Teacher: Class, today we're going to discuss why Pluto is no longer classified as a major planet.
Student 1: Is it because it's too small?
Teacher: That's part of it. It also hasn't 'cleared its neighborhood' around its orbit, meaning there are other objects of similar size in its orbital path.
Student 2: So, it's considered a 'dwarf planet' now?
Teacher: Exactly. The International Astronomical Union set new criteria for planets in 2006, and Pluto didn't meet all of them.
What is the core subject of this discussion?
A) The discovery of new planets
B) The size comparison of planets
C) The reclassification of Pluto as a dwarf planet
D) The exploration of distant celestial bodies