Some version of “my very educated mother just served us nine pizzas” helped generations of students remember the names and order of the nine planets orbiting the sun. Recently, scientists have tried to cancel the pizza, ending Pluto’s membership in planetary club because it is very small, orbits eccentrically and is just one of many icy objects floating in the Kuiper Belt beyond Neptune.

Perhaps because they didn’t want to offend the scores of school kids who came to the defense of Pluto, or more likely because scientific knowledge is changing at a rapid pace, astronomers are now considering a new definition of a planet that will expand our current list to 12.

This makes sense, says Neil Comins, an astronomy professor at the University of Maine. As new objects are discovered, our picture of our solar system changes, requiring new definitions to reflect the new reality.

There are already four very different types of spherical objects that we call planets. Adding more is inevitable as more are discovered.

Pluto was discovered in 1930 and hastily added to the eight planets known before 1900. Since then, astronomers have discovered that Pluto is much smaller than originally thought. In fact, it is smaller than the Earth’s moon. The Hayden Planetarium at the American Museum of Natural History in New York drew angry protests – including voluminous praise for the beloved, but distant planet from school children – when it demoted Pluto in a new exhibit in 2000.

Professor Comins goes along with the International Astronomical Union in suggesting that there be a generic definition of a planet – a large round object that orbits the sun, not other planets – and that adjectives be used to divide the planets into categories. The union may decide today whether a further feature – dominance within its orbit – should also be a requirement.

That’s among the variations its members will vote on in Prague this week. It also would create a new category, maybe plutons or plutonians, maybe dwarf planets, which are differentiated from the eight major planets. Two other Kuiper Belt objects, Xena,

a recently discovered ice ball, and Charon, Pluto’s moon, would be added to this list. Ceres, the largest asteroid, would be known as a dwarf planet.

So, how about my very educated mother cheerily just served us nine pizzas carrying a xylophone? That’s Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and Xena.