PrepTest 103, Section 1, Question 9
Measurements of the motion of the planet Uranus(76 more words not shown)
Which one of the following,(7 more words not shown)
- APluto was not(3 more words not shown)
- BThere is a(12 more words not shown)
- CNeither Neptune nor(6 more words not shown)
- DThe force the(15 more words not shown)
- EUranus' orbit is(9 more words not shown)
Reading the argument
Why does the thing pulling Uranus have to be a planet?
The known outer planets don't have enough mass to account for the measured pull. That gives us a reason to look for another explanation. But “something else” and “another planet” aren't interchangeable. Could a collection of comets or asteroids supply the pull instead?
What we’re looking for
The conclusion says there must be an undiscovered planet in our solar system. To weaken it, I want a plausible source of the force that isn't a planet. We don't have to show that an undiscovered planet is impossible. We just need to undermine the claim that one is required.
Working the answers
- A
When Pluto was discovered doesn't explain the force we're investigating. A late discovery might remind us that planets can go unnoticed, but it doesn't give us a reason to reject this particular argument. We need to address what could be pulling Uranus.
- BCorrect
A belt of comets beyond Pluto offers another source of powerful gravitational pull. That fits our prediction: something other than an additional planet could account for the unexplained force. The argument's evidence may point to another source of gravity, but it doesn't force us to call that source a planet.
- C
We already know that Neptune and Pluto don't have enough mass to exert the measured force. Comparing their mass with Uranus's doesn't identify a different explanation or undermine the need to look beyond them. It leaves the argument's leap to another planet intact.
- D
This compares the Sun's pull on Uranus with its pull on planets closer to the Sun. The unexplained force, however, pulls Uranus away from the Sun and the inner planets. A weaker inward pull is not by itself an explanation of the additional outward pull described in the passage. We still need to know where that force comes from.
- E
Knowing that Uranus's orbit is closer to Neptune's than to Pluto's doesn't change the stated fact that those two planets lack enough mass to produce the measured force. E gives a relative distance, but it doesn't supply the alternative source of the force that would challenge the conclusion.
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