Astronomer: In most cases in which a planet has been detected orbiting a distant star, the planet's orbit is distinct...

ifuhrmanb on September 12, 2019

I think that D would strengthen too. What's wrong with D?

Irina on September 13, 2019

@ifuhrmanb,

The astronomer concludes that the planet's oval orbit is likely the result of close encounters with other planets orbiting the same star supported by the evidence that the comets orbiting the sun were thrown into oval orbits by close encounters with planets in the solar system. The question requires us to find a statement that would strengthen this conclusion. Let's look at the answer choices:

(A) When two planets or other objects have an encounter, usually the smaller one is the more greatly affected.

Incorrect. There are a couple of issues with this answer choice, first of all we cannot quantify how smaller and what exactly is more greatly affected, is this answer choice trying to say that only one planet would be throw into an oval orbit? Or both planets but resulting in distinct orbit shapes? Second, this answer choice appears to corroborate the evidence provided by the astronomer, comets - a smaller object -that have an encounter tend to have oval orbits but the planets in the solar systems still have circular, yet it fails to strengthen the conclusion as it requires us to make an implausible inference that each planet with an oval orbit is a result of a close encounter with a larger planet.

(B) There is no indication that the orbit of any planet in the solar system has been affected by a close encounter with another planet.

Incorrect. This answer choice would weaken the argument. If no planet has been affected by a close encounter with another planet in a solar system, the conclusion that this would be the case for any other star appears unwarranted.

(C) In most cases in which planets have been discovered orbiting a distant star, more than one planet has been found orbiting that star.

Correct. The argument requires that more than one planet is orbiting the same start, otherwise, a close encounter with another planet would be impossible. If this were false, the conclusion would no longer make sense, hence we can conclude that that this fact strengthens the argument.

(D) Most comets in the solar system were thrown into an oval orbit by a close encounter with some other object.

Incorrect. This is a tricky one, notice how the argument tells us that "many comets ..have been thrown into an oval orbit due to a close encounter with the planets orbiting the sun," whereas this fact states that MOST comets have oval orbit due to a close encounter with some other object - not necessarily even orbiting the sun. This fact would weaken the argument, suggesting that an object could get an oval orbit from an encounter with any other object anywhere in the universe, e.g. a passing asteroid for instance, not necessarily the one orbiting the same star, as the astronomer argues.

(E) For each distant star that have been found to have a planet, no other object large enough to affect the planet's orbit has been found orbiting the same star.

Incorrect. This answer choice appears to suggest that no other planets have been found orbiting the same star, hence weakening the astronomer's argument that an oval orbit is a result of a close encounter of two planets.

Let me know if this helps and if you have any further questions.