The International Astronomical Union, the governing body of astronomers that shocked Pluto lovers in 2006 by announcing that Pluto was no longer a planet, will hold its next general assembly in Rio de Janeiro, Brazil, in August. There is a popular movement to have the astronomers re-examine their demotion of Pluto to a dwarf planet at the assembly.

The IAU was founded in 1919. Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation. Among the many tasks of the IAU are the definition of fundamental astronomical and physical constants, unambiguous astronomical nomenclature and promotion of educational activities in astronomy.

The IAU also serves as the internationally recognized authority for assigning designations to celestial bodies and surface features on them.

Less than 4 percent of the IAU's 9,000 members actually participated in the vote to demote Pluto in 2006. Some say that's not enough, and a decision with such cultural and scientific impact should have been based on a wider sampling of experts.

Pluto was discovered by astronomer Clyde Tombaugh in 1930. With a diameter of only 1,423 miles, Pluto is about two-thirds the size of Earth's moon.

Pluto's planethood was never seriously questioned until 1992. That's when astronomers started to find other objects "out there." Pluto's neighborhood, it turns out, is cluttered with icy bodies about the size of asteroids. They orbit the sun in a busy belt, a bit like the asteroid belt, all beyond the orbit of Neptune.

Back in 1951, astronomer Gerard Kuiper predicted such a belt to explain where certain comets came from, and today it bears his name: the Kuiper Belt.

The discovery of the Kuiper Belt troubled many astronomers. They began to wonder if Pluto was just another Kuiper Belt Object. Other astronomers insisted Pluto was both a planet and a Kuiper Belt Object. So the arguments began.