Planetary Classification

Tony Harris asks:

It sounds as though you have some kind of planetary classification scheme set up. Care to share how it works? :please:

Well, yes, yes I do.

(Let me start out by saying that it does not, however, define what a planet is, except inasmuch as the classification scheme’s largest category stops at 2.5e28 (i.e. 2.5 x 1028) kg, which is about 13.2 Jupiter masses, at which point it gets kicked over to the stellar classification scheme at the bottom end of dwarves, brown.

Insofar as there is a definition of a planet used there, it’s dreadfully informal and would run along the lines of:

  • Orbits a star (other than a brown dwarf that is itself part of another star system);
  • Masses less than 2.5e28 kg;
  • Masses enough to be approximately spheroidal;
  • Is cared enough about by travelers to be listed in Leyness’s Worlds, or some other casual reference that isn’t an astronomer’s catalog or a space pilot’s ephemeris.

So, Pluto, despite being a gelidian-class planetesimal by the scheme, is a planet because it has enough historical significance and popular interest that it’s undoubtedly mentioned in the hypothetical Leyness’s Worlds: A Guide To Places No-One’s Discovered Yet, Except The Natives Who Obviously Don’t Count, while the other plutoids or whatever we’re calling them these days aren’t, because they don’t.)

As for the scheme itself: well, it’s kind of long and complicated and unfinished, especially since the universe keeps surprising us with New Facts About Planets that need to be fitted into it as if they’d been there all along, so I hope you’ll forgive me for not sharing the whole thing. But I don’t mind talking a bit about it, so I’ll do that.

The primary distinction it uses when classifying – and it classifies everything from small rocks through moons and on up, not just “planets” – is mass, which has the advantage of encompassing a fair bit of other useful information. Using mass, it defines five basic categories: small bodies (< 6e20 kg; too small to maintain hydrostatic equilibrium); planetesimals (6e20 kg to 9e23 kg; can maintain hydrostatic equilibrium); lithics (9e23 kg to 3e25 kg; tend to clear out their orbits and self-sustain geology); helians (1.8e25 kg to 1e26 kg; big enough to retain helium, but not yet gas giants, and yes, this overlaps with the one before it); and gas giants (5e25 kg to 2.5e28 kg, and so does that). There’s also a sixth class, wanderers, which defines those odd planetary-massed objects that aren’t gravitationally bound to any star and just drift about in the deep.

The overlaps, incidentally, are because none of these top-level categories are intended to be particularly strict. The Imperial Grand Survey learned long ago that the universe is a very complicated place that doesn’t fit itself into neat boxes for their convenience – so the boxes are a mite fuzzy, and worlds are assigned to the class that bests fits them even if technically they’re a little too massive for the top-level category that world sits within.

So, the categories: small bodies are asteroids, comets, and similarly-sized moons, for the most part, subcategorized primarily by composition and solidity (aggregate-class rubble piles vs. silicaceous-class stony asteroids, for example). The comets are also divided based on whether they’re icy bodies orbiting peacefully in the outer system, or which are actively plunging through the inner system.

Planetesimals are mostly large asteroids and moons, divided up by age (are they just forming?), composition (rocky or tarry or icy), level of geological activity and its source (mostly passive, due to eccentric orbits and epistellar heating, due to tidal flexing, due to internal heating, etc.

Vesta, Ceres, Luna (selénian-class), Europa, Titan (galínilacustric-class), even Mars (eutalentic-class), they all fall somewhere into this classification.

Lithics are the big rocky ones with substantial atmosphere. Their subclassifications are much the same as the planetesimals, plus additional ones: age (forming or dying), water content (xeric through thalassic to pelagic – a global ocean, in these terms) and their methane and ammonia equivalents, presence of a biosphere, presence of halogens, and so forth. There are more lithic subclasses than there are subclasses of anything else, just about, partly because they’re more varied, and partly because they also attract a great deal of interest.

Earth (sylithotectonic-class) fits in here.

Helians get the aborted gas giants, plus a variety of superterrestrial rocky worlds with thick, helium-rich atmospheres. Its number of subclassifications is relatively limited, simply because there’s a very fine line between retaining helium at all and ending up among the…

Gas giants, divided principally by mass (subgiants, dwarf giants, mesogiants, supergiants), and then principally by their orbital positioning (epistellar, within the snowline, beyond the snowline) which defines most of their composition and behavior.

Around here, they would include, for example, Jupiter (melíeréan-class) and Neptune (déiran-class).

So, I hope that was an interesting peek into how the IGS planetary classification system works. I’m happy to answer a bit more, if anyone’s interested, but as I said, I don’t really want to put the whole thing out there until it’s suitably finished and polished.

The Eleventh Planet?

(In honor of current events, here, have a Pluto-analog…)

They say one is the loneliest number, but eleven is the loneliest planet. Well, it’s not a planet as such. Múrcár is, galactographically, a gelidean-class planetesimal, massing 1.7 x 1022 kg, and orbiting well beyond the system snowline at an average of 41 au.

This terminological technicality is a great relief to the Imperial Grand Survey’s Board of Nomenclature, since at aphelion, Múrcár’s orbit reaches 56 au from Lumenna, at the far outer edge of Senna’s Belt. Since the stars of the Lumenna-Súnáris System have only a 125 au separation at closest approach, the height of deep summer, they have been known to swap Sennan objects back and forth at this time; and while it has not yet been observed, astronomers believe that Múrcár’s orbit is vulnerable to this phenomenon when conditions are right. And thus the nomenclaturists would prefer, in this special case, not to have to rule definitively that Múrcár is Lumenna XI when it might be Súnáris X only a matter of mere millennia later.

The above, unfortunately, is the most interesting thing that can be said about Múrcár specifically. It is in virtually all ways a typical gelidean-class Sennan object, composed largely of ices (primarily water ice, with a surface admixture of methane, carbon monoxide, and nitrogen ice) surrounding a core of silicaceous material; it is merely the largest – and first discovered – of the objects in Senna’s Belt.

Múrcár was discovered in 1843 by Senna Marasi, an astronomer at the Starspike, during a fortuitous transit of Súnáris. While it was considered for a while the eleventh planet of the Lumenna system, further studies of the region soon showed other bodies, albeit smaller, to also exist there, similar to the e’Luminiaren. The first close-up images of Múrcár were obtained in 2099, in a fly-by by the Peregrine Ardent probe, and it was first visited in 2139, at which time a lander from Outward Bound confirmed much of the speculation about its surface conditions. The first sophont landing did not take place until 2409 (CSS Veiled In Darkness As A Gown), during an exploratory mission primarily focused upon the further-out bodies of the Shards.

Since then, Múrcár has remained essentially uninhabited. It has however, in its time, hosted:

  • A temporary home port and refueling station for comet herders during the ecopoesis of Talentar;
  • An astronomical observatory far from the traffic and noise of the inner system;
  • A monastery, “Emptiness Dome”, for the meditative Children of the Void sect, before the construction of their present home of Blackwatch Station in Almeä System;
  • A repository-vault for the Green Bytes data haven, which moved on due to Múrcár becoming too well known for their purposes;
  • The primary coordination point for the Outsystem Early Warning line, until it was obsoleted by newer technologies and placed far behind the Empire’s borders after the Reunification;
  • The observatory control center for the Barrascán Array (before its replacement by the Very Long Baseline Observer, itself replaced by the Super-Size Synthetic Aperture).
  • A fueling station for outbound lighthuggers of the now-largely-obsolete “snowball” type.

In the present day, however, Múrcár hosts only a single automated, unstaffed, habitat and fuel station, intended for emergency use. Múrcár also serves as a gathering place, market, and communications hub for the various hermits, fringers, and other darkfolk who make their homes in the far outer system, but this activity almost always takes place in Múrcár near space, rather than on the world itself.

– Leyness’s Worlds: Guide to the Core Worlds

Trope-a-Day: New Neo City

New Neo City: Averted, at least within the Empire.  While people are, of course, free to call their cities, estates, habitats, moons, etc., anything they feel like, the Imperial Grand Survey find this trope incredibly annoying – especially predicting how meta this could get looking into the deep future – and flat-out refuse to put any New Neo Names on the maps.  Period.

Speculativism Index

The Speculativism Index, under any of its names, is a crude clionomic hack used by free traders, weirdseekers, adventurers, and various other professions which necessarily interact with the starbound and minority civilizations of the known Galaxy in a more amateurish way than the Grand Survey or the Exploratory Service.

The Speculativism Index is calculated thus: Examining as large a sample as you feel necessary of the civilization’s media stores and libraries, across formats, and avoiding specialty locations, compute the volume of data devoted to various types of speculative fiction and the volume of data devoted to other, non-speculative fiction; then obtain the ratio between them.  This ratio, expressed as the percentage of the former, is the Speculativism Index.

(A large part of the difficulty involved in this is the problem of recognizing what constitutes speculative fiction in an exotic cultural context.  The Speculativism Index of the d!grith, for example, was historically underestimated due to the failure to recognize their popular “speculative accountancy” genre, while more severe problems attended properly interpreting, and therefore classifying, the multibranched “quantum fictions” of the star-dwelling seb!nt!at.  When these were corrected for, the recalculated d!grith Speculativism Index matches their observed performance; the unusually alien psychology of the seb!nt!at remains something of a special case.)

The Index is principally used, by its inventors, as a sales/interaction valuation tool.  As one free trader explained the associated rule of thumb: “Anything under ten, just leave – they’re never going to make it off their world on their own, and they’re not going to thank you for forcibly introducing them to so many things outside their context.  Between ten and thirty, a little backward, so probably more effort than most of us want to deal with, but with work, can shape up into a solid customer.  Thirty to sixty, that’s the respectable Galactic mainstream.  Over sixty… then you’ve got a whole different class of problems.  Then you’re fighting off their enthusiasm.

It has generally been thought in the past that the Speculativism Index was too rough-and-ready a measure to be of use for clionomic purposes.  Recent studies have established, however, that there is a strong correlation between generally accepted estimates of the Speculativism Index of various well-known civilizations and the degree to which they prosper in the meta-society of the Associated Worlds according to various well-known scales (the Integration Coefficient, the Polity Prosperity Index, and the Progress and Innovation Index), and that the Speculativism Index also correlates with the results of the accepted clionomic coefficients of neophilia, xenophilia, and internal cognitive freedom.

Colleagues, I commend this area to your attention.

– Journal of Cliodynamics, Vol. LXXVI