Without a consensus definition for the word planet, these questions provoked years of pointless debate among people for whom counting planets matters. The known universe once contained seven planets. Then what became the solar system contained six. When Uranus was found in 1781, the figure rose to seven again. It was bumped up to 11 with the discovery of the four largest bodies in the zone between Mars and Jupiter. Then it dropped back to seven once again after these four bodies—and any others yet to turn up in the zone—were demoted to asteroids. Once Neptune was spied in 1846, the total became eight.

After the discovery of Pluto, the tally rose to the now-familiar nine. Astronomer Clyde Tombaugh had found Pluto through a dogged search for a long-suspected Planet X beyond Neptune, and everyone initially assumed he had found something large. But refined measurements showed the object to be much, much smaller than originally thought, smaller in fact than six of the satellites in the solar system, including Earth's moon.

Then, for that one week in August, there were 12 planets. The IAU's roundness criterion added Ceres, the only gravitationally round asteroid; Pluto's moon Charon, which is unnaturally large compared with Pluto; and 2003 UB313, temporarily but affectionately called Xena after the leather-clad warrior princess from cable television. Now we are officially back to eight—the nine you memorized in grade school, minus Pluto.

Courtesy of NASA and ESA
If my overstuffed e-mail in-box is any indication, this game of planetary enumeration remains a deep concern of elementary school students and the mainstream media. After all, counting planets is what allows you to invent clever mnemonics to remember them in order of increasing distance from the sun, such as "My Very Educated Mother Just Served Us Nine Pizzas." Or its likely successor: "My Very Educated Mother Just Served Us Nachos."


But such exercises have stunted the curiosity of an entire generation of children by suggesting that memorizing a sequence of names is the path to understanding the solar system. The word planet seems to hold an irrational sway over our hearts and minds. That level of fascination made sense in the days before telescopes could observe details in planetary atmospheres, before space probes had explored Mars and bulldozed into a comet, and before we understood the history of asteroid and comet collisions, linking celestial bodies large and small. But today, the rote exercise of planet counting rings hollow and stands in the way of appreciating the full richness of our cosmic environment.

Suppose other properties are what matter to you. Interested in cyclones? You might lump together the thick, dynamic atmospheres of Earth and Jupiter. Fascinated by the chemistry of life? Icy moons like Jupiter's Europa and Saturn's Enceladus may be the best extraterrestrial places to find liquid water, a crucial ingredient for biology. Or perhaps instead you care about ring systems, or magnetic fields, or size, or mass, or composition, or proximity to the sun, or formation history. And the discovery of planets around other stars has exposed entire new categories like "hot Jupiters"—giant gassy worlds heated to near incandescence by their astonishing proximity to their suns.

These classifications say much more about an object's identity than whether its self-gravity made it round or whether it is the only one of its kind in the region. Why not rethink the solar system as multiple overlapping families of objects? Then the way you organize them by their characteristics is up to you. The fuss over Pluto doesn't have to play out as a death in the neighborhood. It could mark instead the birth of a whole new way of thinking about our cosmic backyard.

War Over the Worlds

Only 424 of the 2,412 astronomers registered to vote at the International Astronomical Union's General Assembly in Prague cast their ballots (right) for the new definition of planet. Nonetheless, the outcome sparked a frenzy of media coverage, online chatter, and even protests. (One grassroots group of 300 astronomers declared, "We, as planetary scientists and astronomers, do not agree with the IAU's definition of a planet, nor will we use it.") DISCOVER asked leading astronomers for their reactions.

"From a science point of view, Pluto is still Pluto. It is the first known in an entirely new class of Pluto-like objects that are waiting for discovery." Richard Binzel of MIT, Pluto expert and member of the IAU Planet Definition Committee

"The IAU's planet definition resolution is an example of science at its worst—it's sloppy internally inconsistent, and designed backwards, that is, to produce a desired result that only eight planets are in our solar system. I am embarrassed for the IAU." Alan Stern of the Southwest Research Institute, leader of the New Horizons mission to Pluto

"I am absolutely delighted at the decision to place Pluto in its proper context as a glorious member of the Kuiper belt objects. Astronomers made a mistake in the 1930s in calling Pluto a planet. Now finally Pluto is being placed in its rightful category." Geoffrey Marcy of the University of California at Berkeley, noted planet hunter

"The new proposal reflects how most scientists working on the outer solar system think about these bodies. Trying to set them up as planet serves no scientific purpose. The main reasons for labeling big Kuiper belt objects as planets are nostalgia—Pluto used to be a planet, how could it not be one now?—and self-interest." David Jewitt of the University of Hawaii, codiscoverer of the first Kuiper belt object