Recent observations suggest that small, earthlike worlds form a very

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!
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Claim: Small planets like Earth are a small percent of planets orbiting stars (other than Sun) in our galaxy.
- Of 200 planets examined, almost all are much larger than Earth and their star much smaller than Sun.

Which would weaken the justification of the claim?

A. There are millions of planets orbiting stars around which astronomers have not attempted to detect planets.

This says that there are millions of planets which have not been examined. Nevertheless, the sample of 200 examined show a particular characteristic. Nothing says that the sample is not representative. Sampling is a valid technique if sample is a good representation of the population. Also, we don't know whether the stars being talked about are from this galaxy.

B. The best current astronomical theories predict that almost all planets around other stars are probably hundreds of times larger than the earth.

This supports our 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.

E. The observations would have detected any small, earthlike worlds orbiting the stars around which larger planets have been detected.

This tells us that small planets would have been easy to detect too. This helps our claim.

Answer (D)

The argument implies that sample of 200 planets is representative of planets in entire galaxy and that percentage of earth-like planets in the galaxy is very low. We need to weaken the claim.

A indicates that there are millions of other stars whose planets have not been detected. No one can sample millions of stars anyway, and this choice does not indicate why the sample taken is not representative.

B & E do not weaken the claim.

C is trap - it states that an earth-like planet is more likely to be found orbiting a sun-like star. In the absence of information about percentage of sun-like stars in the galaxy, it is not possible to comment on percentage of earth-like planets. Also, the argument says

D correctly point out that smaller planets (in relation to the size of star) are hard to detect. Thus the 200 planets detected cannot truly indicate the percentage of earth-like stars (as it is likely that only bigger planets were detected while many earth-like planets lie undetected).

Dear Karishma,
Can you explain in more detail?
Regards,
Basim

Posted from my mobile device

Official Explanation

Argument Evaluation

This question asks you to identify a claim that would, if true, weaken the justification for the conclusion that only a small percentage of the total number of planets in our galaxy are formed by earthlike worlds.

The only justification given for this conclusion is that, of the over 200 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.

Any evidence suggesting that the planets the astronomers have detected may be unrepresentative of planets in general would weaken the justification this claim gives to the conclusion.

A. This may look like it weakens the justification. After all, if the total number of planets were significantly smaller than millions, then the sample size of over 200 planets would allow us to make the inference with more confidence. Nevertheless, particularly when accounting for the vagueness of the conclusion, the size of the sample is large enough to give us a reasonable degree of certainty. More importantly, though, note that the conclusion is restricted to planets orbiting stars in our galaxy. There is nothing in this answer choice to suggest that the planets it refers to are actually in our galaxy.

B. This choice strengthens the justification for the conclusion.

C. This choice does not weaken the justification for the conclusion. The only way it might do so is if it provided information showing that astronomers have mainly looked at planets orbiting an unrepresentative sample of stars, that is, a sample that is more heavily populated with planets orbiting stars that are smaller than most stars in the total star population. We are given no reason to believe this is the case.

D. Correct. This gives us a reason to think that the sample may be unrepresentative. Planets more earthlike in size may be less likely to be detected than the much larger stars that astronomers have detected.

E. This claim strengthens, rather than weakens, the argument. It implies that the astronomers' detection methods would not have inadvertently underrepresented the number of earthlike worlds.

The correct answer is D.

What is with the wording of the new edition of CR questions? I feel like the answer choices are extremely difficult to comprehend.

What does this even mean?
A. There are millions of planets orbiting stars around which astronomers have not attempted to detect planets.

Astronomers have not tried to detect planets out of the millions of planets that orbit the many number of stars? Isn't it supposed to be trying to detect earth-like planets?

Moreover, the OG explanation for why A is incorrect includes "More importantly, though, note that the conclusion is restricted to planets orbiting stars in our galaxy. There is nothing in this answer choice to suggest that the planets it refers to are actually in our galaxy."
Where does the passage explicitly narrow in on our galaxy, and exclude other galaxies?

hi experts,
I have the same problem with A , please help

VeritasKarishma GMATNinja nightblade354 and other experts please help us out here!
Why is A wrong? Even after reading the OG explanation I'm not at all convinced. A seems fair. A vs D is hard. They're so close.
Any help would be appreciated. Thanks in advance

VeritasKarishma GMATNinja generis GMATGuruNY daagh mikemcgarry
Can you please explain why option A is wrong and option C is correct ?

New Video: Fair warning, I do a better job of explaining why A is wrong then explaining why D is correct!

Also, NEVER MAKE ASSUMPTIONS TO JUSTIFY ANSWERS! THIS IS WHY I HATE THIS QUESTION!


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Answer: D

Evidence : 200 planets detected, All those are bigger than Earth.
Conclusion : Earth-like planets form very low % of all planets.


Can we be sure of what could be the size of those planets if at all astronomers attempted to detect them ? NO

This one strengthens the argument.

Argument is about planet size irrespective of their respective sun's size.


Makes sense. Those smaller planets (earth-like) are more difficult to detect. Hence, we cannot come to any conclusion on how many such planets are present.

This one too strengthens the argument as it says that the observations reflect the actual presence of planets.

Also, I want to share my two cents on this question to clear up the doubt on A and D on the forum:

Let's look at A-
It says that "Astronomers have not attempted to detect"- which means that they just have not attempted to detect, if they plan on attempting to find those more planets out there then there is an equal possibility that those may/may not be "earthlike/small". Hence, we don't get 100% weakened on this. I don't fully agree with the analysis around "our galaxy" that other people have given here. I think that is not a very deterministic error.

Option D solves this uncertainty-
It simply bolsters the belief that those "small/earth-like planets" are hard to locate/detect due to which we can infer that the astronomers have only been able to detect large and heavy planets.

Deferring to experts like GMATNinja ManhattanPrep @karishmaveritas egmat MagooshExpert to share their thoughts and to find if they agree with my above thought process

Thanks!
Hope I was able to add some value BACK to this forum!
mk96

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.
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Thank you for explanation. However, I think that the confusion point in (D) is that it does not tell that "the smaller the planet, the harder to detect", it simply says that "the smaller the planet RELATIVE TO ITS STAR it orbits, the harder to detect". So, links the size of planet to the size of its star. May be the planet is really small and detectable, but relative to its star it is big.

This is exactly my reasoning for eliminating D. What if the planet is small and earthlike but is equal to the size of its star. So now we can't say for certain that this planet will not be detected by the astronomers.

Correct option : D

A. There are millions of planets orbiting stars around which astronomers have not attempted to detect planets.
Wrong: it says have not attempted, if attempted, more number will be found and as per passage their might be possiablity, the large planet ratio will increase, this option keep confusion, hence irrelavant

B. The best current astronomical theories predict that almost all planets around other stars are probably hundreds of times larger than the earth.
Wrong: This option looks like stregthen

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.
Wrong: Stars is smaller in size, that means orbiting planet is larger in size than star, this option looks like strenghten

D. The smaller a planet is relative to the star it orbits, the more difficult it is for astronomers to detect.
Correct : a weaken, becasue not detected, small size planet not found, makes the claim demean

E. The observations would have detected any small, earthlike worlds orbiting the stars around which larger planets have been detected
Wrong : Strenghten

ThatDudeKnows avigutman GMATNinja If we knew that these millions of planets are in our galaxy, then would we say the data is unrepresentative (because of a small sample) or that we still do not know if it is representative or not ( given that nothing else is there to support why 200 is representative or not of millions of planets)? This for choice A

Sample size arguments can be tricky for exactly the reason that you've fallen into, but let's see if we can get a little clarity.

Imagine a bag of marbles. Millions of marbles. Some are red and some are blue, but we have no idea what percentage of each. You reach in the bag 200 times and pull out a marble each time. Three are blue. That seems to suggest that very few are blue, right? 200 out of millions is a small percentage of the overall, but it's still 200 trials, which is a lot, so the odds are really good that it is a representative sample of the millions.

Imagine a galaxy of planets. Millions of planets. Some are red and some are earthlike, but we have no idea what percentage of each. You reach in the galaxy 200 times and pull out a planet each time. Three are blue. That seems to suggest that very few are blue, right? 200 out of millions is a small percentage of the overall, but it's still 200 trials, which is a lot, so the odds are really good that it is a representative sample of the millions.

The argument's conclusion is that recent findings SUGGEST blahblahblah. Just because there are a ton of other planets to look at without any reason to expect them to be red doesn't weaken the suggestion.

If we only picked three marbles from the bag, sure, sample size matters. But quality of sample size is more a function of the number of things IN the sample than it is the number of things NOT IN the sample, and 200 is a lot for a binary distribution.

There are several possible reasons for why a sample might not represent the general population that you're trying to sample.
In this case, the correct answer plays on sampling bias (the planets in our sample is selected in a non-random manner, due to difficulties with detection).
I don't recall ever seeing an official GMAT problem in which the size of the sample was the weakness of a statistical analysis.
You'll probably benefit from learning about different statistical biases:
https://en.wikipedia.org/wiki/Selection_bias
I made a short video on one of them, which you can view here:
https://youtu.be/zI51kLcEJDw