28 ASTRONOMY • MAY 2018An organicallyearly 2017, the two of
us, along with a few others, refreshed
the debate on the definition of planet
in scientific nomenclature. The
International Astronomical Union’s
(IAU) historic definitional vote in 2006
recognized only eight solar system
planets, and this has brought new focus
to some underlying issues of importance
to planetary science. Specifically, this
debate touches on how words acquire
their meaning and shape our thinking
in both science and everyday life.
Accordingly, the definition of planet is
about much more than whether students
learn Pluto’s name in a list of planets.
In science, two languages describe the
natural world: words (in our case, in
English) and mathematics. Here, we’ll
focus on words. Words possess power
beyond communication: Word choice
affects how we conceptualize, organize,
synthesize, and contextualize informa-
tion. Words are also how we scientists
and educators communicate science to
the public. In other words (so to speak),
words structure our understanding of
the world. This mental structure is what
educational psychologists call a schema.
Scientists define words as part of our
scientific nomenclature with an eye
toward schematic usefulness to concep-
tualize, organize, synthesize, and contex-
tualize information about nature.
Nomenclatures’ definitions arise organi-
cally: Scientists choose words andphrases to describe their
work, and write them in
peer-reviewed journals and peri-
odicals, and speak them aloud in scien-
tific conferences and classrooms.
Precedent is a key element in forming
definitions (just ask a lawyer!) that both
ref lect and promote a useful schema for
understanding the natural world.
Conversely, scientific definitions are
almost never and should never be
handed down authoritarian-
style from a central voting
body, particularly when sci-
entists of different disciplines
have different uses for the
same word. The artificial
authority behind the few
voted definitions in exis-
tence, such as the IAU’s
planet definition, should be
viewed with skepticism and
even dismissal. Science func-
tions through individual
experts making conclusions
and coming to consensus,
rather than being instructed
on what has been decided.
For instance, as far as I (Kirby) know as
a planetary geologist, no one has ever
voted on the definition for a barchan
sand dune. Yet, through usage and prec-
edent, a definition for barchan exists
based on its introduction in the scien-
tific literature in 1881 by Alexander
von Middendorf. Britannica’s useful def-
inition for barchan sand dune is based
on the word’s precedent in the literature;
the definition is a “crescent-shape sanddune produced by the action of wind
predominantly from one direction ...
with a gentle slope facing toward the
wind and a steep slope, known as the
slip face, facing away from the wind.” (It
so happens that barchan sand dunes are
all over the place on Mars!)
That definitions arise through pro-
fessional and common usage are one
blow against the legitimacy of the IAU’s
definitional vote. Another blow arises
from the fact that scientists
of one discipline should not
presume to define words for
another. An illustration
stems from considering the
word metal. Astronomers
use it to describe elements in
stars heavier than helium.
Metallurgists use the word in
the more common way, yet
astronomers and metallur-
gists don’t fight over the def-
inition — each user
community knows what they
mean when they use the
word metal. What would
happen if the metallurgical
community declared an official defini-
tion of metal and then publicly scorned
astronomers for using a different defini-
tion, saying, “I wish they would just get
over it”?
Just as different definitions of metal
serve different communities, we, as plan-
etary scientists, find it useful to define a
planet as a substellar mass body that has
never undergone nuclear fusion and has
enough gravitation to be round due toWords possess
power beyond
communication:
Word choice
affects how we
conceptualize,
organize,
synthesize, and
contextualize
information.PLUTO: NASA/JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/SOUTHWEST RESEARC
H INSTITUTE;
ASTRONOMY
: ROEN KELLY