The Pluto Files: The Rise and Fall of America's Favorite Planet (17 page)

A P
ERSONAL
R
ECOMMENDATION FOR
E
DUCATORS

Y
ES, IT REALLY IS OFFICIAL
. P
LUTO IS
NOT
a red-blooded planet, as voted in August 2006 by the general assembly of the International Astronomical Union. Pluto is now a “dwarf” planet.

How rude.

The vote overturned the Planet Definition Resolution proposed by the Planet Definition Committee, which had stated simply that round objects in orbit around the Sun are planets. Pluto is a round object. Therefore, Pluto is a planet. This first attempt to define “planet” would have given everyone the right to utter Pluto and Jupiter in the same breath even though Jupiter is 260,000 times larger than Pluto. Plutophiles had about a week to rejoice before learning the sad news that Pluto fails a new criterion—that a legitimate planet must also dominate the mass of its orbital zone. Poor Pluto is crowded by thousands of other icy bodies in the outer solar system.

Embarrassing as it was to us all, the term
planet
had not been formally defined since the time of ancient Greece.

In 1543, Nicolaus Copernicus published his thesis—his newfangled, Sun-centered (heliocentric) universe, which confounded the wanderer classification scheme. Instead of being stationary and in the middle of things, Earth moved around the Sun just like everybody else. From that moment on, the term
planet
had no official meaning at all. Astronomers just silently agreed that whatever orbits the Sun is a planet. And whatever orbits a planet is a moon.

Not a problem if cosmic discovery freezes in time. But shortly thereafter, we learned that comets orbit the Sun too, and are not, as long believed, local atmospheric phenomena. Are they planets too? No, we already had a name for them: comets. They’re the icy objects on elongated orbits that throw a long tail of evaporated gases as they near the Sun.

How about the craggy chunks of rock and metal that orbit the Sun in a region between Mars and Jupiter? Hundreds of thousands roam there. Are they each a planet too? While first called planets—beginning in 1801 with the discovery of Ceres—it became rapidly clear as dozens more were discovered that this new community of objects deserved its own classification. They came to be called asteroids.

Meanwhile, Mercury Venus, Earth, and Mars form a family of their own, being relatively small and rocky, while Jupiter, Saturn, Uranus, and Neptune are large and gaseous, have many moons, and bear rings.

And what’s going on beyond Neptune? Beginning in 1992, icy bodies were discovered that look and behave a lot like Pluto. Yet another swath of populated real estate was discovered, akin to the discovery of the asteroid belt two centuries before. Known as the Kuiper belt, in honor of the Dutch-born American astronomer Gerard Kuiper, who championed its existence, this region of the solar system contains Pluto, one of its largest members. But Pluto had been called a planet since its discovery in 1930. Should all Kuiper belt objects be called planets?

Without a formal definition for the word
planet
, these questions created years of debate among people for whom counting planets matters.

If my overstuffed in-box is any indication, planetary enumeration remains a major pastime of the elementary schools and a deep concern of the print and broadcast media. Counting planets is what allows you to invent clever mnemonics to remember them in sequence from the Sun, such as “My Very Educated Mother Just Served Us Nine Pizzas.” Or its possible successor: “My Very Educated Mother Just Served Us Nachos.” Here’s one that may grow on us all: “My Very Educated Mother Just Said Uh-oh No Pluto.”

Where do you go from there? Because of exercises such as this, elementary school curricula have unwittingly stunted an entire generation of children by teaching them that a memorized sequence of planet names is the path to understand the solar system. The word
planet
itself continues to garner profound significance in our hearts and minds. This was surely justifiable before telescopes let us observe planet atmospheres; before space probes landed on planet surfaces; before we learned that icy moons make fertile targets for astrobiologists; before we understood the history of asteroid and comet collisions. But today, the rote exercise of planet counting rings hollow and impedes the inquiry of a vastly richer landscape of science drawn from all that populates our cosmic environment.

Suppose other properties matter to you instead. Suppose you care about ring systems, or size, or mass, or composition, or weather, or state of matter, or proximity to the Sun, or formation history, or whether the cosmic object can sustain liquid water or liquid anything. These criteria represent demographic slices that reveal much more about an object’s identity than whether or not its self-gravity made it round or whether or not it’s the only one of its kind in the neighborhood.

Why not think of the solar system as families of objects with like properties, and the cut through these properties is yours to take. Interested in cyclones? You get to talk about the thick atmospheres of Earth and Jupiter together. How about auroras? That conversation would include Earth, Jupiter, and Saturn, since all three have magnetic fields that guide charged solar particles to their poles, rendering their atmospheres aglow. How about volcanoes? That would include Earth, Mars, Jupiter’s moon Io, and Saturn’s moon Titan, whose eruptions are likely driven by ice and not by lava. How about wayward orbits? That would have to include comets and near-Earth asteroids, themselves putting life on Earth at risk. The list of ways to envision and organize the solar system is long and likely endless.

Imagine a solar system curriculum that begins with the concept of density. A big idea for third graders, but not beyond their grasp. Rocks and metals have high density. Balloons and beach balls have low density. Divide the inner and outer planets in this way, as cosmic examples of high and low density. Have fun with Saturn, whose density, like that of a cork, is less than that of water; unlike any other object in the solar system, Saturn’s material would float in a bathtub.

At no time are you counting things. At no time are you worried about the definition of a taxonomic category. At no time are you left in search of a mnemonic on the premise that to understand the solar system you must memorize the proper names for things.

Eventually, you might be curious about the joint criteria of roundness and isolation. It’s undiscriminating enough to combine into the same category tiny, rocky, iron-rich Mercury and large, massive, gaseous Jupiter—at which point you remember that way back in August of the year 2006, the International Astronomical Union created a name for that class of object. You search the organization’s archives, find the word
planet
, and then quickly move on to the rest of what piques your interest in the solar system.

A
T A
J
ULY
2008
MEETING IN
O
SLO
, N
ORWAY, THE
E
XECUTIVE
Committee of the IAU approved a proposal
⁴⁹
from the IAU Committee on Small Body Nomenclature for the name
plutoid
to classify all dwarf planets that one might find orbiting beyond Neptune. As of this printing, two dwarf planets orbit there—Pluto and Eris—with many more surely to come. This obviously makes Pluto a plutoid. By IAU rules, Ceres, another dwarf planet in the solar system, does not receive this honor because it orbits between Mars and Jupiter in the asteroid belt. Odd that Pluto’s moon Charon is excluded, as are any yet-to-be-discovered round moons of dwarf planets beyond Neptune. An arbitrary, but enforceable, rule.

Alan Stern, now of the University Space Research Association, does not like the plutoid class for many reasons astrophysical, but when interviewed, he typically leads with “It sounds like ‘hemorrhoid.’”

He and I finally agree on something.

By August 2008, Pluto’s pitbull, Mark Sykes, was at it again. He and others organized a conference for planetary experts to discuss what to do about classification schemes for the solar system, with little to no regard for IAU proclamations. Held at the Applied Physics Laboratory of Johns Hopkins University in Maryland, the conference included a heavily touted public forum titled “The Great Planet Debate.”
⁵⁰
Given my history with the subject, I felt somewhat accountable for things, so I agreed to debate Sykes himself, in a reprise of our impromptu conversation in my office six years earlier. But this time we had a moderator—NPR’s
Science Friday
host, Ira Flatow. On entering the auditorium, Mark and I were showered with “Let’s Get Ready To Rumble” entry music, commonly played as professional wrestlers enter the ring.

I am pleased to report that Mark was much more polite and cordial than I had ever remembered him to be. And at times, I was the one who was manic. But in the end, we did not converge on the definition of a planet. We both agreed, however, that the IAU had body-slammed Pluto on this one. And that a more enlightened solution to the problem awaited us all.

In an attempt
to achieve closure, I took a pilgrimage to Orlando’s Disney World. I felt duty-bound to alert Pluto (the dog) of my role in his demotion. After initial dismay, or at least what looked like dismay in a creature that cannot frown, Pluto and I became fast pals, and he has accepted his uncertain fate with grace and dignity.

Meanwhile, nobody is quite sure how Pluto (the ex-planet) feels about all this except, perhaps, cartoonists. Nearly 4 billion miles away, Pluto, a cosmic object by any name, offers the last word:

Letter from Siddiq Canty, Mrs. Koch’s second-grade class, Roland Lewis Elementary School, Tampa, Florida (spring 2008).

Pluto Data (2008)
⁵¹