PrepTest 114, Section 2, Question 25
In our solar system only one of the(41 more words not shown)
The argument is questionable because(10 more words not shown)
- AIf a planet(8 more words not shown)
- BOur solar system(10 more words not shown)
- CThe conditions necessary(7 more words not shown)
- DLife similar to(11 more words not shown)
- EMost other planetary(7 more words not shown)
Reading the argument
One in nine here doesn't mean one in nine everywhere. The argument gives a ratio for our solar system, then uses it across the universe. Why should this one system be typical? Perhaps life-supporting planets are much rarer elsewhere. Perhaps they're more common. The local ratio doesn't tell us which.
Imagine that the relevant ratio elsewhere were one in a million instead of one in nine. That is an alternative to consider, not an astronomical fact we're adding. It shows why the calculation depends on an assumption about other systems. Even a very large universe doesn't, by itself, justify carrying our particular ratio across to it.
What we’re looking for
The question asks which unjustified assumption makes the argument questionable. I'd look for the assumption that other planetary systems resemble ours in the respect that matters: how many of their planets are fit to sustain life.
Working the answers
- A
Being fit to sustain life doesn't guarantee that life actually arises. The conclusion concerns suitable planets, not planets already inhabited. A adds a claim about life developing that the calculation doesn't require.
- BCorrect
The argument treats our solar system as a useful guide to many others. That's what lets it take the one-in-nine ratio and use the number of systems in the universe to reach its much larger total.
The relevant similarity isn't that the systems look alike in every detail. It's that our ratio gives us a basis for estimating how common life-supporting planets are elsewhere. We haven't been given evidence for that assumption. Pointing out the gap doesn't prove the conclusion false; it shows why this local figure doesn't establish it.
- C
The argument could use a ratio of suitable planets without explaining every condition life needs in order to begin. Our objection is about whether that ratio applies beyond this solar system. A general claim that the conditions are well understood doesn't identify that assumption.
- D
The conclusion allows some form of life, not only life similar to Earth's. It also doesn't depend on such life evolving under very different conditions. The questionable step is carrying the local proportion into other systems, not establishing every kind of environment where life could evolve.
- E
Nine planets in every system isn't required to apply a one-in-nine proportion. A system with eighteen planets could have two suitable ones, for example, and preserve that ratio. The number of planets and the fraction that can sustain life are different. The unsupported assumption concerns the latter.
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