No matter what category you put it in, Pluto is as interesting as ever.

Many of my young readers are disappointed that Pluto is no longer a planet. I think its important for them to realize that the body is as interesting as ever. That’s why I revised an old “Ask Dr. Fred” question on my website for children’s science and retitled it “Why Isn’t Pluto a Planet Anymore?”

Here’s the introduction to that revision:

August 24, 2006, was a significant day in the history of planetary science. If you read the newspaper headlines, you might think it was famous as the day that Pluto was thrown out of the planetary family. But if you read more about the 2006 meeting of the International Astronomical Union, you will discover that the story is a bit different than that. August 24, 2006, was really the first day that there was an official scientific definition of the word “planet.”

Until that date, scientists used the word planet, and many people called themselves planetary scientists. But no major scientific organization had come up with an official definition. For a long time, they didn’t have to, because everyone seemed to agree on what bodies should be included in the list of planets.

But then, the more observations people made of Pluto, the less certain they were about including it in the list. They discovered it was smaller than they thought at first, even smaller than Mercury. They knew its path around the Sun was more elongated than the other planets, so elongated that it is closer to the Sun than Neptune for about 20 years of its 248-year orbital period. They knew that Pluto’s orbit was unlike the other planets’ and the asteroids, which lie in nearly the same plane as Earth’s path. From Mercury to Neptune those bodies never leave a thin disk-shaped region around the Sun, but Pluto’s orbit takes it well above and below that disk.

When Clyde Tombaugh discovered Pluto in 1930, he and everyone else called it the ninth planet. But by the time Tombaugh died in 1997, some astronomers were expressing doubts about it. They realized that it was probably one of many large icy bodies in a distant region of the Solar System called the Kuiper (KI-per) Belt. They even suspected that it might not even be the largest Kuiper Belt object, just the first and largest one found up to that time.

That made Pluto’s status as a planet the kind of question that most scientists love to disagree and argue about. When scientists argue, it doesn’t mean they’re angry with each other. It just means they look at the same evidence and draw different conclusions.

Dr. Fred thinks arguments like that are fun, especially when someone disagrees with something that no one has questioned for years, such as the number of planets in the Solar System. When a large Kuiper Belt Object known as Quaoar (kwa-oh-wahr) was discovered in 2002, people started to talk about changing Pluto’s status. That’s when Dr. Fred decided to include this question in his “Ask Dr. Fred” pages. He kept adding updates as new large bodies were discovered in the outer Solar System, but on day Pluto was “demoted,” he knew the old article had to be rewritten completely.

To read that rewritten article click here.