Reheated cabbage about planets

Popular Science has a recent item about whether Pluto is a planet, prompted by a short paper from the NASA New Horizons team (Runyon, et al.). The paper argues for redefining ‘planet’. In the Popular Science article, Sara Chodosh tries to show “why this matters”. The back-and-forth about Pluto, she writes, is a sign that “we’re still learning”. But the problem is that the short paper doesn’t make any new arguments or reflect any new findings.

Unconvincing objections

Runyon et al. provide three arguments against the 2006 IAU definition of ‘planet’.

First: The IAU definition only applies to objects orbiting our sun.

Although this is a weakness of the IAU definition, it is easy to generalize the IAU definition by not requiring a primary orbit around our sun but instead around some star.

Second: The IAU definition requires “zone clearing”, but nothing clears its orbit perfectly and absolutely. Every actual planet continuously encounters new flotsam which crosses its orbit.

At most, this shows that there is some vagueness. The neighborhood of planet’s orbit is dominated primarily by a planet’s gravity, in a way that can be quantified and made precise as needed. For example: Pluto crosses the orbit of Neptune, and we could just as well say that Neptune crosses the orbit of Pluto. Neptune is the planet of the two, however, because the interaction is overwhelmingly dominated by Neptune.

Third: Zone clearing is harder the further out you go. So the IAU definition makes the requirement for planethood “distance-dependent, requiring progressively larger objects in each successive zone.”

On the face of it, this isn’t even an objection. It’s just a fact about the IAU definition. Runyon et al. see it as an objection because they are committed to having planethood be an intrinsic feature of an object. They write:

In keeping with emphasizing intrinsic properties, our geophysical definition is directly based on the physics of the world itself rather than the physics of its interactions with external objects.

Chodosh glosses the point this way:

All of this is part of their larger argument: a planet isn’t a planet because it’s in a particular orbit or because it has a particular size. A planet is a planet because of its physical properties.

The problem is that physical interactions with other things are just as real as internal physical features. So to define ‘planet’ in terms of orbit clearing is still to define it in terms of physical properties.

Planets occur as parts of solar systems. Although we identify them as individuals, it’s not crazy to think that one’s status as a planet depends on its place in that system. Insofar as we are interesting in the growth and structure of solar systems, context ought to make a difference.

Their definition is uncompelling

Runyon et al. propose this definition:

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a  spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

First: Defining planets just in terms of roundness was one of the possible outcomes back in 2006. So the proposal here doesn’t reflect a new possibility or anything new we’ve learned.

Second: This definition yields boundaries just as vague as an orbit-clearing definition. How wobbly can something be and still be “spheroidal”? How precisely must one describe a thing for it to be “adequately described”?

Third: This definition would include not just contested objects like Pluto and Eris, but also at least one asteroid in the asteroid belt (Ceres).

Fourth: This definition would include objects regardless of their context, so it would include things we usually think of as moons. Not just Io and Ganymede, but Earth’s own moon.

Runyon et al. write that part of their motivation is to reflect “both sound scientific classification and people’s intuitions”. Although moons might sometimes be called ‘planets’ in astronomical discourse, ordinary language certainly has it that moons and asteroids aren’t planets. It’s a matter of settled usage. And astronomers’ usage really can’t be the constraint here, since anything orbiting anything else counts as a “minor planet”.

Of course, there are physically interesting generalizations to be made about anything large enough that its gravity pulls it into a spheroidal shape. So the roundness definition picks out a natural kind just as much as the orbit clearing definition does. Rather, the question is how we should use the word ‘planet’.

What’s at issue here is more a matter of usefulness than discovery. Although Runyon et al. would like a definition which helps them publicly justify the mission to study Pluto, their broad definition would have its own public problems. Among the more than a hundred spheroids in our solar system, eight (or nine) of them are noteworthy for their orbit dominance. If we loosen up the word ‘planet’ to include all the spheroids, we need some way to concisely distinguish spheroids which have their primary orbit around the star and which dominate that orbit. Astronomers, for their part, may slip back and forth using ‘planet’ for either category in different contexts.

If there is any lesson to draw from this, a reason “why this matters”, it’s that it underscores that taxonomy is more important than nomenclature. We want our categories to reflect the natural kind structure of the domains we are studying, even if language flops around a bit.


A  GEOPHYSICAL  PLANET  DEFINITION. K.D. Runyon, S.A. Stern, T.R. Lauer, W. Grundy, M.E. Summers, K.N. Singer Lunar and Planetary Science XLVIII (2017)

I have written quite a lot about planets (e.g., in a book and at a blog) and blogged about grumbling from the New Horizons team before.

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