On August 24, 2006, at its XXVIth General Assembly
in Prague, the International Astronomical Union (IAU) reclassified
Pluto as a ‘dwarf planet’. To understand this action, and the Voyage
team's decision to include a Pluto Unit in replicas of the exhibition,
it is useful to put Pluto’s long journey in an historical context.
| Pluto was discovered in
February 1930 by Clyde Tombaugh (1906-1997) at the Lowell Observatory
in Flagstaff Arizona. Over the next few decades it became clear that
Pluto was fundamentally different from the other planets in the Solar
System. It was a solitary, comparatively small icy world beyond the gas
giants Jupiter, Saturn, Uranus and Neptune. Its orbit around the Sun
was far more inclined to the plane of Earth’s orbit than any other
planet, and its orbit was far more non-circular (eccentric) than the
other planetary orbits with the exception of Mercury.
Pluto appeared to have characteristics more in line with comets—up to a
trillion icy objects beyond Neptune, each typically a few kilometers
across. Yet early size estimates for Pluto indicated a diameter of a
few thousand kilometers. (We now know Pluto has a diameter of about
2,300 km, which is big enough to hold 100 million comets.)
Pluto also seemed to be stepping up to the plate as a planet. In 1978
its moon Charon was discovered, and in 1988 an atmosphere was detected.
Surely planets have moons and atmospheres.
But in 1992 another Pluto-like object designated 1992 QB1 was
discovered beyond Neptune; in 1993 the Galileo spacecraft flew by the
asteroid Ida and found it was orbited by a small moon (officially named
Dactyl); and models indicated that Pluto’s atmosphere may only exist
when it is close to the Sun—like comets. So by the mid-1990’s, what was
clear was that there were other Pluto-like objects though smaller, even
asteroids have moons, and an atmosphere does not necessarily a planet
make. Pluto wasn’t a planet in the sense of Earth-like or Jupiter-like
worlds, and it wasn’t a comet. Maybe it was something else.
It turned out that 1992 QB1 was the first of a new class of object
detected—the Trans-Neptunian Objects (TNOs), also called Kuiper Belt
Objects (KBOs). They are larger than comets, ranging in size from
approximately 100 km to 1,000 km, and we began finding lots and lots of
The debate over whether to take away Pluto’s planet status has been
around since at least 1992 with the discovery of 1992 QB1. Depending on
whom you ask, the debate was always tempered by the lack of a crisp
definition for ‘planet’, some compassion for Clyde Tombaugh by the
astronomical community, and maybe even Pluto’s longstanding place as
the ninth planet in the hearts of children and the public.
By 2005, the debate on Pluto’s planet status was rising
to a fever pitch. By then nearly a dozen 1,000 km diameter TNOs were
known. It made far more sense to say that the first TNO detected was
not found in 1992, but rather in 1930 by Tombaugh. Pluto no longer
would be a solitary errant planet but the largest of this new class of
object, the TNOs. But still Pluto managed to duck under the cover of
historical precedent. After all it was a planet for 75 years.
In April 2006 things changed. Researchers using the Hubble Space
Telescope announced that one of the large TNOs, 2003 UB313, was
actually bigger than Pluto. And so the battle lines were
drawn. Either we had to add more planets to the Solar System and face
the possibility of dozens more planets as more large TNOs were
discovered, or Pluto had to be ‘demoted’.
At their meeting in August, the IAU first floated the idea of 12
planets, which would have added 2003 UB313, Pluto’s Charon (which is
massive enough compared to Pluto for Pluto-Charon to be considered a
binary planet system), and the largest asteroid Ceres located between
Mars and Jupiter. But that was not to be. In a major reversal over the
course of a week the IAU issued a resolution
that officially reduced the number of planets to eight.
The IAU decision was not without controversy even among the planetary
science community. The IAU issued a new definition for planet that
seemed to many to be rushed, political, and scientifically lacking.
The IAU added additional complexity to the situation by creating a new
category of Solar System object called ‘Dwarf Planet’. The IAU gives
dwarf planet status to a body that is not one of the eight planets, is
not a satellite of a planet, but is massive enough for gravity to cause
it to attain a spherical shape. Designated as dwarf planets were Pluto,
Ceres, and 2003 UB313—which the discovery team had dubbed Xena, but has
now be formally named Eris. As of this writing there are 10 known TNOs
(other than Pluto, Charon, and Eris) with a diameter of about 1,000 km
or greater. All are potential candidates for designation as a dwarf
As of September 25, 2006, the Minor Planet Center at the Smithsonian
Astrophysical Observatory listed 1,014 Trans-Neptunian Objects on its
daily updated website, which includes Pluto as of September 7,
2006. The Minor Planet Center (MPC) is charged with officially keeping
track of such things under the auspices of Division III of the
International Astronomical Union (IAU).
|The debate over Pluto’s status as a planet was
raging at the time of the 2001 Voyage installation on the
National Mall in Washington, DC. The latest controversy at the time was
the opening of the Rose Center at the Hayden Planetarium in New York in
early 2000, and Pluto was noticeably absent from the planetary family.
What to do about Pluto was top of mind for us back then. Voyage
needed to teach visitors about the Solar System—all components of the
Solar System. To this end, a Unit was placed between the model Mars and
Jupiter that was dedicated to both asteroids and comets. It was located
at the center of the asteroid ‘belt’. Comets were addressed her as well
for good reason. At about this location from the Sun, solar heating
causes cometary water ice to vaporize forming the gaseous coma around
the nucleus, and the solar wind and pressure from sunlight begin to
sweep gas and dust into two tails.
In 2001, regardless of what was going to happen
regarding Pluto’s status, we needed to talk about the Trans-Neptunian
Objects. Even if still called a ‘planet’, Pluto was clearly related to
the TNO population, and in fact was likely the largest one (at the
time). The Pluto storyboard used the debate as a teachable moment. It
is where we introduced the TNOs to the visitor and the possible linkage
In 2006 nothing has changed except that we now call Pluto a dwarf
planet and fully recognize its membership in the TNO class of objects.
Regarding replicas of the exhibition, the Pluto Unit will still be
included as the logical place to talk about the controversy, the TNOs,
the new classification of dwarf planet, and pay homage to tiny
Pluto—who’s probably wondering what all the hoopla is all about back on