In which year was Pluto officially demoted from the status of a major planet to that of a dwarf planet?

Introduction / Context:
For many decades, students learned that the solar system had nine planets, including Pluto. However, advances in astronomy and the discovery of other similar objects forced scientists to reconsider the definition of a planet. This led to Pluto being reclassified as a dwarf planet. The question tests knowledge of the specific year in which this historic decision was made.

Given Data / Assumptions:

• Pluto was long considered the ninth planet of the solar system.
• The International Astronomical Union, often shortened as IAU, sets official definitions for celestial bodies.
• In the mid two thousands, the IAU adopted a new definition of a planet.

Concept / Approach:
In 2006, the IAU met and voted on a formal definition of a planet. According to this definition, a planet must orbit the Sun, be massive enough to be nearly round and have cleared its orbit of other objects. Pluto satisfies the first two conditions but does not fully clear its neighbourhood because it shares space with other Kuiper Belt objects. Therefore, Pluto was reclassified as a dwarf planet after this decision.

Step-by-Step Solution:
Step 1: Recall that the reclassification of Pluto happened during an International Astronomical Union meeting. Step 2: Remember that the decision took place in 2006 in Prague. Step 3: Connect this date with the widespread news that school textbooks would need to be updated from nine planets to eight planets. Step 4: Compare the given years and select 2006 as the year matching this event. Step 5: Confirm that no later year corresponds to the official first demotion of Pluto.

Verification / Alternative check:
Astronomy timelines and educational articles consistently state that on 24 August 2006, the IAU voted to define the term planet more precisely. As a result, Pluto was placed into a new category called dwarf planets, along with objects such as Eris and Ceres. News reports and textbooks from that time recorded this scientific change widely, which confirms that the key year is 2006.

Why Other Options Are Wrong:
Option a, 2005, is too early and refers to the period when debate was still ongoing and no formal redefinition had been adopted.
Option c, 2008, and option d, 2009, are later years and do not match the original decision, although some follow up clarifications and discoveries about dwarf planets occurred later.

Common Pitfalls:
Many students remember that the change happened in the two thousands but confuse the exact year. Another pitfall is mixing up the date of Pluto discovery in 1930 with the date of its reclassification. Some may also think that Pluto was removed from the list of planets without any official process, but in reality the IAU followed a formal voting procedure.

Final Answer:
Pluto was officially demoted to dwarf planet status in 2006.