Logical Reasoning

PT103 · S1 · Q9 Measurements of the motion of the

A free, expert breakdown of this official LSAT Logical Reasoning question.

  • Save & drill this skill — build targeted practice sets from questions like this one

  • Video walkthroughs — watch every question solved step by step

  • 81 official LSATs — as questions, timed sections & full-length tests

Start practicing free

Measurements of the motion of the planet Uranus seem to show Uranus being tugged by a force pulling it away from the Sun and the inner planets.

Conclusion (therefore)

In addition to the known planets, there must be at least one planet in our solar system that we have yet to discover.

Evidence

Measurements of Uranus show that there is a force tugging it away from the Sun and inner planets.

But Neptune and Pluto, the two known planets farther than Uranus, don't have enough mass to account for the tugging.

Evaluate

Since we're trying to solve a science mystery here, we can use our Explain Curious Fact framework.

Curious Fact ... why does it seem like Uranus is being tugged away from the Sun at a force stronger than the gravitational pull of the two farther planets (Neptune and Pluto)?

Author's Explanation ... there must be at least one planet in our solar system that we have yet to discover (and that planet exists farther from the Sun than Uranus and accounts for the rest of the outward tugging force we're measuring).

The two pressure points are always 1. Alternate Explanations for the curious fact 2. the plausibility of the author's explanation

Goal

On Weaken (and every other question type other than Strengthen), the correct answer is more likely to deal with an alternate explanation.

So let's primarily look for a different way to explain where this outward tugging force is coming from (a black hole? a planet outside our solar system? an asteroid belt? comets?).

But we'll keep an open mind for answers that would otherwise be trying to degrade the plausibility that there's another planet in the solar system we've yet to discover.

9.

Which one of the following, if true, most seriously weakens the argument?

  1. Pluto was not discovered until

    Strengthens, if anything

    The date of discovery doesn't change anything about this conversation. It's not providing an alternate explanation for what's tugging Uranus outward, and it isn't hurting the author's storyline that there's an undiscovered planet out there. If anything, the fact that we didn't even realize Pluto existed until this past century helps the plausibility of the author's notion that there might be yet another planet we still haven't discovered.

    1% picked this

  2. Correct

    There is a belt of

    Why this is right

    This provides the Alternate Explanation we were looking for. We're trying to figure out what force is pulling Uranus away from the Sun. The author thinks it must be some undiscovered planet, but this answer raises the possibility that what's really providing the outward gravitational pull is the belt of comets beyond Pluto.

    Skill tested: Weaken · how this choice captures the argument's function is the move to repeat next time.

    80% picked this

  3. Neither Neptune nor Pluto is

    No Impact: comparing mass

    This doesn't pose an alternate explanation for what is pulling Uranus away from the Sun, nor does it hurt the storyline that there's an undiscovered planet in the solar system. Thus, it does nothing for the conversation. We don't care which of these three planets is most or least massive. We actually don't care about Neptune and Pluto at all anymore, since we were told that they are not massive enough to be the mystery source of what's tugging Uranus away.

    3% picked this

  4. The force the Sun exerts

    No Impact

    Those of us familiar with gravity will not be surprised to hear that the farther something is from the Sun, the less gravitational pull the Sun exerts. Since this is a common sense fact about the world, it could probably never be correct on Strengthen / Weaken / Paradox (where the correct answer should be a new thing we didn't already know). This doesn't pose an alternate explanation for the outward tug on Uranus, nor does it hurt the storyline that there's an undiscovered planet in our solar system.

    14% picked this

  5. Uranus’ orbit is closer to

    No Impact

    Those of us familiar with our Solar system will not be surprised to hear that Uranus is closer to Neptune than it is to Pluto, the outermost planet in the solar system (although it's not actually considered a planet any more). Since this is a common sense fact about the world, it could probably never be correct on Strengthen / Weaken / Paradox (where the correct answer should be a new thing we didn't already know). This doesn't pose an alternate explanation for the outward tug on Uranus, nor does it hurt the storyline that there's an undiscovered planet in our solar system. We already knew that Neptune and Pluto aren't the solution to the mystery of the outward tugging on Uranus, so there's no reason we'd be interested in hearing anything more about them.

    2% picked this

Continue the review in LSAT Lab

Save this question, watch the video walkthrough, and drill similar questions in your LSAT Lab account.

Save and practice this skill
LSAT Lab

Turn this review into a targeted study plan.

Save this question, drill more like it, watch the video walkthrough, and track your progress in your LSAT Lab account.

Start practicing free