Pluto demoted under new definition of planet
(CBC) - Little Pluto, formerly the solar system's smallest planet, has been stripped of its status by the International Astronomical Union, reducing the number of planets to eight.
The new guidelines - introduced in Prague on Thursday after a week of debate by the 2,500 astronomers at the organization's conference - define what is a planet and what is not. Pluto didn't make the cut.
Pluto has been considered a planet since its discovery in 1930. Pluto is now considered a "dwarf planet," and the eight others - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune - are now called "classical planets."
Pluto, which is smaller than Earth's moon, doesn't fit the new criteria for a classical planet: "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighbourhood around its orbit."
Pluto's orbit is inclined relative to the rest of the solar system and crosses over the orbit of Neptune, disqualifying it as a classical planet.