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.

An artist's rendering of Pluto.
(NASA/JPL-Caltech/Associated Press)

Pluto has been considered a planet since its discovery in 1930. Under the new guidelines, it's now considered a "dwarf planet," leaving eight planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Pluto, which is smaller than Earth's moon, doesn't fit the new criteria for a 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 doesn't qualify because its orbit is inclined relative to the rest of the solar system and crosses over the orbit of Neptune.

NASA said Thursday that Pluto's designation as a dwarf planet would not affect its $700-million US New Horizons spacecraft mission, launched earlier this year. The spacecraft will reach Pluto after a 9½-year journey.

The IAU also introduced a third class, "small solar-system bodies," for objects smaller than dwarf planets, such as asteroids and comets, that orbit the sun.

Astronomers have been working without a solid definition of a planet since the days of Copernicus. The new definitions fill that void.

Just a week ago, the IAU's leader suggested a different definition for planet that would have included Pluto, its moon Charon, the asteroid Ceres and a recently discovered object, 2003 UB313, which is slightly larger than Pluto and was nicknamed Xena.

The plan was unpopular among the astronomers at the conference and led to the long debate.

Not first time 'planet' demoted

It wasn't the first demotion for a body formerly considered a planet.

Ceres was considered a planet when it was first discovered in 1801.

However, after the size of Ceres was determined and other bodies were found in a belt between Mars and Jupiter, it lost its status. The term "asteroid," meaning "star-like," was coined to describe them.

Three other "planets" became "asteroids" at the same time.

Among other implications, Thursday's new definition means students will have to abandon current mnemonic devices that helped them remember how the planets are arranged in order from the sun.

For example, "Mark's Very Extravagant Mother Just Sent Us Ninety Parakeets" helped them recall that the order was Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

Perhaps they can now return to the pre-1930 mnemonic: Mary's Violet Eyes Make John Stay Up Nights.