In 2006, the International Astronomical Union changed the definition o

­In 2006, the International Astronomical Union changed the definition of planet. Controversially, the new definition disqualifies Pluto as a planet. Such a change is not unprecedented. The first asteroids discovered were initially classified as planets. But astronomers soon realized that asteroids are much smaller than the other objects captured by their definition and so numerous that it is impractical to consider them all planets. So planets were redefined to be bodies larger than asteroids.Similarly, astronomers now know that Pluto belongs to a vast population of bodies—several much larger than the largest asteroid—called Kuiper belt objects (KBOs). Eris, a KBO discovered in 2005, is larger than Pluto. If Pluto were a planet, then Eris would also be one, along with many other large KBOs. The number of official planets again threatened to become unwieldy. So, as before, the term planet needed redefinition.

The new definition of planet effectively requires a planet to have more mass than all other bodies in its orbital zone combined (roughly speaking, two bodies share an orbital zone if their orbits cross). Pluto, Eris, and the asteroids do not. But each planet in our solar system has at least 5,000 times the combined mass of the other bodies in its orbital zone. Thus, the definition both restricts the number of planets and captures an important natural division between planets and all other bodies in our solar system.­

1. According to the passage, Eris

A. satisfies the International Astronomical Union's new definition of a planet
B. is an asteroid larger than Pluto
C. was classified as a planet when it was first discovered
D. is the largest Kuiper belt object, but smaller than any planet
E. has no more mass than the combined mass of the other bodies in its orbital zone


2. The passage suggests that, of the following, which would be most likely to lead the International Astronomical Union to support a further change to the definition of planet?

A. Many Kuiper belt objects are larger than any asteroid.
B. Each of many dozens of Kuiper belt objects is more massive than all other bodies in its orbital zone combined.
C. No Kuiper belt object qualifies as a planet under the new definition.
D. Several planets orbiting other stars have more than 5,000 times the combined mass of the other bodies in their orbital zones.
E. A new object that is orbiting the sun is discovered and found to have more mass than the combined mass of the other objects in its orbital zone, but not 5,000 times as much.


3. In the context of the passage as a whole, the main function of the first paragraph is to

A. present a historic precedent for the recent change in the definition of planet
B. clarify why the first asteroids discovered were initially classified as planets
C. argue that asteroids should not be classified as planets
D. describe certain details of the new definition of planet
E. explain why Pluto does not qualify as a planet­