The International Astronomical Union has decreed that Pluto is not a "planet" by (re)defining the word. I put "re" in parentheses because it's not clear that "planet" was ever really defined other than as a list with nine members. So now there's a new definition that results in Pluto being stripped of its status, much to everyone's dismay except (some) astronomers.
So what is Pluto now? A "dwarf planet". But apparently "dwarf" isn't an adjective describing Pluto's size, it's part of a whole new term: "dwarf planets" aren't "planets". So what have we got?
The decision establishes three main categories of objects in our solar system.
* Planets: The eight worlds from Mercury to Neptune.
* Dwarf Planets: Pluto and any other round object that "has not cleared the neighborhood around its orbit, and is not a satellite."
* Small Solar System Bodies: All other objects orbiting the Sun.
The problem with the old definition of "planet" is that it included Pluto but had no specific criteria that would have prevented hundreds of other bodies (some, like 2003 UB313, larger than Pluto) from logically falling to the same category. Despite everyone's affection for Pluto, no one really wanted to expand the list of planets to that length... and scientists tend to dislike arbitrary classifications.
Unfortunately, the new definition appears to be based on the idea that a planet must "clear the neighborhood around its orbit", which is somewhat ambiguous. Says an astronomer in charge of exploring Pluto:
"I'm embarassed for astornomy," said Alan Stern, leader of NASA's New Horizon's mission to Pluto and a scientist at the Southwest Research Institute. "Less than 5 percent of the world's astronomers voted." ...
Stern, in charge of the robotic probe on its way to Pluto, said the language of the resolution is flawed. It requires that a planet "has cleared the neighborhood around its orbit." But Earth, Mars, Jupiter and Neptune all have asteroids as neighbors.
"It's patently clear that Earth's zone is not cleared," Stern told SPACE.com. "Jupiter has 50,000 trojan asteroids," which orbit in lockstep with the planet.
So, again, we seem to be stuck with an unclear definition that, though more aesthetically pleasing to some, doesn't add up to more than "I know it when I see it".