Recent observations suggest that small, earthlike worlds form a very

Can someone tell me why C is not the right answer? It says:

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.

Since astronomers have only been looking at planets that orbit stars much smaller than the sun, C is exposing a weakness in "data collection", by mentioning that astronomers are looking at the wrong data; they should be looking at a star "similar to the sun" and not "much smaller than the sun". That way, they would be able to find planets more likely to be earthlike in size.

Of course we don't know "how many" such stars exist, but the point is that C does prove a flaw in data collection; isn't that the point of this question?

AjiteshArun KarishmaB GMATNinja GMATGuruNY

Thanks Ajitesh. I found C really difficult to eliminate.

Honestly, I find C also suggesting that they're looking at a biased sample.

They are looking at "stars much smaller than the sun". C says (implies) that this is a biased sample because planets earthlike in size are not likely to be found by looking at stars much smaller than the sun. Is this not a bias?

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)­

But where does it in the passage it says "our galaxy"? All it says is "in the galaxy".

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!