PrepTest 143, Section 1, Question 15
Astronomer: In most cases in which a planet(75 more words not shown)
Which one of the following,(8 more words not shown)
- AWhen two planets(21 more words not shown)
- BThere is no(23 more words not shown)
- CIn most cases(20 more words not shown)
- DMost comets with(19 more words not shown)
- EFor each distant(25 more words not shown)
Reading the argument
Close encounters can produce oval orbits. Are they what produced these oval orbits?
The example from our solar system gives the astronomer a possible explanation: close encounters with planets have shifted comets into oval orbits. Maybe encounters between planets did something similar around distant stars. But we should still ask whether those planets could have acquired oval orbits in some other way.
What we’re looking for
For a strengthener, my first thought is evidence ruling out other ways of producing the shape. If close encounters were the only way, that would make the explanation much stronger. That's more than we need, though. The conclusion only says some of the planets probably acquired their orbits this way. Evidence making that account more plausible could be enough.
Working the answers
- A
This tells us which object is usually affected more when an encounter happens. It doesn't establish that the planets under discussion had such encounters or give us a reason to attribute their oval orbits to them. Relative size doesn't fill that gap.
- B
A lack of evidence for encounters between planets around our sun doesn't support the proposed history of planets around distant stars. The astronomer uses our comets as an example of what encounters can do; B doesn't add evidence that the proposed encounters happened elsewhere.
- CCorrect
This is a more modest form of support than my initial prediction. For a planet to encounter another planet orbiting the same star, another planet has to be available. Finding multiple planets in most of the observed systems makes that proposed interaction more plausible.
Imagine instead that a system had only one planet and never had another. There would be no fellow planet for the encounter described. C gives us evidence of possible partners. It doesn't show that they passed close to one another or prove that encounters caused the oval orbits, but it supports an opportunity the explanation needs. We don't have to establish the only possible cause to strengthen the argument.
- D
This adds information about comets around our sun. We already have evidence that close encounters can put those objects into oval orbits. The unresolved question is whether encounters between planets explain some distant planets' orbits. More detail about the original comet example doesn't supply the evidence about those other systems that C does.
- E
Read “other object” to include another planet. E says no other object large enough to affect the discovered planet's orbit has been found around that star. That doesn't eliminate just a competing explanation; it withholds evidence of the very partner the astronomer's explanation calls for.
Not finding another object doesn't prove one never existed. Still, this lack of a discovered partner doesn't support the proposed encounter. C goes in the useful direction by showing that other planets are present.
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