The final world on our journey through the Solar System is the planet Pluto. Pluto is unlike any other planet. It is not dense like the inner planets, and it is not large and gaseous like the outer planets. Instead, it is made of mostly ice. Compared to every other planet and even several of the Solar Systems moons Pluto is tiny. Pluto is really more like a comet than a planet.
If Pluto is more like a comet than a planet, why do we call it a planet?
The answer to this question has more to do with history than science. In the late 1800s and early 1900s, an astronomer named Percival Lowell used his telescope to draw many pictures of the surface of the planet Mars. Percival Lowell also noticed that the planet Neptune seemed to wobble funny. He thought that perhaps the reason Neptune wobbled was because of another yet undiscovered large planet which revolved around the Sun outside of Neptune’s orbit.
He hired a young man named Clyde Tombaugh to examine thousands of pictures of the sky looking for the missing planet. In 1930, 14 years after the death of Percival Lowell, Clyde Tombaugh finally found an object which he thought was the planet they had been looking for.
Clyde Tombaugh announced to everyone that he had discovered a new large planet. After a few months they named the new planet Pluto. The PL stands for Percival Lowell. Pluto is so far away that people did not realize for decades that Pluto was not large. In fact, it is very, very small.
We now know that Neptune does not wobble like Percival Lowell thought it did. The wobbles he thought he saw were caused by his broken telescope. If Pluto had been discovered by someone besides Clyde Tombaugh who was looking for a planet, it would have been called a comet. Instead, we are stuck with a planet that is really a comet.
In 1998 there was a meeting where astronomers discussed officially making Pluto a comet. It was decided that it would be too hard. Thousands of books, pictures, computer programs, and museum displays would have to be changed. In 2006, the status of Pluto was once again debated. This time Pluto was not so lucky. On August 24th, 2006, Pluto was officially downgraded from planet to dwarf planet by the International Astronomical Union (IAU).
Since the discovery of Pluto we have discovered other large comets even further from the Sun.
Assignment # 2:
In your opinion, do you think we should call Pluto a planet or a comet? (There is no right or wrong answer to this question).
Check your answer
There is no right or wrong answer to this question.