Okay, everyone, it's official. Pluto is not a planet. As decreed in August
2006 by a vote of the General Assembly of the International Astronomical
Union (IAU), Pluto is now a "dwarf planet." On the other hand, who does
the IAU think it is—indeed, who do we astronomers think we are—to
be able to demote Pluto? Isn't that sort of like California declaring that
Liechtenstein isn't a country?
The word planet
seems to hold an irrational sway over our hearts
and minds. That made sense in the days when, along with stars, they were
the only familiar objects in space—before telescopes could observe
the birth of distant galaxies, before space probes had bulldozed into a
comet, and before we understood the history of cosmic collisions that links
celestial bodies large and small.
In these past four weeks, we've looked at the Solar System as scientists
do: at its structure and composition, its origin, and its contents, rocky
and gassy. Let's look at Pluto once again—not in terms of rigid classification
or nursery-school mnemonics but in the context of its scientific importance—and
at how learning more about Pluto contributes to the human endeavor of understanding
What's a planet, anyway?
All the Sturm und Drang
about Pluto stems from a simple problem.
The label planet
originated in ancient Greece. The word simply meant
"wanderer" and referred to the seven prominent celestial objects—Mercury,
Venus, Mars, Jupiter, Saturn, the Sun, and the Moon—that moved against
the background of stars.
Life got more complicated in 1543, when Nicolaus Copernicus described
a newfangled Solar System. In his heliocentric universe, instead of remaining
stationary in the center, Earth moved around the Sun, just like the other
bodies. At that moment, planet
lost its astronomical meaning. Astronomers
tacitly agreed that whatever orbits the Sun is a planet and whatever orbits
a planet is a moon.
|Evolving Our Understanding
Petrus Apianus's Earth-centric engraving of the Solar System (left) from
1540 shows the planets and the Sun orbiting Earth, with a band of constellations
around the perimeter. In 1543, Copernicus's revolutionary heliocentric
system (center) paved the way for modern astronomy, including way-finding
diagrams like the plaque (which includes Pluto) on the Pioneer 10 spacecraft
(right), now heading into interstellar space. ©Library of Congress/NASA
This wouldn't be a problem if cosmic discoveries had ended with Copernicus.
But shortly thereafter, we learned that comets, too, orbit the Sun and
are not local atmospheric phenomena, as was long believed. Comets
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 chunks of rock and
metal that orbit the Sun between Mars and Jupiter in the asteroid belt?
When Ceres, the first such object, was detected by Giuseppe 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 these small bodies made of rock and minerals asteroids,
and have now cataloged tens of thousands of them.
Even the traditional planets don't fit into one neat category. The rocky
planets (Mercury, Venus, Earth, and Mars) form a family because they are
relatively small and rocky, while the gassy planets (Jupiter, Saturn, Uranus,
and Neptune) are large, gaseous, have many moons, and bear rings.
And who's counting?
The number of planets dropped to six when the Sun and the Moon were deleted
and Earth was added. 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
again after these four bodies—along with others in the zone yet to
be discovered—were demoted to asteroids. Once Neptune was spied in
1846, the total became eight.
When astronomer Clyde Tombaugh found Pluto in 1930, after a dogged search
for a long-suspected Planet X beyond Neptune, the tally rose to the now-familiar
nine. 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.
The Kuiper Belt thickens the plot
The story took another twist in 1992, when David C. Jewitt of the University
of Hawaii and Jane Luu of the Massachusetts Institute of Technology began
to detect a swath of frozen objects on the Solar System's fringes, out
beyond Neptune. This region of icy bodies was named the Kuiper Belt in
honor of the Dutch-born American astronomer Gerard Kuiper, who predicted
its existence. Pluto is one of its largest members. Akin to the asteroids
in the belt between Mars and Jupiter, these bodies nevertheless made up
another category of objects in the Solar System. Over 800 other Kuiper
Belt objects have since been cataloged. Should they all be called planets?
|The Kuiper Belt
A disk-shaped region of icy debris beyond the orbit of Neptune, the Kuiper
Belt likely contains remnants of the early Solar System, as does the asteroid
belt. Because many asteroids and comets never formed planetary bodies that
melted, they record early Solar System processes—a record that has
been erased elsewhere. ©NASA/JHU
So we find ourselves at the International Astronomical Union General Assembly,
meeting in Prague in August 2006. At first the IAU seemed ready to defend
Pluto's planetary standing. On August 16, after many meetings over the
course of a year, its seven-member Planet Definition Committee stated that
round objects in orbit around the Sun are planets. Roundness (though not
necessarily a perfectly spherical shape), reasoned the committee, indicated
a balance between the gravitational forces pulling matter inward and the
internal pressure pushing outward within a celestial body: a scientifically
significant state called hydrostatic equilibrium.
Since Pluto qualifies, this would have given everyone the right to place
Pluto and Jupiter in the same category, even though Jupiter is 250,000
times larger. The draft resolution would also have rendered at least three
additional objects eligible for planet status, objects that had achieved
hydrostatic equilibrium but had previously been deemed "too small."
So for that one week in 2006, there were 12 planets. The IAU's roundness
criterion added Ceres, the largest asteroid; Pluto's moon Charon, which
is unusually large relative to Pluto; and another Kuiper Belt object, 2003
UB313, affectionately dubbed Xena after the leather-clad warrior princess
from cable television, but now officially named Eris, after the Greek goddess
Plutophiles had about a week to rejoice before the astronomers refined
their definition: a planet must also be the most massive object in its
orbital 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. This criterion also eliminated Ceres, Charon, and Eris. To soothe
the Pluto boosters, the IAU elected to call it a dwarf planet, without
clearly qualifying what that is.
|And Then There Were Eight
In 2006, the International Astronomical Union published a draft illustration
of the Solar System containing 12 planets (bottom). One week later, a final
illustration was published (top), with four of those objects reclassified
as "dwarf planets." ©IAU
How much should counting count?
So today we're officially back to eight planets—the nine we memorized
in grade school, minus Pluto.
Counting planets does encourage clever mnemonics, such as "My Very Educated
Mother Just Served Us Nine Pizzas"—or its likely successor: "My Very
Educated Mother Just Served Us Noodles." Or Nectarines. Or Nopalitos! It
could be argued that such counting exercises have stunted the curiosity
of an entire generation of children. Counting and memorizing just stands
in the way of appreciating the full richness of our cosmic environment,
right? On the other hand, it's well known that the concreteness of lists
and lyrics helps students tie abstract concepts to tangible learning tools.
The best solution probably rests in the middle ground. For now, a dwarf
planet is defined as a Solar System body that orbits the Sun, is near-spherical
in shape, isn't a satellite, and shares the region around its orbit with
other celestial bodies. And who knows how long that classification will
The best question of all: What questions intrigue you?
Imagine a Solar System curriculum that begins with the concept of density—a
big concept for third graders, but not inaccessible. 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 would
You might wonder about the joint criteria of roundness and isolation.
They're general enough to be shared by both tiny, rocky, iron-rich Mercury
and massive, gaseous Jupiter. But what if other characteristics or phenomena
pique your interest? Suppose, for example, that you're interested in cyclones.
The thick, dynamic atmospheres of Earth and Jupiter are fertile breeding
grounds for these storms, so they could be lumped together under that criterion.
Fascinated by the chemistry of life? Icy moons like Jupiter's Europa and
Saturn's Enceladus may be the best extraterrestrial destinations in the
search for liquid water, a crucial ingredient for life as we know it. Perhaps
you think ring systems are cool, or magnetic fields, or size, or mass,
or composition, or proximity to the Sun, or formation history. Each attribute
could serve as a vector for exploring the bodies that populate the Solar
These classifications say much more about an object than whether it is
round, or unique in its neighborhood, or what category we assign it to.
Why not rethink the Solar System as multiple, overlapping families of objects?
Then the way you organize them is up to you. The fuss over Pluto doesn't
have to play out as a death in the neighborhood. Instead, it could mark
the birth of a whole new way of thinking about our cosmic backyard.
No matter how the scientific debate about Pluto rages in the years to
come, it will remain a beloved little icy dirtball to millions—and
a catalyst to scientific curiosity and excitement. And if you're a Pluto
lover, you can rest assured that the dwarf planet won't be forgotten. Guess
what the American Dialect Society declared as the 2006 Word of the Year?