hydrostatic equilibrium, regardless of its
orbital parameters. This is the definition
we presented at the 2017 Lunar and
Planetary Science Conference. Indeed,
planetary scientists already use and teach
such a geophysical definition of planet to
promote a useful mental schema about
the round and non-round worlds we
study: At least 119 peer-reviewed papers
in professional, scientific journals implic-
itly use this definition when they refer to
grown
planet. Similarly, stars’ size and spectral
diversity between red dwarf stars and blue
supergiant stars parallels planets’ diversity
between small Kuiper Belt dwarf planets
and giant planets.
Definitions, like numerical measure-
ments, have uncertainty, or as scientists
like to call it, an “error bar.” Many small
quasi-round worlds fall into that uncer-
tainty on the small end of the size spec-
trum, and deuterium-fusing large worlds
subtypes of planets and how the solar sys-
tem is naturally organized outward from
the Sun, using a handful of planets as
examples. This is analogous to learning the
organization of the periodic table of the
elements without having to memorize all
or even most of the 100+ names.
Along with this teaching strategy,
scientists, educators, and students should
ignore illegitimate scientific definitions
that arise via voting, such as the IAU’s
Should we really define a word by voting? by Kirby D. Runyon and S. Alan Stern
round worlds (including moons) as plan-
ets. The publication history for these
papers spans decades, hailing from both
before and after the 2006 IAU vote. This
overwhelming precedent cements the geo-
physical definition’s legitimacy in profes-
sional planetary science.
We realize that more than 100 objects
in the solar system fit this geophysical
planet definition, yet this does not dilute
the word’s usefulness. Rather, subcatego-
ries of planets help us form a mental
schema to recognize planets’ diversity and
then draw conclusions and insights based
on groupings of planets with similar
properties. A few useful examples of
diversity among planets include terrestrial
planets (Mars), giant planets (Uranus),
dwarf planets (Pluto, Eris, etc.), and satel-
lite planets (Europa). Each subcategory of
planet helps us recognize similarities and
differences: the domain of comparative
planetology. For instance, Enceladus (an
icy dwarf satellite planet) and Neptune (a
giant planet) are very different types of
planets in size, gravity, and orbits. Yet
both are round, contain high amounts of
water, and are located within our solar
system’s Middle Zone. (This usage further
illustrates the nonsensical claim by the
IAU that dwarf planets are not planets;
rather, dwarf planets are a subcategory of
planets just as giant planets are.) Just as
having approximately 400 billion objects
that fit the definition of star in our Milky
Way Galaxy does not diminish the useful-
ness of the word star, likewise having
many planets in our solar system does not
diminish the usefulness of the word
(brown dwarfs) fall into the error bars on
the large end. However, the geophysical
definition of planet has low enough
uncertainty to still be useful to us.
The geophysical definition further
proves its worth when considering exo-
planets orbiting other stars outside our
solar system. As a thought experiment,
assume our Milky Way Galaxy has a con-
servative 100 billion possible planetary
systems anchored by at least one star.
Assume a conservative 100 dwarf planets
like Pluto or Eris in each system. That’s
10 trillion dwarf planets in just our gal-
axy. If one assumes five giant planets per
planetary system, that’s only 500 billion
giants in the galaxy compared to 10 tril-
lion dwarfs. Thus, dwarfs outnumber
giants 20 to 1 and are the rule rather
than the exception. Re-formulating our
schema of what a planet is facilitates
such insights.
This new schema for planet — properly
defined by expert planetary scientists —
will powerfully work itself out in grade
school classrooms. Rather than teaching
students the names of all the planets,
teachers should emphasize the types and
Kirby D. Runyon
is a postdoctoral
planetary geologist
at the Johns Hopkins
Applied Physics
Lab specializing
in image analysis
to understand the
evolution of planetary
landscapes.
planet definition. Instead, they should
adopt definitions that arise naturally
through usage by experts in the field,
which reflect and promote a useful
mental schema about the natural world
and a more accurate picture of how
science operates.
Other scientists may find a different
definition useful, such as one more con-
cerned with orbits and gravitational
effects on smaller worlds, as proposed by
the IAU. However, such scientists should
not look to the IAU’s vote to cement their
preferred definition, but should rather
use and teach the definition they find
useful. In parallel, they should not
begrudge other scientists’ criteria for
what makes a definition useful to them.
Just as in the example about the use of the
term metal, each user community should
use planet definitions useful to them
without deferring to a central voting
authority. And, just as other definitions
arise organically, the definition of planet
may now be considered organic, drawing
to a close this public hand-wringing
debate and thawing hearts that had fro-
zen toward the planet Pluto.
S. Alan Stern is a plan-
etary scientist who pri-
marily studies the outer
solar system. He is also
the principal investigator
on NASA’s New Horizons
mission and formerly the
associate administrator
for NASA’s Science
Mission Directorate.
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