My very eager mother just served us nine pizzas.

The crazy sentence used by generations of children as a mnemonic for learning the order of the planets was quickly revised once Pluto lost its spot in the majors in the summer of 2006.

Most educational Web sites have long since been updated to reflect tiny Pluto's reclassification as a dwarf planet.

Books, however, aren't as quick to catch up. Anything less than two years is a speedy turnaround time for a major publisher.

As a result, there has been an invisible asterisk floating next to the astronomy section in every print library.

But now the books are starting to come, and one of the first for kids was well worth the wait.

Elaine Scott's "When Is a Planet Not a Planet? The Story of Pluto" is a remarkable work that adults as well as youngsters will find fascinating and accessible (Clarion Books, $17, ages 8 and older).

Not only the saga of Pluto, it's also a terrific introduction to astronomy and a fascinating look at the way scientists work.

The cool cover should pull readers right in. Pluto, for once, is front and center. The other planets trail back toward a tiny glowing sun.

Scott packs ages of astronomical history into a few easy-to-digest pages. She describes the work of early astronomers, such as Ptolemy, Copernicus and Galileo, and how their discoveries led to the identification and naming of each planet.

Next comes a chapter with a simply written, clear explanation of scientific process and how it has been applied to the study of the planets. With school science fairs coming up, this is a great way to show children the role of observation, hypothesis and theory in real-life science.

Finally, we get to the brouhaha over Pluto. Scott explains that scientists realized there had never really been a good answer to the question: "What is a planet?" So they got busy.

With the groundwork laid in earlier chapters -- and clear, compelling explanation of the eventual new three-part planet definition -- many youngsters and adults will understand for the first time what the heck happened to Pluto.

Basically, it came down to the fact that the ex-planet didn't have enough size and gravity to be able to "clear its neighborhood" -- or push other celestial objects out of its orbit.

Wisely, Scott points out that even the new definition might not last forever. "Science is exciting, because it continually changes as new information is discovered," she writes.

The book is liberally illustrated with gorgeous space images, news photos and historical pictures. There's even a group shot of the Planet Definition Committee, looking like folks who might have had some fun together while they were laboring over unearthly matters.

There's a good glossary, bibliography and index.