It's Official! Pluto Is Not A Planet!

Astronomers at the International Astronomical Union (IAU) General Assembly in Prague, decided on Aug. 24, 2006 that the Solar System has eight planets, not nine, and Pluto is not one of them. Instead, Pluto has been designated a "dwarf planet."

424 astronomers were involved in the vote.

"I'm embarrassed for astronomy. Less than 5 percent of the world's astronomers voted," said Alan Stern, leader of NASA's New Horizons mission to Pluto and a scientist at the Southwest Research Institute. He expects the decision to be overtuurned by the astronomy community. Other astronomers have criticized the definition as ambiguous.

The assembly ruled has ruled, in order to qualify as a planet, a world must meet three criteria:

It must have enough mass and gravity to gather itself into a ball. It must orbit the sun. It must reign supreme in its own orbit, having "cleared the neighborhood" of other competing bodies.

So Jupiter, for example, which circles the sun supreme in its own orbit, is a planet - no adjective required. Pluto, on the other hand, shares the outer solar system with thousands of Pluto-like objects, some bigger, some smaller. Because it has not "cleared its own neighborhood," it is now to be called a dwarf planet.

There is one flaw in that argument, though; Earth, Mars, Jupiter and Neptune all have asteroids as neighbors and can hardly be said to have cleared their neighborhoods. True, those asteroids are a sparse population, but research in recent years has discovered over 1,000 Near Earth Objects, bodies that could potentially impact our planet in the future. Not so much a case of Earth serenely sailing a pristine path as occasionally getting a bug-splat on its windscreen as it orbits through space.

"It's patently clear that Earth's zone is not cleared," Stern said. "Jupiter has 50,000 Trojan asteroids," which orbit in lockstep with the planet. "It won't stand," he said. "It's a farce."

The historian and astronomer emeritus at Harvard, Owen Gingerich, who led the committee that proposed the initial definition, has called the new definition "confusing and unfortunate" and said he was "not at all pleased" with the language about clearing the neighborhood. Gingerich also did not like the term "dwarf" planet.

Mike Brown, of Caltech, recently found an object with the snappy moniker of 2003 UB313, which is larger than Pluto.

"As of today I have no longer discovered a planet," he said. But Brown called the result scientifically a good decision.

"The public is not going to be excited by the fact that Pluto has been kicked out," Brown said. "But it's the right thing to do."

This decision clarifies the vocabulary of planetary astronomy while simultaneously upturning 76 years of "Pluto is a planet" pop-culture. Will non-specialists heed Pluto's demotion? That remains to be seen. Meanwhile, according to the IAU, the Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune; and three dwarf planets: Ceres, Pluto and 2003 UB313.

Quite a number of textbooks and classroom charts will now have to be updated to take account of Pluto's change in status.

Named after the god of the underworld in Roman mythology (and not a cartoon dog), Pluto orbits the Sun at an average distance of 5.9 billion kilometres (3.7 billion miles) taking 247.9 Earth years to complete a single circuit of the Sun.

The New Horizons spacecraft is due to fly by Pluto and the Kuiper Belt in 2015.