Recent observations suggest that small, earthlike worlds form a very low percentage of the planets orbiting stars in the galaxy...

GMAT Critical Reasoning : (CR) Questions

Source: Official Guide

Critical Reasoning

Weaken

HARD

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Recent observations suggest that small, earthlike worlds form a very low percentage of the planets orbiting stars in the galaxy other than the sun. 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.

Which of the following, if true, would most weaken the above justification of the claim that earthlike worlds form a low percentage of the total number of planets?

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

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

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.

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

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

Solution

Passage Analysis:

Text from Passage	Analysis
Recent observations suggest that small, earthlike worlds form a very low percentage of the planets orbiting stars in the galaxy other than the sun.	What it says: Recent findings show that earthlike planets are rare around other stars in our galaxy What it does: Opens with the main claim that we'll need evidence to support What it is: Author's main conclusion based on astronomical observations
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.	What it says: Out of 200+ detected planets, nearly all are much bigger than Earth and orbit smaller stars What it does: Provides specific evidence to back up the claim about earthlike worlds being rare What it is: Supporting evidence from astronomical data Visualization: 200+ detected planets → Almost all are 100x+ larger than Earth

Argument Flow:

The argument starts with a broad conclusion about earthlike planets being rare, then immediately provides specific observational data to support this claim. It flows from general claim to specific evidence.

Main Conclusion:

Small, earthlike worlds make up a very low percentage of planets orbiting other stars in our galaxy.

Logical Structure:

The evidence (200+ detected planets are almost all much larger than Earth) is used to support the conclusion (earthlike worlds are rare). This is a straightforward evidence-to-conclusion structure where the sample data is meant to represent the broader galactic population.

Prethinking:

Question type:

Weaken - We need to find information that would reduce our belief in the conclusion that earthlike worlds form a low percentage of planets around other stars

Precision of Claims

The argument makes a quantitative claim about percentage (very low) based on a specific sample (200+ detected planets) and qualitative comparisons (hundreds of times larger/heavier, orbiting smaller stars)

Strategy

To weaken this argument, we need to find information that suggests the sample of 200+ detected planets might not be representative of all planets out there. The key weakness could be in the detection methods - maybe our current technology is biased toward finding certain types of planets and missing earthlike ones. We should look for scenarios where the evidence doesn't actually support the conclusion about the true percentage of earthlike worlds

Answer Choices Explained

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

This choice points out that there are millions of unobserved planets around other stars. However, this doesn't weaken the argument because we have no reason to believe that planets around unobserved stars would have a different size distribution than planets around the 200+ observed systems. The argument is making a claim about the general percentage of earthlike worlds based on a substantial sample, and the existence of unobserved planets doesn't suggest our sample is biased or unrepresentative.

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

This actually strengthens rather than weakens the argument. If astronomical theories predict that planets around other stars are mostly much larger than Earth, this supports the conclusion that earthlike worlds are rare. The question asks us to weaken the argument, so a choice that provides additional independent support for the conclusion moves us in the wrong direction.

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.

While this tells us something about where we might be more likely to find earthlike planets (around sun-like stars rather than smaller stars), it doesn't challenge the representativeness of our current sample or suggest that our detection methods are missing earthlike worlds. The observed data still shows that almost all detected planets are much larger than Earth, regardless of what star types they orbit.

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

This directly attacks the representativeness of our sample by revealing a detection bias. If smaller planets are systematically harder to detect than larger ones, then our sample of 200+ detected planets would be skewed toward larger planets, regardless of the true distribution of planet sizes. This means the observed low percentage of earthlike worlds in our sample doesn't necessarily reflect the actual percentage in the galaxy - we might just be missing most of the small planets due to technological limitations. This creates significant doubt about the conclusion.

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

This would actually strengthen the argument by confirming that our detection methods are comprehensive enough to find small, earthlike worlds if they were present around the observed stars. If our instruments would have detected earthlike planets but didn't find them, this supports rather than weakens the conclusion that such planets are rare.

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