Recent observations suggest that small, earthlike worlds form a very

How is D the correct answer choice? It mentions that smaller planets relative to the size of the stars are difficult to detect. Th option is not talking about all the small planets. It is talking about the size of the planets relative to the size of the stars.

Our argument says that the stars are smaller as compared to the sun. What if the size of the stars and the planets are almost the same? How is D making sense then?

Which would weaken the justification of the claim?

C. A planet orbiting a star similar to the sun would be more likely to be earthlike in size than would a planet orbiting a much smaller star.

We don't know how many stars are Sun-like. If Sun-like stars form a very low percentage, Earth like planets will form a very low percentage too. We need to weaken the justification i.e. the observation.

D. The smaller a planet is relative to the star it orbits, the more difficult it is for astronomers to detect.

This says that our sample may not be representative of the population. If smaller planets are harder to detect, only large planets might have been detected. Hence, of the 200 planets, most were large because they were easy to find. Perhaps, smaller planets are harder to find and hence were not a part of the 200.

Answer (D)

In a sampling CR, information about A SAMPLE is used to draw a conclusion about A WHOLE GROUP.
To weaken the conclusion, the correct answer will typically indicate WHY the sample might not accurately reflect the whole group.

Case 1: https://gmatclub.com/forum/guidebook-writer-i-have-visited-hotels-throughout-the-country-and-hav-80358.html
Here, the OA suggests that poorly constructed hotels have likely been torn down.
Implication:
The visited hotels in the passage do not accurately reflect ALL hotels built before 1930.

Case 2: https://gmatclub.com/forum/people-who-h ... 23222.html
Here, the OA suggests that zoo employees with allergy issues are likely to switch to another occupation.
Implication:
The zoo employees in the survey do not accurately reflect ALL zoo employees with allergy issues.


D: The smaller a planet is relative to the star it orbits, the more difficult it is for astronomers to detect.
Here, earth-like planets in the galaxy are likely to be so small that they cannot be detected.
Implication:
The detected planets in the passage do not accurately reflect ALL planets in the galaxy.



A: There are millions of planets...which astronomers have not attempted to detect.
Here, there is no indication WHY the planets in red might differ from those detected by the astronomers.
Eliminate A.

Granted that (A) is weak, but (D) too is no better!

I am not sure how valid are my concerns, and i would appreciate your views.

I agree that some of these new questions are tricky! As usual, the exact language of the passage, question, and answer choices will help to eliminate the incorrect answers.

Let's first take a look at the passage:

• Conclusion: "Recent observations suggest that small, earthlike worlds form a very low percentage of the planets orbiting stars in the galaxy other than the sun."
• Justification for this conclusion: "Of over two hundred planets that astronomers have detected around other stars, almost all are hundreds of times larger and heavier than the earth and orbit stars much smaller than the sun."

The question asks which answer choice would "would most weaken the above justification of the claim that earthlike worlds form a low percentage of the total number of planets." There are a couple of things of note in this question:

• It points us toward the justification for the claim. So, we are trying to weaken that particular piece of the passage, or the link between the justification and the passage's conclusion.
• We need to find the answer that most weakens the justification -- this leaves open the possibility that multiple answer choices weaken the justification. We may need to eliminate the weaker weakeners (is your head spinning yet?), and keep the strongest weakener.

Take a look at (A):

To clarify the meaning of this sentence, note that the things "around which" astronomers have not attempted to detect planets are the "stars" mentioned earlier in the sentence. In other words, astronomers haven't attempted to detect planets around certain stars, and around these stars are millions of planets.

At a glance, this seems to weaken the force of the justification in the passage! Astronomers have only detected 200 planets, so perhaps these are not representative of the millions of other planets out there. Maybe a higher percentage of the not-yet-detected planets are earthlike, which would undermine the author's conclusion that a very low percentage of planets in the galaxy are small and earthlike.

After a bit more thought, though (A) is a pretty weak weakener (for the exact reason mentioned in the OE, which I'll try to explain a bit).

The author's conclusion is focused on the planets in our galaxy, while (A) just tells us that we haven't yet detected a bunch of planets out there somewhere in space. Maybe the 200 planets that were already detected are all within our galaxy, while the millions of undetected planets are outside of our galaxy. In this case, the justification in the passage would not be weakened very much, because the data from the 200 planets would be much more relevant to the conclusion than would the new information provided by (A).

We don't know for sure whether this is true, so we just have to keep in mind that (A) may weaken the justification provided in the passage, or it may not.

Now take a look at (D):

The justification in the passage for the author's conclusion is that "almost all" of the 200 planets detected by astronomers are much heavier than the earth and orbit much smaller stars. From this, the author concludes that a very low percentage of planets in the galaxy are small/earthlike.

But wait -- what if astronomers just suck at detecting small/earthlike planets in the first place? Then the justification (that almost all of the detected planets are big) doesn't mean much. It just shows that we are good at detecting one kind of planet, and bad at detecting another. The link between the 200 detected planets and the conclusion is pretty much destroyed.

(D), if true, makes the justification provided in the passage kind of worthless. (A), in comparison, is the weaker weakener -- because we don't know whether the millions of undetected planets are in our galaxy, we don't know how much it impacts the force of the passage's justification.

For this reason, (A) is out and (D) is the right answer.

I hope that helps!