“Some days, I think about Pluto and feel sad,” Felix sighed, “Pluto was once the ninth planet in our solar system. It was loved by everyone. It was the smallest planet in the solar system and had a moon half its size. It also had a tilted orbit. But its planetary status was taken away by the astronomers.” 

Orak smiled and said, “In 2006, the International Astronomical Union (IAU) changed the definition of a planet. IAU promotes astronomy. Their new definition classified Pluto as a dwarf planet. A dwarf planet is a celestial body that resembles a small planet but doesn’t qualify as a planet. 

But if it makes you feel any better, not all astronomers agree with the new definition of planets. They believe Pluto should be a planet.”

“Tell me more?” Felix said, visibly excited. 

“According to IAU’s definition, a planet must have cleared the space around its orbit. It means a planet should clear the debris and other celestial bodies in and around its path. But Pluto doesn’t fulfill the condition. It is different from the eight planets of the solar system,” Orak said. 

“So they demoted Pluto?” Felix huffed and puffed. 

“IAU’s intention was not to demote Pluto,” Orak said, “Astronomers were discovering more and more celestial bodies similar to Pluto in the area. So, the decision was either to add those objects to the list of planets or reclassify Pluto. They chose to reclassify Pluto as a dwarf planet. Dwarf planets are also an important class of objects, Felix. 

But some astronomers believe that a planet need not clear its orbit. Planets should be defined by their geology. Geology is the study of the physical structure of a celestial body. For example, rocks.” 

“Does Pluto have an interesting geology?” Felix said. 

“Yes, Pluto has mountains made of water ice. It has fields of frozen nitrogen. It also has methane snow-capped peaks. It has dunes and volcanos,” Orak said. 

“For a long time, the definition of planet was more inclusive,” Orak said, “For example, when Galileo observed the sky through his telescope, all large moving bodies were considered planets. But as we got closer to space and studied the objects, they were reclassified as moons, asteroids, dwarf planets. 

The definition of planets has changed many times for different reasons. The IAU’s definition is just the latest change.” 

Felix perked up, “So who’s to say that it won’t change again?”