orbit hasn’t changed as far as we know.

(E) SUPER dangerous to predict the future on any Inference question. How can we PROVE that in the future we’ll find out Pluto is even smaller than we now think? Maybe our current observation is correct (or maybe we’ll revise our estimate upwards).