Poor Pluto.

Astronomers have just rewritten planet history, making even the newest textbooks out-of-date. After years of debate, scientists have concluded that Pluto is not a planet. Instead, it belongs to a brand-new category.

When scientists first spotted Pluto in 1930, they labeled it a planet because it appeared similar to other planets. The rock-and-ice body revolved in an orbit around the Sun, and was estimated to be Earth-size.

Since the discovery, scientists have learned that Pluto is much smaller--only one-fifth the size of Earth. Plus, it is just one of thousands of similar objects within a swath of space beyond Neptune called the Kuiper (KIE-puhr) belt. These findings caused scientists to debate Pluto's planet status. Why the uncertainty? "Scientists had never defined the word planet," says Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology.

This summer, a group of selected astronomers wrote a planet definition. Under the new rules, Pluto is no longer a planet (see What Is a Planet? at right). Scientists labeled it a "dwarf planet"--a designation for planet-like objects that don't fit the new definition.

ALL ABOUT PLUTO

DIAMETER: 2,300 kilometers (1,430 miles) AVERAGE DISTANCE FROM THE SUN: 5.9 billion kilometers (3.7 billion miles) DAY LENGTH (Time it takes to rotate once on its axis): 6.4 Earth days YEAR LENGTH (Time it takes to orbit once around the Sun): 248 Earth years