Pluto: An Update on the Honorary Planet


NASA, from the New Horizons spacecraft.

I wrote about Pluto a little over a year ago, in ‘Pluto: The Honarary Planet’, after a paper appeared criticizing the International Astronomical Union’s decision in 2006 to demote Pluto to some lesser status, perhaps ‘minor planet’, ‘dwarf planet’, or ‘planetoid’, thus reducing the ‘official’ list of true planets in our Solar System to eight.

This decision led to a decade of wrangling over terminology.  The IAU’s recommendation, to restrict ‘planet’ to bodies that orbited our Sun (not other stars!) and that cleared lesser debris out of their paths, was ridiculous on its face.  Last year’s paper, by one Kirby Runyon, offered a definition based on ‘intrinsic’ properties, eliminating any consideration of orbit, but including any “sub-stellar mass body that has never undergone nuculear fusion” and is spheroidal in shape.  This bolstered planetary objects to over a hundred in our Solar System alone, including moons.  I suggested we simply conform to popular usage and refer to ‘planets’ as “those more-or-less big (spherical) objects that orbit the Sun (or a sun),” and those that orbit other planets as ‘moons’.

Astronomers David Grinspoon and Alan Stern (they call themselves ‘planetary scientists’, an unfortunate imitation of the faux climatologists who label themselves ‘climate scientists’), writing in The Washington Post (fortunately was not behind the paywall), report on a presentation at the annual Lunar and Planetary Science Conference in Houston:

It stated: “In keeping with both sound scientific classification and peoples’ intuition, we propose a geophysically-based definition of ‘planet’ that importantly emphasizes a body’s intrinsic physical properties over its extrinsic orbital properties.” After giving a precise and nerdy definition, it offered: “A simple paraphrase of our planet definition — especially suitable for elementary school students — could be, ‘round objects in space that are smaller than stars.’”

This of course conflates planets and moons.  Grinspoon and Stern defend this usage:

We find ourselves using the word planet to describe the largest “moons” in the solar system. Moon refers to the fact that they orbit around other worlds which themselves orbit our star, but when we discuss a world like Saturn’s Titan, which is larger than the planet Mercury, and has mountains, dunes and canyons, rivers, lakes and clouds, you will find us — in the literature and at our conferences — calling it a planet.  This usage is not a mistake or a throwback. It is increasingly common in our profession and it is accurate.

Elevating planetary satellites to ‘planets’ themselves may be professionally gratifying, but to my mind it still carries the ‘intrinsic over extrinsic’ debate too far, ignoring orbits.  Titan, I submit, may be larger than Mercury, but it is still a moon.  The average layman, when he hears about ‘the planet Titan’, may well wonder what star it circles, or what science-fiction novel it inhabits.  Is the Earth’s Moon a ‘planet’?  It is, after all, spherical, and has mountains and plains, if not rivers and clouds.

What about the ‘spherical’ criterion?  They write,

Most essentially, planetary worlds (including planetary moons) are those large enough to have pulled themselves into a ball by the strength of their own gravity. Below a certain size, the strength of ice and rock is enough to resist rounding by gravity, and so the smallest worlds are lumpy. This is how, even before New Horizons arrives, we know that Ultima Thule [a body out in the Kuiper Belt beyond Pluto] is not a planet. Among the few facts we’ve been able to ascertain about this body is that it is tiny (just 17 miles across) and distinctly nonspherical. This gives us a natural, physical criterion to separate planets from all the small bodies orbiting in space — boulders, icy comets or rocky and metallic asteroids, all of which are small and lumpy because their gravity is too weak for self-rounding.

Is it simply gravity that makes bodies spherical?  Likely so, though I wonder whether gravity or the force of collision has to create enough heat for the components to flow.  In any case, until we discover a truly irregular body orbiting a star and large enough to be called a planet, ‘spherocity’ is a reasonable criterion (the asteroid Vesta is a possible candidate).  But let’s continue to distinguish between planets and moons, following common usage.  The rule ought to be: don’t make things more arcane than they need to be.  We want the kids to be interested, not confused by pettifoggery. /LEJ

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