A Death in the Solar System

Say good-bye to the old nine planets. Say hello to a whole new celestial family.

By Neil deGrasse Tyson|Monday, November 27, 2006
RELATED TAGS: SOLAR SYSTEM

It's official. Pluto is not a red-blooded planet. As decreed in August by a vote of the General Assembly of the International Astronomical Union, Pluto is now a dwarf.

At first, the IAU seemed ready to defend Pluto. On August 16, the union's seven-member Planet Definition Committee released a draft Planet Definition Resolution, which stated that round objects in orbit around the sun are planets. Pluto is a round object in orbit around the sun. Therefore, Pluto is a planet. This definition would have given everyone the right to utter "Pluto" and "Jupiter" in the same breath, even though Jupiter is 250,000 times larger. The draft resolution would also have opened the door to granting planet status to at least three objects that had, until recently, been considered unworthy.

Plutophiles had about a week to rejoice before the astronomers returned from their deliberation with a change of heart. According to the final IAU definition, a planet must still be round but must also dominate the mass of its orbital zone. In other words, a full-fledged planet must have no competitors in its zone.. Poor Pluto is crowded by thousands of other icy bodies in the outer solar system, some bigger than Pluto itself, so it fails the test. To soothe Pluto's boosters, the IAU elected to call it a dwarf planet, without entirely quantifying what that is.

All this embarrassment stems from a simple problem. The term planet had not been defined since the time of ancient Greece, when the label originated. The word simply meant "wanderer" and referred to the seven prominent celestial objects that moved against the background of stars. They were Mercury, Venus, Mars, Jupiter, Saturn, the sun, and the moon. So influential were these celestial travelers on classical culture that the names of our seven days of the week can be traced back to them.

Life got more complicated in 1543, when Nicolaus Copernicus described a newfangled, heliocentric universe. Instead of remaining stationary in the middle, Earth moved around the sun just like the other bodies. From that moment onward, planet had no official meaning, and astronomers tacitly agreed that whatever orbits the sun is a planet and whatever orbits a planet is a moon.

This would not be a problem if cosmic discoveries had ended in 1543. But shortly thereafter, we learned that comets orbit the sun, too, and are not, as long believed, local atmospheric phenomena. Comets are icy objects on elongated orbits that throw off a long tail of gases as they near the sun. Are they planets too? How about the craggy chunks of rock and metal that orbit the sun in a region between Mars and Jupiter? When Ceres, the first of these objects, was sighted by Giuseppi Piazzi in 1801, everyone called it a planet. With the discovery of dozens more, however, it became clear that this new community of objects deserved its own classification. Astronomers called them asteroids and now have cataloged tens of thousands of them.

Even the traditional planets don't fit into one neat category. Mercury, Venus, Earth, and Mars form a family because they are relatively small and rocky, while Jupiter, Saturn, Uranus, and Neptune are large, gaseous, have many moons, and bear rings.

The story took another twist in 1992, when David C. Jewitt of the University of Hawaii and Jane Luu of MIT began to detect frozen objects on the solar system's fringes, out beyond Neptune. They found a new swath of space traffic, akin to the asteroid belt discovered two centuries before. Known as the Kuiper belt, in honor of the Dutch-born American astronomer Gerard Kuiper, who predicted its existence, this region of the solar system contains Pluto, one of its largest members. But Pluto has been called a planet since it was discovered in 1930. So should all Kuiper belt objects be called planets?

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.

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 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

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