A decade after Pluto was ousted from the planet lineup, a supporter of the now dwarf planet is fighting to restore its title.

Johns Hopkins University scientist Kirby Runyon wants to make one thing clear: Regardless of what one prestigious scientific organization says to the contrary, Pluto is a planet.

So, he said, is Europa, commonly known as a moon of Jupiter, and so is the Earth's moon, and so are more than 100 other celestial bodies in our solar system that are denied this status under the prevailing definition of "planet."

The definition approved by the International Astronomical Union in 2006 demoted Pluto to "non-planet," thus dropping the consensus number of planets in our solar system from nine to eight. The change - a subject of much scientific debate at the time and since - made no sense, said lead author Runyon.

Icy, rocky Pluto had been the smallest of the nine planets, its diameter under three-quarters that of the moon and nearly a fifth of Earth. Still, said Runyon, Pluto "has everything going on on its surface that you associate with a planet. ... There's nothing non-planet about it."

Runyon and his co-authors argue for a definition of "planet" that focuses on the intrinsic qualities of the body itself, rather than external factors such as its orbit or other objects around it. They define a planet as "a sub-stellar mass body that has never undergone nuclear fusion" and that has enough gravitational heft to maintain a roughly round shape. (Even if it bulges at the equator because of a three-way squeeze of forces created by its own gravity and the influence of both a star and a nearby larger planet.)

This definition differs from the three-element IAU definition in that it makes no reference to the celestial body's surroundings. That portion of IAU's 2006 formula - which required that a planet and its satellites move alone through their orbit - excluded Pluto. Otherwise, Pluto fit the IAU definition: It orbits the sun and it is massive enough that the forces of gravity have made it round.

The proposed new geophysical definition omits stars, black holes, asteroids and meteorites, but includes much of everything else in our solar system. It would expand the number of planets from eight to approximately 110.

That expansion is part of the appeal of the new definition, Runyon says. He says he would like to see the public more engaged in solar system exploration. As the very word "planet" seems to carry a "psychological weight," he figures that more planets could encourage that public interest.

The new definition, which does not require approval from a central governing body, is also more useful to planetary scientists. Most of them are closely affiliated with geology and other geosciences, thusmaking the new geophysical definition more useful than the IAU's astronomical definition.

He has some reason to be optimistic, as the new definition has already been adopted by Planet Science Research Discoveries, an educational website founded by scientists at the University of Hawaii at Manoa.

"I want the public to fall in love with planetary exploration as I have," Runyon said. "It drives home the point of continued exploration."

The study will be presented at Lunar and Planetary Science Conference in Texas.