Pluto, a distant world on the frigid outskirts of the solar system, was downgraded eight years ago when scientists changed its status from full-fledged planet to planetary dwarf.

Scores of Pluto fans booed the decision and terminology -- dwarf planet -- imposed by the International Astronomical Union, which voted in Prague to reduce the number of planets to eight.

Controversy has yet to wane. There's a Facebook page devoted to reinstating Pluto. Elsewhere, a permanent online petition still invites the public to voice disagreement with the union. And the editor of a popular astronomy magazine is calling for a presidential-style debate to settle Pluto's status in the heavens -- once and for all.

The maelstrom mounts as Earthlings prepare for a first-ever rendezvous: An American data-gathering spacecraft will make a historic Pluto flyby next year. Ironically, the New Horizon space probe was launched the year the far-flung orb was demoted.

The spacecraft will hurtle breathtakingly close to the Plutonian surface, having traveled seven years and 4.5 billion miles to reach Pluto, deep in the icy Kuiper belt.

Astronomers are certain new insights will emerge from the mission, but that probably won't change Pluto's status, they say.

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Still contentious

"Clearly, Pluto is a touchy subject," said Fred Walter, a professor of astronomy at Stony Brook University.

As Walter sees it, powerful scientific evidence undergirded Pluto's downgrade from planet to dwarf.

"It has the most extreme orbit of any of the planets, at least when it was a planet," Walter said, noting Pluto, about the size of Earth's moon, is highly dependent on a larger, full-fledged celestial body, a guardian planet in the cosmos.

"Pluto isn't gravitationally independent," Walter added. "It's gravitationally tied to Neptune."

But just as mystery has shrouded Pluto because of its multibillion-mile distance from Earth, semantics have affected it, too, Walter said.

"We haven't really fully demoted Pluto. The word 'planet' is still there," he said, referring to the term dwarf planet.

"But if you were Pluto," he asked, "would you rather be the runt among planets, or the king of the dwarf planets?"

Another name for Pluto, Walter added, is trans-Neptunian object.

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Denton Ebel, who chairs the Earth and Planetary Sciences Department at the American Museum of Natural History in Manhattan, cringes at the thought of "the Pluto discussion."

Discord, he asserted, is inevitable when the subject is Pluto. He doesn't think it's a full-fledged planet, either.

He scoffs at Pluto proponents who say scientists are prejudiced just because it's small. "It's not in the same class of objects as Earth and Mars and the other bodies we think of as planets," Ebel said. "There's an object in the Kuiper belt that is larger than Pluto, and it isn't a planet.

"There are lots of objects out there and we are still finding new ones. But everything can't be a planet."

In recent years, space hunters have found hundreds of new objects in the Kuiper belt, Pluto's home, a region far beyond the solar system. Ebel refers to the area as the deep freeze, a vast expanse populated by comets and alien planetlike objects.

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On Pluto itself, the average temperature is minus 380 degrees. And, it's not just dark, it's superdark: Sunlight is 1,000 times fainter than on Earth.


First seen in 1930

Scientists who oppose Pluto's return to full planetary status say it may have been misdesignated upon discovery in 1930 by Clyde Tombaugh at the Lowell Observatory in Arizona.

Tombaugh died in 1997, but a portion of his ashes was placed aboard the New Horizon probe and are en route to the world once called Planet X. In 1930, there were no official definitions of a planet.

Before 1996, Walter said, "no one ever sat down and determined what a planet is."

The astronomical union has three key criteria for a celestial body to make the cut. Dinky Pluto had trouble making the grade.

The union's rules say a planet must orbit the Sun and be massive enough to exist in "hydrostatic equilibrium," which means having its own potent gravitational pull. Hydrostatic equilibrium is the reason planets are spherical.

The third criterion: A planet must "clear the neighborhood" of smaller bodies within its orbit. This complicated wording means a planet must be dominant in its orbital zone, Ebel said, and Pluto isn't because it's in a complex dance with several satellites surrounding it.

Yet, Pluto's ban from the solar system still stokes powerful emotions in legions of fans.

"In my heart, I know that it really can't be a planet any more," said Ken Spencer, an amateur astronomer and president of the Astronomical Society of Long Island. "I was really sad to see it demoted. But after reading why, it's hard to argue with those reasons."

Carol Paty, an assistant professor of earth and atmospheric sciences at the Georgia Institute of Technology, said while it's likely the New Horizon mission will beam back data that scientists never knew, she doubts Pluto will be reinstated.

David Eicher, editor of Astronomy magazine, thinks Pluto is a planet despite the union's stipulations, and is calling for an open public debate.

He asks: Shouldn't a planet be considered a full-fledged planet regardless of its size?

Eicher would like to see Alan Stern, the New Horizon spacecraft's principal investigator, debate astrophysicist and television personality Neil deGrasse Tyson, director of the Hayden Planetarium in Manhattan.

Stern has gone on record saying Pluto should be returned to its former status. Tyson also has gone on the record and says the astronomical union got its vote right.

Neither Stern nor Tyson could be reached Tuesday for comment.

"This issue needs to be settled logically, and with a careful face-to-face debate over the meaning of what the IAU has done," Eicher said.

But Walter thinks emotion about Pluto falls unavoidably along an age divide. "When I teach astronomy 101, I lecture on whether Pluto is a planet.

"I don't tell the students what Pluto is. I let them vote. And overwhelmingly they say Pluto is not a planet.

"It's only older people who think differently," he said. "This is a sociological issue, not a science issue because people don't want to give up what they learned in school."