Lesson plans change with Pluto's demotion

The new IAU guidelines define a planet as "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit."

Pluto is now considered a dwarf planet.

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune are "classical planets," Martin said.

Two other celestial objects join Pluto as dwarfs: the asteroid Ceres, which was a planet in the 1800s before it was demoted, and 2003 UB313, an icy object slightly larger than Pluto dubbed "Xena" by its discoverer, Michael Brown of the California Institute of Technology.

"Pluto is just a state of mind, anyway. No one's ever been there," said Jeff Ridgeway, who works in the children's department at Washington County Free Library in Hagerstown.

Ridgeway said he remembers his children learning a song about the nine planets from Blue's Clues when they were toddlers.

But change is inevitable, he said.

"It's just the fact that we're able to see more with modern technology. It changes our view of the universe," he said.

Martin said he will incorporate the change into his lessons.

"It benefits education. They can see how the process works," he said.

Scientists are always refining their knowledge, and the new planetary guidelines are a perfect example of science in action, he said.

The Associated Press contributed to this story.