PRAGUE, Czech Republic (AP) -- The ninth rock from the sun faces the prospect of being demoted to a "dwarf planet" on Thursday if the International Astronomical Union approves new planetary definitions.

Some scientists say Pluto never deserved to be a planet in the first place. It's smaller than Earth's moon, has a funky way of orbiting the sun, and lurks so far out on the fringes of the solar system even the powerful Hubble Space Telescope has to squint to see it.

Pluto is no stranger to controversy. In fact, it's been dogged by disputes ever since its discovery in 1930.

Discovered by Clyde Tombaugh of Arizona's Lowell Observatory, Pluto was classified as a planet because scientists initially believed it was the same size as Earth. It remained one because for years, it was the only known object in the Kuiper Belt, an enigmatic zone beyond Neptune that's teeming with comets and other planetary objects.

Pluto got an ego boost in 1978 when it was found to have a moon that was later named Charon. The Hubble turned up two more, which this past June were christened Nix and Hydra.

But in the 1990s, more powerful telescopes revealed numerous bodies similar to Pluto in the neighborhood. New observations also showed that Pluto's orbit was oblong, sending it soaring well above and beyond the main plane of the solar system where Earth and the other seven planets circle the sun.

That prompted some galactic grumbling from astronomers who began openly attacking Pluto's planethood.

At one point, things looked so bad for Pluto, the international union said publicly in 1999 that rumors of Pluto's imminent demise were greatly exaggerated and there were no plans to kick it out of the cosmic club.

A year later, the Hayden Planetarium at New York's American Museum of Natural History was accused of snubbing Pluto by excluding it from a solar system exhibition.

Pluto took another hit after Michael Brown of the California Institute of Technology discovered 2003 UB313, a slightly larger Kuiper Belt object. What's the point, some astronomers wondered, in keeping Pluto as a planet?

Its future brightened earlier this year, when NASA sent the New Horizons spacecraft to Pluto to get a closer look at the ball of rock and ice. The Hubble has managed to glimpse only its most prominent surface features; New Horizons, if all goes well, will arrive in 2015.

As recently as last week, the IAU -- the official arbiter of heavenly bodies -- appeared ready to reaffirm Pluto's planet status.

Richard Binzel, a professor of planetary science at the Massachusetts Institute of Technology and a member of the international union's planet definition committee, had contended that Pluto met key tests of planetary physics "by a long shot" and had earned its status.

On Thursday, looking weary, he was asked whether he'll mourn if Pluto winds up demoted.

"I don't know. Ask me later," he said.