LSAT Explanation PT 31, S3, Q25: In our solar system only one

LSAT Question Stem

The argument is questionable because it presumes which one of the following without providing justification? 

Logical Reasoning Question Type

This is a Flaw question. 

Correct Answer

The correct answer to this question is B. 

LSAT Question Complete Explanation

First, let's analyze the argument in the passage. The argument can be broken down as follows:

Premise 1: In our solar system, only one of the nine planets (Earth) qualifies as fit to sustain life.

Premise 2: There is an astonishingly large number of planetary systems in the universe.

Conclusion: The number of planets fit to sustain some form of life is extremely large.

The question type for this problem is a Flaw question, which asks us to identify the assumption that the argument presumes without providing justification. Before we discuss the answer choices, let's come up with an "Evaluate" question about the argument. An appropriate "Evaluate" question for this argument would be: "Are other planetary systems similar to our solar system in terms of the ratio of planets that can sustain life?"

Now, let's go through the answer choices.

a) If a planet is Earthlike, then life will arise on it.

This answer choice is incorrect because the argument is only concerned with planets being "fit to sustain life," not whether life will actually arise on them.

b) Our solar system is similar to many other planetary systems in the universe.

This is the correct answer. The argument assumes that the 1:9 ratio of life-sustaining planets in our solar system can be applied to other planetary systems without providing any justification for this assumption.

c) The conditions necessary for life to begin are well understood.

This answer choice is incorrect because the argument does not rely on understanding the conditions necessary for life to begin. The argument is based on a simple observation of the ratio of life-sustaining planets in our solar system and applying that ratio to other planetary systems.

d) Life similar to Earth's could evolve under conditions very different from those on Earth.

This answer choice is incorrect because it weakens the analogy between our solar system and other planetary systems, which the argument relies on. The author assumes that life would be possible elsewhere in similar proportions to how it's possible in our solar system.

e) Most other planetary systems in the universe have nine planets.

This answer choice is incorrect because the argument does not assume that most planetary systems have the same number of planets as our solar system. The argument is based on applying the 1:9 ratio to other planetary systems, not the raw number of planets.

In conclusion, the correct answer is B, as the argument presumes without providing justification that our solar system is similar to many other planetary systems in the universe.