Michigan Test for Teacher Certification (MTTC) Secondary Integrated Science Practice test

Which planet was reclassified as a minor planet in 2006?

1. Neptune

2. Jupiter

3. Pluto

4. Mars

The correct answer is: Pluto

Pluto was reclassified as a minor planet in 2006 due to the establishment of a clearer definition of what constitutes a planet by the International Astronomical Union (IAU). This decision was primarily based on three criteria that an object must meet to be classified as a planet: it must orbit the sun, be spherical in shape due to its own gravity, and have successfully cleared its orbit of other debris. While Pluto meets the first two criteria, it does not fulfill the third, as it shares its orbit with other objects in the Kuiper Belt. Consequently, the IAU designated Pluto as a "dwarf planet," a subcategory of minor planets. This reclassification reflects a significant shift in our understanding of the solar system and illustrates the evolving nature of astronomical definitions. Other options, such as Neptune, Jupiter, and Mars, remain classified as planets, as they meet all the criteria set out by the IAU.