Planets
adapted from NASA

1 Our understanding about the universe and our place in it has changed over time. New information can cause us to rethink what we know and reexamine how we classify objects to better understand them. New ideas and views can come from questioning an existing theory.
What is a Planet?
2 This simple question doesn't have a simple answer. Defining the term planet is important, because such definitions reflect our understanding of the origins, architecture, and evolution of our solar system.
3 Pluto, discovered in 1930, was identified as the ninth planet. Pluto is much smaller than Mercury and is even smaller than some of the planetary moons. Though Pluto kept its planetary status through the 1980s, things began to change in the 1990s with some new discoveries.
4 Technical advances in telescopes led to better observations and improved detection of very small, very distant objects. In the early 1990s, astronomers began finding numerous icy worlds orbiting the Sun in a doughnut-shaped region called the Kuiper Belt beyond the orbit of Neptune—out in Pluto's realm. With the discovery of the Kuiper Belt and the icy bodies known as Kuiper Belt Objects, or KBOs, it was proposed to think of Pluto as the biggest KBO instead of a planet. Pluto triggered a scientific debate—a vigorous debate that continues to this day.
The Planet Debate
5 The International Astronomical Union (IAU), a worldwide organization of astronomers, took on the challenge of classifying the newly found KBOs. In 2006, the IAU passed a resolution that defined planet and established a new category, dwarf planet. Pluto is now a dwarf planet as recognized by the IAU.
The New Definition of Planet
6 The most recent definition of a planet was adopted by the IAU in 2006. It says a planet must do three things:
It must orbit a star.
It must be big enough to have enough gravity to force it into a spherical shape.
It must be big enough to have its gravity clear away any other objects of a similar size near its orbit around the Sun.
Debate—and Discoveries—Continue
7 Astronomers and planetary scientists did not universally agree with these definitions. In fact, the lively planethood debate continues till this day. As our knowledge deepens and expands, the more fascinating the universe appears. Whether our definitions of planet can be applied to newly found objects in the future remains to be seen.
1
Select the correct answer.
Based on the selection, what is the author's message about the new definition of planets?

A.
It is uncertain whether the definition of planet will remain the same in the future.
B.
Pluto should not have been the reason why IAU changed the definition of planets.
C.
The new definition for planets needs to be considered as settled moving forward.
D.
The IAU used poor methods when setting a new definition for planets.