Pluto no longer a planet

Now that Pluto has been demoted, the big question remaining is: What did my very educated mother just serve us?

As anyone who wasn't snoozing during sixth-grade science class knows, "My very educated mother just served us nine pizzas" is a mnemonic to help memorize the planets in order from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

But a NASA Web site pointed out Thursday, "On Aug. 24, 2006, the International Astronomical Union (IAU) formally downgraded Pluto from an official planet to a dwarf planet."

Meeting in Prague, Czech Republic, the IAU voted that to be a real planet something must orbit the sun, be large enough for gravity to form into a sphere and must have cleared other things out of the way in its orbital neighborhood.

Pluto was discovered by a young Arizona astronomer, Clyde Tombaugh, in 1930. Ever since, schoolchildren worldwide have learned that it was a planet.

In 1930, "it was thought to be a huge planet," said Patrick Wiggins, NASA solar system ambassador to Utah. But as telescopes improved and astronomers were able to make more precise measurements, "it just kept shrinking and shrinking."

Today, he added, Pluto is "known to be smaller than Earth's moon."

But Tombaugh was a kindly and engaging old man. "Nobody wanted to go up to Clyde and say, 'Hey, your planet's not really a planet,"' Wiggins added.

That changed when Tombaugh died in 1997, almost 91 years old. Then scientists began to discover other objects nearly as big as Pluto orbiting in the Kuiper Belt, a region far from the sun peppered with chunks of material left over from the formation of the solar system.

A crisis came with the discovery of "Xena," announced last year. This object, detected by a team headed by Mike Brown of the California Institute of Technology, is bigger than Pluto.

If Pluto is a planet, surely "Xena" should be too, went the logic. For a time, the IAU agreed with that idea, proposing that the solar system has 12 planets, including Pluto, Charon (Pluto's moon), Xena and the largest asteroid, Ceres.

But on Thursday the IAU voted to limit classic planets to the first eight discovered, with others called "dwarf planets" that need only be round to qualify.