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The author concludes that there must be at least one undiscovered planet in our solar system. This is because measurements show that Uranus is being pulled away from the Sun by some force, and the planets further from the Sun than Uranus are too small to provide that force.

The author assumes that the cause of the force pulling Uranus away from the Sun has to be a planet, rather than some other celestial object or phenomenon.

At best, this answer choice suggests that because Pluto was only discovered recently, perhaps there are more planets that haven't yet been discovered. Even if that argument were sound, it would strengthen the conclusion, not weaken it -- but the date Pluto was discovered is really irrelevant to whether there is another undiscovered planet in our solar system.

This gives us a potential alternative explanation for the force pulling Uranus away from the Sun: a belt of comets, rather than some other planet. This weakens the author's hypothesis.

We don't know how how massive an object has to be to exert a pull on Uranus, and besides, we already know Neptune and Pluto don’t have the mass to exert the force pulling Uranus away from the Sun. This doesn’t weaken anything in the author’s argument.

Even if this is true, we still need an explanation for the force pulling Uranus away from the Sun. We are trying to challenge the “another planet” hypothesis, which this answer choice doesn't do.

This is irrelevant: we already know neither Neptune nor Pluto can be the object pulling Uranus away from the Sun, so how close Uranus is to either planet doesn't matter to us.

PrepTest 103 - Section 1 - Question 9

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