Omaha, Nebraska - Pluto may be no more than a distant, icy rock in the minds of international scientists who stripped it of planethood. But the dwarf planet's downgrade is creating a teachable moment in US classrooms.

So some museums have to adjust exhibits and publishers update text while other scientists protest to keep the puny rock a planet.

That's just science, many teachers said on Friday at a regional meeting of the National Science Teachers Association.

"Pluto's still the same Pluto," said Gerry Wheeler, executive director of the NSTA. "It's still up there doing exactly the same thing."

Pluto's regrouping with newly named Eris and the asteroid Ceres will help teachers define what is - and isn't - a planet.

"I've been saying Pluto's not a planet for years," said Donna Governor, a middle-school science teacher from Cumming, Georgia.

Governor teaches the solar system by having students measure and label orbits of each planet on a long roll of toilet paper. On one end is the sun; 250 sheets away is Pluto's outer orbit.

By modelling and comparing Pluto's orbit to orbits of other bodies, students can grasp the timing of its discovery in 1930 and why scientists reclassified it years later, Governor said.

"The science community wants change," she said. "All of these new revisions mean we know more than we did before."

Other Pluto-inclusive materials for teachers include flash cards, pictures, textbooks and online lessons. A booth representing Nasa's educational arm distributed solar system trading cards designed for kindergarten- through third-grade students.

"Pluto is the smallest planet in our solar system. It is made of ice and some rock," Pluto's card says.

Educational materials won't change overnight, but Nasa will re-evaluate its once appeals to keep Pluto in the club die down, Nasa spokesperson Robert Mirelson said.

Complaints about the redefining of what constitutes a planet and appeals of Pluto's planetary worthiness surfaced almost as soon as the International Astronomical Union voted in August.

Pluto didn't qualify under the new definition because its orbit overlaps with Neptune's. A planet must "clear the neighbourhood around its orbit," under one part of the IAU's new rules.

A T-shirt designed by Bob Summerfield, who founded a nonprofit that teaches astronomy in Pennsylvania, reads: "What was the IAU thinking?"

"It's a bad definition," Summerfield said, adding the new definition should exclude Earth and other planets whose orbits aren't cleared.

Others don't like the ruling because of a sentimental attachment to Pluto.

Shortly after the vote, the City Council in Madison, Wisconsin, adopted a tongue-in-cheek resolution declaring Pluto "Madison's ninth planet." Friends and colleagues of the late Clyde Tombaugh, who discovered the former planet, gathered at New Mexico State University to protest its title-stripping. Scientists in Myanmar, also known as Burma, declared they would still consider Pluto a planet, despite the IAU vote.

"I think there's some nostalgia, too," Governor said of people unwilling to let go of Pluto. "Mickey Mouse has a cute dog." - Sapa-AP