Astronomy

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Should we really define a word by voting? by Kirby D. Runyon and S. Alan Stern

grown

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

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

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

(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

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

planet definition. Instead, they should

adopt definitions that arise naturally

through usage by experts in the field,

which ref lect 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.

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.

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.