The conclusion of the argument is: "the chances that intelligent life will emerge on a planet are, therefore, low."
Why? We are told that when a planetary system forms, the chances of a planet that is capable of supporting life being formed is high, while the chances of a large planet the size of Jupiter or Saturn being formed is low. Without large planets like these, intelligent life would have never arisen on Earth because the large planets' gravitational forces prevented Earth from being frequently struck by large comets.
So the reason why these large planets were needed in the case of Earth was that their gravitational forces prevented Earth from being frequently struck by large comets. However, we do not know how common large comets are in planetary systems. It's possible that only the planetary system that Earth is a part of has many large comets. If that is the case, then the fact that the occurrence of large planets is low may not necessarily mean that the chances of intelligent life emerging will also be low. If, however, large comets are very common, then it would be true that the chances of intelligent life emerging would be low, since the large planets needed to deter the large comets have a small chance of forming.
Thus, answer choice (D)--"how high the chances are that planetary systems will contain many large comets"--is our correct answer because it is most useful in helping us evaluate the argument.
Hope that was helpful! Please let us know if you have any other questions.
JosephJanuary 17, 2016
Could you explain the reasoning behind B being incorrect?
MehranJanuary 24, 2016
The problem with (B) is that it is not useful in evaluating the argument. It is discussing the possibility of intelligent species surviving if a comet struck their planet.
First, notice this argument is about LARGE comets. (B) just says "comet."
Additionally, this argument is about intelligent life being able to arise (i.e. evolve) on a planet that is frequently struck by large comets.
Whether such intelligent life would survive if a comet struck their planet has no bearing on whether or not that intelligent life could have evolved on a planet in a planetary system without large planets such as Jupiter and Saturn protecting it from being struck by large comets.
Hope this helps! Please let us know if you have any other questions.
