PLUTO DECLARED A DWARF PLANET: Today at the International Astronomical Union (IAU) General Assembly in Prague, astronomers decided that the Solar System has eight planets, and Pluto is not one of them. Instead, Pluto is a "dwarf planet."
To be a planet, the assembly ruled, a world must meet three criteria:
(1) It must have enough mass and gravity to gather itself into a ball.
(2) It must orbit the sun.
(3) It must reign supreme in its own orbit, having "cleared the neighborhood" of other competing bodies.
So, e.g., mighty Jupiter, 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. Because it has not "cleared its own neighborhood," it is a dwarf planet.
This decision clarifies the vocabulary of astronomy while simultaneously upturning 76 years of "Pluto is a planet" pop-culture. Will non-astronomers pay any attention to Pluto's demotion? We'll see. 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.
IAU Press Release
I wonder if schools will start teaching that there are only 8 planets left? They'll have to get new acronyms.