EXPLANATION QUESTION #2

The question asks which scenario would most likely lead the International Astronomical Union (IAU) to further change the definition of a planet. To answer this, we need to understand the current definition's rationale and what might prompt a revision.

Key Points from the Passage
1. Current Definition (2006):
- A planet must:
- Orbit the Sun.
- Be spherical (have sufficient mass to assume hydrostatic equilibrium).
- "Clear its orbit" (have more mass than all other bodies in its orbital zone combined).
- Pluto and Eris don't meet the third criterion because they share their orbital zones with many other Kuiper Belt Objects (KBOs).
- Planets in our solar system have at least 5,000 times the combined mass of other bodies in their orbital zones.
2. Reason for the $\mathbf{2 0 0 6}$ Change:
- Without the new definition, many KBOs (like Eris) would qualify as planets, making the number "unwieldy."
- The definition aims to capture a "natural division" between planets and other bodies.

What Would Prompt a Further Change?
A further change would likely occur if the current definition:
- Fails to maintain a practical number of planets (e.g., if too many new objects qualify).
- No longer reflects a "natural division" (e.g., if many objects barely meet the mass dominance criterion).
- Excludes objects that "should" be planets or includes those that "shouldn't" be.

Evaluating Each Option
Option A: Many Kuiper belt objects are larger than any asteroid.
- This was already true before 2006 (Eris is larger than Pluto, which is larger than asteroids). The IAU didn't base the definition on size alone.
- Unlikely to prompt change: Size isn't the issue; orbital dominance is.

Option B: Each of many dozens of Kuiper belt objects is more massive than all other bodies in its orbital zone combined.
- This means dozens of KBOs would qualify as planets under the current definition.
- The 2006 change aimed to avoid an "unwieldy" number of planets. This would directly contradict that goal.
- Most likely to prompt change: The IAU would likely tighten the definition (e.g., raise the mass dominance threshold) to reduce the number.

Option C: No Kuiper belt object qualifies as a planet under the new definition.
- This is already true (Pluto/Eris don't qualify). No pressure to change.
- Unlikely to prompt change.

Option D: Several planets orbiting other stars have more than 5,000 times the combined mass of the other bodies in their orbital zones.
- This aligns with the current definition (our planets meet this threshold). No conflict.
- Unlikely to prompt change.

Option E: A new object is discovered with more mass than others in its orbital zone, but not 5,000 times as much.
- The current definition doesn't require a 5,000:1 ratio; it just notes our planets meet this. The actual requirement is simply "more mass."
- This object would qualify as a planet, but unless many such objects exist, it wouldn't pressure a change.
- Less likely than B to prompt change.

Why Option B is Best
The 2006 change was driven by the impracticality of classifying many KBOs as planets. If many dozens of KBOs suddenly met the mass dominance criterion, the IAU would face the same "unwieldy" problem they tried to solve. They'd likely revise the definition (e.g., requiring a higher mass ratio like 5,000:1) to restore a clear division.

Why Others Are Inferior
- A, C, D: Don't challenge the definition's practicality or natural division.
- E: Only one object wouldn't pressure change; the issue is scale (as in B).

Final Answer
B. Each of many dozens of Kuiper belt objects is more massive than all other bodies in its orbital zone combined is most likely to lead the IAU to further change the definition.

CorrecT Answer is: B