Planetology is the study of a planet, moon or asteroid as a whole, including all its component elements, planetary formation, evolution, and taxonomy.
- 51 Peg planet - Text by M. Alan Kazlev
A planet, that shows orbital periods shorter than 15 terran days. These were among the first extrasolar planets discovered by Terragen astronomers, during the pre-singularity period (early Information Age). Included in this category are 51 Peg itself, Tau Bootis, 55 Cancri, and Upsilon Andromedae, which have orbital periods of just 4.2, 3.3, 14.7 and 4.6 days, respectively. The orbits are all small, with radii less than 0.1 A.U. They commonly range in mass from 0.44 Jupiters (for 51 Peg) to 3.64 Jupiters (Tau Bootis). They move in circular or near-circular orbits.
- Apparent Brightness of the Local Sun - Text by Steve Bowers
To standard Terragen eyes, suns with a different spectral type from Sol do not seem as bright from the surface of a terraformed planet, since less of their radiation falls within the visible range.
- Arean Type planet - Text by John M. Dollan
Mars-like worlds where the atmosphere and hydrosphere has largely disappeared due to the cessation of magnetic activity.
- AreanLacustric Subtype worlds - Text by John M. Dollan
Young Mars-like worlds with moderate amounts of ocean cover
- AreanTundral Subtype - Text by John M. Dollan
Cold Arean type worlds, often with considerable reserves of ice
- AreanXeric Subtype - Text by John M. Dollan
Mature, unusually hot and dry Arean type worlds.
- Artificial Planets - Text by Steve Bowers
A number of very energy-costly, hi-tech or god-tech methods that allow the construction of new planets or planet-like objects to order.
- Artificial Worlds - Text by Anders Sandberg, John M. Dollan, and Steve Bowers
Artificial worlds can be divided into habitats (when a habitat can be called a world is of course always a matter of taste, but the formation of a self-sustaining climate and ecology without the need for constant maintenance is usually the relevant point), mega-habitats (>100 km), Banks orbitals, ISOs, Dyson swarms, and other structures (like Kepleria). Categorization is usually simply by basic design, with no need for more elaboration.
- Asteroid - Text by M. Alan Kazlev
Generally any small object that is not a satellite of a planet, and is too small to be considered a planet. Asteroids larger than 50km are classified as Planetoids under the NoLWoCS system.
- Astrum - Text by M. Alan Kazlev
Radial-patterned features on Venusian type planet. The plural of the word is "astra".
- Atmosphere - Text by M. Alan Kazlev
The gaseous envelope surrounding a planetary object, moon or star, (or possibly the habitable surfaces of an artificial structure. A typical atmosphere consists of a mixture of gases. Some of the most common are hydrogen, helium, methane, nitrogen, oxygen, argon, carbon dioxide, water vapour, and ammonia. Often an atmosphere is divided into zones of similar pressure, temperature, and composition.
- Aurora - Text by M. Alan Kazlev
A radiant emission caused by the bombardment of the atmosphere with charged particles that are being guided along the planet's magnetic lines of force. Also known as northern or southern lights, or simply lights.
- AzuriJovian Type - Text by John M. Dollan
Warm clarified blue gas giants with temperatures between 350K and 800K
- Badlands - Text by M. Alan Kazlev
Generic term for any barren, severely eroded places on terraformed or quasi-terraformed, garden or quasi-garden terrestrial planets, where the land is useless for habitation, farming or other human purposes. Most badlands are terrible desolate windswept places, but in some cases soft rock layers are sculpted into beautiful forms, and exposed rock layers may contain fossils and other interesting finds.
- BathyPelagic Subtype - Text by John Dollan
Gaian Type world in which there are deep oceans covering 85-100% of the surface.
- Belt, Asteroid - Text by M. Alan Kazlev
Generally, a band of asteroids which encircles a star, often where a planet would normally have formed. It may form the main "world" in a stellar system, as in the Barnard Belt, or it may simply be a region between planets where no planet coalesced, as in the Sol Belt.
- Belt-Zone Circulation - Text by M. Alan Kazlev
The atmospheric circulation typical of Jovian planets. Dark low pressure belts and light high pressure zone circle the planet parallel to the equator.
- Belts - Text by M. Alan Kazlev
Dark cloud bands in the atmosphere of Jovian (especially SubJovian and Eujovian) planets. These are low pressure regions of descending gas, representing deep cloud banks, and are located hundreds of kilometers beneath the clear hydrogen upper atmosphere. c.f. zone.
- Biome - Text by Stephen Inniss and M. Alan Kazlev
A major ecosystem that extends over a broad region, has a characteristic climate, and presents a typical common appearance due to the suite of organisms that are adapted to it. In the context of a garden world or a large megahab it is a subset of the overall ecosphere. The classical biomes known to the humans of Old Earth have been widely replicated through the Terragen Sphere, but many more exotic biomes have also been discovered or resurrected or invented.
- Biosphere, Planetary - Text by M. Alan Kazlev
The organic component of a Garden World; includes all living organisms (whether natural or bionanites or both) and all organic matter that has not yet decomposed.
- Biospherics - Text by M. Alan Kazlev
The study, design, construction, and maintenance, of artificial biospheres.
- Bolide - Text by M. Alan Kazlev
A meteor, asteroid, or comet that hits the surface of a planet, usually with catastrophic effect.
- CADRS Planetary Classification System, The - Text by Radtech497
Comprehensive planetary classification system.
- Campian Subtype - Text by John M. Dollan
Gaian worlds with less than 50% ocean cover
- Carbon Worlds - Text by John M. Dollan and Steve Bowers
Carbon-rich Terrestrial worlds
- Carbonaceous type asteroid - Text by M. Alan Kazlev and John M. Dollan
Stony carbon-rich and volatile-rich meteorite or asteroid, some of the earliest-formed matter in a solar system, and a popular resource for asteroid miners.
- Centaurian Type Asteroid - Text by John M. Dollan
Object found in the outer part of a planetary system, generally between 5 and 30 AU
- Cerean Type Rock dwarf - Text by John M. Dollan's Planet Classification List
Small rocky planetoid; represents one of the most common types of body in many solar systems.
- Chaos (planetology) - Text by M. Alan Kazlev
A distinctive area of broken terrain on the surface of a moon or planet.
- Chlorine Worlds - Text by Stephen Inniss
Chlorine worlds are like typical Eugaian worlds, with one important difference: the planet has an unusually high proportion of chlorine, and the process of photosynthesis releases free chlorine in significant quantities.
- Chondraceous Type - Text by John M. Dollan in his Planet Classification List
Asteroid of carbonaceous chondrite in composition. Although these are typically found in the outer regions of the inner solar system, they may also occur as evolved cometary bodies that have long since lost their volatiles.
- Chthonian Class Worlds - Text by John M. Dollan
Gas giant worlds, formerly HyperthermalJovians, which have lost their volatiles through evaporation
- Climatology - Text by M. Alan Kazlev
The study of long-term weather patterns and climate (including temperature and precipitation), especially on Gaian Type planets.
- Comet - Text by M. Alan Kazlev
A minor celestial icy body that orbits a star in a highly elliptical path. It is made up of a nucleus (solid, frozen ice, gas and dust), a gaseous coma (water vapour, CO2, and other gases) and a tail (dust and ionized gases). Its long tail of gas and dust always points away from the sun, because of the force of the solar wind. The tail can be up to 250 million kilometres long, and is most of the visible part of the comet. Comets are highly prized as a source of water and other useful habitat and terraforming materials.
- Continent (geography) - Text by Stephen Inniss
Originally, one of six or seven very large contiguous landmass on Old Earth, separated or largely separated from other continents by ocean. Some Gaian class worlds, some terraformed worlds, and some extremely large habitats such as Banks Orbitals may also have land masses that are regarded by their inhabitants as continents.
- Continent (geology) - Text by Stephen Inniss
On Old Earth, or on Gaian style worlds exhibiting similar patterns of plate tectonics, a large platform of metamorphic rock and largely granitic igneous rock, covered over much of its area by relatively thin layers of sedimentary rocks.
- Core (Planetary/Stellar) - Text by M. Alan Kazlev
Innermost layer of a celestial body (such as a planet, moon, or star). It is usually under great pressure and of very high temperature, many millions of degrees. In stars, this is the dense central region where nuclear reactions occur.
- Corona (planetology) - Text by M. Alan Kazlev
An ovoid-shaped feature on the surface of a moon or planet.
- Cryojovian Type worlds - Text by John M. Dollan and Steve Bowers
Cold, mostly cloudless, gas giants in the outer parts of a planetary system.
- Cytherean type worlds - Text by John M. Dollan
Hot greenhouse worlds
- Decaying Orbit - Text by M. Alan Kazlev
An orbit in which the orbiting object is slowly spiralling towards the primary. The radius of the orbit is decreasing over time. Usually this only occurs if there is an atmosphere, strong magnetic field or solar wind that slows the object. A satellite in a decaying orbit eventually crashes into the planet it is orbiting, or burns up after entering the atmosphere.
- Ecopoesis - Text by M. Alan Kazlev
The science and art of designing, shaping, sculpting, or modifying ecosystems.
- EoArean Subtype - Text by John M. Dollan in his Planet Classification List
Young Mars-like Type planet with substantial atmosphere and surface water.
- EoGaian Subtype - Text by John M. Dollan in his Planet Classification List
A young terrestrial world. Sometimes called Archean Subtype.
- EoVulcanian Subtype - Text by John M. Dollan in his Planet Classification List
Vulcanian Type planet. These are worlds that are highly active geologically due to their initial stages of formation. This is the stage in which the planetary crust first forms, and the planet becomes geologically differentiated.
- Epipelagic - Text by M. Alan Kazlev
On a Gaian or terraformed world, the upper part of an ocean, in which there is enough light to support photosynthetic phytoplankton or equivalent xenobiota or bionano.
- EpistellarJovian Type - Text by John M. Dollan in his Planet Classification List
Hot, dark gas giants with temperatures between 900 Kelvin and 1400 Kelvin.
- EuArean Subtype - Text by John M. Dollan in his Planet Classification List
Typical mature Mars-like world with minimal atmosphere and hydrosphere.
- EuCytherean Subtype - Text by John M. Dollan in his Planet Classification List
Major sub-type of Cytherean Type Planet. The greenhouse effect is quite pronounced, with temperatures sometimes exceeding 1,000 degrees Fahrenheit. Geological activity varies, depending on planetary mass. Life is absent. indeed, these worlds are perhaps the most inhospitable known, thanks to high temperatures and high surface pressures.
- EuEuropan Subtype - Text by John M. Dollan
Major sub-type of Europan Type planet. Distinguished by a rocky core with an ice mantle. A subsurface ocean 10 to 100 kilometers in depth is almost always present, due to tidal stretching of the world. Surface movement is constant, and obliterates all impact scars in a short amount of geologic time. Life may be present, but is likely to be primitive, if it is even beyond the microbial stage. Atmospheres are transient.
- EuGaian Subtype - Text by John M. Dollan
A Eugaian world, commonly known as a Garden World, with a rich biosphere.
- EuJovian Type - Text by John M. Dollan in his Planet Classification List
Cool gas giants, with clouds predominantly consisting of ammonia.
- EuPelagic Subtype - Text by John M. Dollan in his Planet Classification List
Major sub-type of Gaian Type planet. There is 100% water coverage of the surface, with extensive submerged continental shelves. If there is any land, it is in the form of small, isolated islands.
- Europan Type - Text by John M. Dollan in his Planet Classification List
Worlds with a rocky core and thick ice mantle and crust. There may be a layer of slush or even a deep ocean of liquid water under the thick ice crust.
- Exosphere - Text by M. Alan Kazlev
In planetology, the outermost layer of a planetary atmosphere; in galactography the edge of the Technosphere, the edge of Terragen space.
- Falculae - Text by M. Alan Kazlev
 bright patches on the surface of a star that are associated with sun spots
 or any region which is brighter than the surrounding area on a planet or a moon
- Ferrinian Type - Text by John M. Dollan in his Planetary Classification List
Worlds composed of more than 80% core, usually iron and other siderophilic elements.
- Fossil Fuel - Text by M. Alan Kazlev
Naturally-occurring, energy-rich carbon-based substance, such as shale, petroleum, coal, or natural gas, in a Gaian Type world's crust that was formed from ancient organic material. During the Industrial, Atomic, and early Information ages on Old Earth fossil fuels were burned in a criminally negligent manner, resulting in drastic climate change and ecological crisis that was only repaired during the late Interplanetary Age.
- Gaian Type - Text by John M. Dollan
Any Earth-like terrestrial world, of which there are many diverse forms depending on water content, composition and temperature.
- GaianTundral Subtype - Text by John M. Dollan
Cold Gaian worlds with periodic, or persistent, ice ages.
- Garden World - Text by Steve Bowers and Stephen Inniss
A planet similar to Earth and hospitable to Terragen life. More broadly, any planet that supports a complex biosphere and macroscopic life forms. May be either a natural Gaian Type planet or a world that has been terraformed, although the term is more often applied to the former.
- Gas Giant - Text by M. Alan Kazlev
See Jovian Class.
- Geography - Text by M. Alan Kazlev
The study of how natural geological forces, and introduced nanecologies, individuals and societies, and megascale projects shape the surface of a planet.
- Geological Time, Geological Time Scale - Text by M. Alan Kazlev
The history of a Terrestrial Class planet in terms of its formation and major stages of development. It is usually measured in many millions of years. The divisions used (from the largest (longest time) to the smallest (shortest time period) are: eon, era, period, epoch, and age.
- Geophysics - Text by M. Alan Kazlev
The study of the structure and dynamic behaviour of terrestrial moons and planets. Includes elements of geology, applied physics, and planetology; study of the geosphere mapping the internal planetary structure; the properties of rock within a planet; planetary formation, development, tectonics and seismology; the nature, history and evolution of the planetary magnetic field if any; and options for terraforming and/or megascale engineering.
- Geosphere - Text by M. Alan Kazlev
In a Terrestrial Class planet, the rocky mass of the world, everything apart from atmosphere and ocean. In the case of Gaian Type terrestrial planets includes continental and oceanic crust as well as the various layers of the planet's interior. The interior of the Terrestrial Class in general and the Gaian Types in particular is layered both chemically and mechanically. While some terrestrial class worlds have a static geosphere, in almost all Gaian Types worlds and some other terrestrial types the geosphere is active, with the crust is in a constant state of motion that gives rise to movement of the continents. The study of the geosphere is known as geology.
- Geyser - Text by M. Alan Kazlev
Jet of hot liquid or gas that shoots from a crack in the crust of a Terrestrial Class planet or moon.
- Greenhouse Effect - Text by M. Alan Kazlev
Increase in the global temperature of a planet as heat energy from sunlight is trapped in the atmosphere. Methane, carbon dioxide and water vapor all are greenhouse gases. A runaway greenhouse effect results in a Venus Type planet.
- Hadean Type Planetoid - Text by John M. Dollan
Planetoidal Class Metal Dwarf. Found mostly around hot stars.
- Halo Orbit - Text by M. Alan Kazlev
Circular orbits around the unstable Lagrange points (L1, L2, and L3), where the plane of the orbit is perpendicular to the axis of two large objects. They are as stable as orbits around a true celestial body.
- Hephaestian type worlds - Text by Mark Ryherd
Hot planets with molten rocky surfaces
- Hermian type worlds - Text by John M. Dollan
Dense, inner system worlds
- Hot Neptunes - Text by Steve Bowers
Planets between 10 x Earth's mass and 0.08 x Jupiter's mass which orbit close enough to have a temperature of 800 Kelvin or more
- HydroJovian Type - Text by John M. Dollan, with comments by Steve Bowers
Temperate gas giants with clouds predominantly consisting of water vapour
- Hydrosphere - Text by M. Alan Kazlev
The totality of water, whether liquid or ice, on a Terrestrial Class planet. In Gaian Type planets, a combination of the right mass, chemical composition, atmosphere, and the right distance from the Sun permits water to exist mainly as a liquid. The high heat capacity of this large volume of water (some 1 to 1.5 million cubic kilometres in a typical Eugaian world) buffers the planet surface from large temperature changes. In Europan Type worlds however, water exists mostly as ice (although sometimes also as slush or liquid).
- Hyperbarian class worlds - Text by Steve Bowers
Very dense planets with cores up to 100 x Earth's mass.
- HyperthermalJovian Type - Text by John M. Dollan
Very hot gas giants, with temperatures above 1400 Kelvin. Includes so-called 'Bluegrey worlds', 'Puffy worlds' and 'Comet worlds'.
- HyperXeric Subtype - Text by John M. Dollan in his Planet Classification List
Gaian Type planet in which water covers less than 10% of the surface.
- Ikarian type world - Text by Steve Bowers
Worlds in markedly eccentric orbits
- Intertidal Zone - Text by M. Alan Kazlev
Biome where the sea meets the land, during high tide it is submerged, in low tide exposed to the air. Home to many familiar and unique biological organisms. Many artificial biospheres simulate tides in order to generate intertidal zones.
- Ionian Type - Text by John M. Dollan in his Planet Classification List
Similar to the Vulcanian Type but of Europan constitution. These worlds are geologically active due to varying degrees of tidal flexing. Often these worlds are so active, that their surface morphology changes within a century.
- Ionosphere - Text by M. Alan Kazlev
The highest but one layer of a Gaian Type planet's atmosphere, part of the thermosphere, and located between the mesosphere and the exosphere. It typically starts at about 60-100 km and continues for several hundred kilometers. It contains ions and free electrons. Auroras may occur in the ionosphere.
- Janusian type worlds - Text by Steve Bowers
Named after the moon of Saturn, these are pairs of worlds with 1:1 resonant orbits which regularly exchange momentum.
- Jovian Class - Text by M. Alan Kazlev
Also known as Gas Giants.
- Lacus - Text by M. Alan Kazlev
On a Selenian Type world, a small plain.
- Lacustric Subtype - Text by John M. Dollan
Humid Gaian worlds with low topography and 50-80% ocean coverage
- Landing Site Name - Text by M. Alan Kazlev
Any ancient historical landing site on a colonized world. Originally referred to features at or near Historical Apollo landing sites on Luna om Solsys, most have long since been built over. Abbreviation: LF.
- Large Ringed Feature - Text by M. Alan Kazlev
On a Terrestrial Class world, a cryptic ringed feature. Abbreviation: LG
- Lenticula - Text by M. Alan Kazlev
On a Europan Type world, surface features in the form of small (several kilometers) dark spots (pl. lenticulae).
- LithicGelidian Type - Text by John M. Dollan in his Planet Classification List
Worlds with a rocky core and an extensive icy mantle.
- Littoral Zone - Text by M. Alan Kazlev
In a natural or artificial biosphere consisting of both large bodies of water, ample dry land, and natural and/or artificial tides, this is the intertidal zone is where the sea meets the land.
- Long-period Comet - Text by M. Alan Kazlev
A comet moving on a nearly parabolic orbit and thus having an orbital period of hundreds of thousands of years. Some haloist and spacer clades build their habitats on such comets.
- Macula - Text by M. Alan Kazlev
On the surface of a planet or star, a dark spot. May be irregular in shape. (pl. maculae; abbreviation MA)
- Magma - Text by M. Alan Kazlev
Molten rock within the mantle or crust of a terrestrial planet from which igneous rock forms.
- Magnetic Storm - Text by M. Alan Kazlev
A temporary perturbation (disruption) of a planet's magnetic field, caused by solar flares. Since they disturb the ionosphere, magnetic storms can disrupt local EM radio transmissions. Magnetic storms usually last for a few days. During a magnetic storm, auroras increase over polar regions. Corona is known for its spectacular aurora displays.
- Magnetopause - Text by M. Alan Kazlev
The boundary between the area in which a planet's magnetic field dominates and the magnetic field of the rest of the Solar System dominates.
- Magnetosphere - Text by M. Alan Kazlev
A planetary magnetic field. Not all planets have a magnetosphere. In terrestrial-type planets it is cause by the molten iron-nickel core and aligned with the north and south poles, and helps to shield against cosmic radiation. The magnetosphere extends some distance into space (depending on the strength of the local planetary magnetism) and is influenced by the solar wind. It is teardrop shaped - being longer in the side facing away from the Sun (the magnetotail.)
- Mantle - Text by M. Alan Kazlev
The layer of a large terrestrial planet located between the crust and the molten core.
- Mare - Text by M. Alan Kazlev
On a Terrestrial Class world, a "sea" or large circular plain (usually of darker basalt) (pl. maria; abbreviation ME).
- Mass Extinction - Text by M. Alan Kazlev
A process in which huge numbers of species on a garden world die out suddenly.
- MesoGaian Subtype - Text by John M. Dollan
Earth-like worlds with primitive biospheres.
- MesoJovian Type - Text by John M. Dollan and Steve Bowers
Major Type of Jovian Class of planet. Masses from 0.21 to 8.0 that of Jupiter
- Metallic type Asteroid - Text by John M Dollan
Metal-rich or 'sideritic' asteroids.
- Meteorology - Text by M. Alan Kazlev and Stephen Inniss
The study of the weather, either on planets or on/in megascale habitats.
- MicroJovian Type - Text by John M. Dollan in his Planet Classification List
Small hydrogen/helium worlds from 10 Earth masses to 0.08 Jupiter masses
- Milankovitch Cycles - Text by M. Alan Kazlev
Fluctuations in temperature on Garden Worlds, especially ice ages, caused by slight variations in the amount of sunlight reaching the planet caused by the eccentricity of its orbit around its primary. The orbital eccentricity changes the planet's average distance from the sun and therefore slightly changes the amount of sunlight reaching the Earth. Eccentricity cycles last over 100,000 years. The theory was first proposed by the Old Earth human Milutin Milankovitch in 31 AT (1938 c.e.).
- Non-Luminary World Classification Scheme - Text by John M. Dollan, with additions by Steve Bowers
The Non-Luminary World Classification Scheme, or NoLWoCS, is a standard classification method used to identify planetary bodies, minor worlds, and artificial structures.
- Ocean - Text by Stephen Inniss
Narrowly defined, a large body of saline water on a Gaian Type planet. More generally, extensive bodies of water such as those found beneath the ice of moons or dwarf planets, or liquid layers in the atmospheres of large gas or ice giants, or extensive bodies of other liquid such as the methane-ethane mix on the worlds inhabited by the Muuh or the ammonia-water mix known to the Soft Ones.
- Oortean Type Asteroid - Text by John M. Dollan Planet Classification List
Asteroidal Class. Cometary bodies which can be found throughout the solar system. They may also be constituent members of outer planet ring systems, or they may be outer planet moons.
- Orbital Inclination - Text by M. Alan Kazlev
The angle between the plane of an orbit and the plane of the ecliptic.
- Paludial Subtype - Text by John M. Dollan
Humid Gaian worlds with less than 50% ocean cover.
- Panthalassic Type - Text by John M. Dollan and Steve Bowers
Giant superterrestrial waterworlds.
- Parhelia - Text by Steve Bowers
Parhelia, or sundogs, are caused when atmospheric crystals become aligned in the atmosphere, typically in still conditions, and a localized bright spot occurs, often rainbow coloured. On earth-like worlds bright sundogs occur at 22° radius from the local sun, with additional parhelial arcs, circumzenithal arcs and pillars in good observing conditions.
- PelaCytherean Subtype - Text by John M. Dollan and Steve Bowers
Hot ocean planets similar in size to Earth.
- PelaGelidian Subtype - Text by John M. Dollan
Icy Ocean worlds.
- Pelagic Subtype - Text by John M. Dollan in his Planet Classification List
Gaian Type planet. Water covers 85 to 99% of the surface.
- Perihelion - Text by M. Alan Kazlev
The perihelion is a planet or comet's closest approach to the star it is orbiting.
- Permafrost - Text by M. Alan Kazlev
Semi-permanent underground ice (usually on a Gaian Type planet).
- Planet - Text by M. Alan Kazlev
A solid (or partially liquid) body usually found orbiting a star. Planets are too small to generate energy by nuclear reactions. One of six categories of matter that make up solar systems, nebula, and galaxies, and can be harvested for interplanetary or interstellar development (the other categories are interstellar gas, dust, planetisimals, brown dwarfs, and stars).
- Planetoid Class - Text by John M. Dollan in his Planet Classification List
Generally, rocky (more rarely metallic or icy) bodies, either irregular or regular in shape. Mostly large asteroids, some small moons, about 51 to 1,000 kilometers along the longest axis.
- Planetologist - Text by M. Alan Kazlev
One who studies a planet, whether a naturalist or as part of a surveying or scientific research team.
- Planetology - Text by M. Alan Kazlev
The study of a planet, moon or asteroid as a whole, including all its component elements, planetary formation, evolution, and taxonomy.
- Plate Tectonics - Text by M. Alan Kazlev
The constant destruction and renewal of the surface of a tectonically active Terrestrial Class, usually Gaian Type planet, caused by the motions of the planet's lithosphere. The crust is fractured into large plates, which slowly move (usually about a centimetre a year) relative to each other.
- Plume - Text by M. Alan Kazlev
On a terrestrial class planet or moon, a plume, an upwelling of hot rock from the interior. Abbreviation: PU
- Poseidonal Subtype - Text by John M. Dollan in his Planet Classification List
Europan Type planet or moon. If the world is massive enough and experiences severe enough tidal stressing, the icy surface and mantle may be melted to a depth of hundreds of kilometers, and the world will be enveloped in a thick atmosphere. Surface ice will be present, but spotty, giant icebergs adrift on a global ocean. Life is almost always present, but rarely very advanced.
- PostGaian Subtype - Text by John M. Dollan
Old Gaian Worlds that are losing their biosphere and hydrosphere
- Proterozoic Eon - Text by M. Alan Kazlev
Representing the "middle period" of the life-history of a Gaian Type world, with life mostly still at the microbial level. During this stage of planetary evolution, the atmosphere changes from reducing to oxygenated, the modern regime of continental drift begins, warm conditions replaced by "Snowball Earth", following that the short-lived Edicarian biota and the appearance of first metazoa (multicelluar animals). While the Earth was passing through it's Proterozoic stage, elsewhere in the galaxy, the Archivists, Mruta, Jacks, Halogenics, and very probably other significant xenosophont empires all arose and disappeared.
- Protoplanet - Text by M. Alan Kazlev and Steve Bowers
Massive object resulting in the colescence of planetesimals in the solar nebula which eventually becomes a planet. Has a thick atmosphere and greater mass than a formed planet. Because of the potential for megascale engineering, protoplanets are often highly sought after by interstellar development corporations and certain clades and hyperturings.
- Pyrohydrothalassic - Text by Steve Bowers
Hot Water Worlds.
- Pyrothalassic Type - Text by John M. Dollan
Hot, rocky superterrestrials
- Ray - Text by M. Alan Kazlev
Ejecta from meteoric impacts forming streamers radiating from some craters; A bright streak of material ejected from a crater on a Selenian Type or similar planet.
- Relative Cosmic Abundance of Elements - Text by Stephen Inniss
The universe consists almost entirely of hydrogen and helium, but of the impurities created by the processes in stars made intelligent life possible, and their relative natural abundance continues to shape the evolution of biology, technology, and society.
- Reticulum - Text by M. Alan Kazlev
On a Venusian Type planet, reticular or netlike pattern (plural : reticula).
- Rift Valley - Text by M. Alan Kazlev
Long straight deep valley produced by the separation of crustal plates. Rift valleys only occur where there are active or incipient plate tectonic stresses, most usually on the surface of Gaian Type planets. However rift-like features can also be seen in the ice of Europan Type worlds.
- Rille - Text by M. Alan Kazlev
Type of valley that occurs on a Selenian Type planet or moon.
- Rima - Text by M. Alan Kazlev
On a Selenian Type planet or moon, a fissure (plural : rimae).
- Seismic Waves - Text by M. Alan Kazlev
Waves passing through the interior or surface layers of a planet due to a seismic disturbance, such as an earthquake or large meteorite impact.
- Seismology - Text by M. Alan Kazlev
Study of vibrational waves passing through planets, revealing internal structure.
- Selenian Type Worlds - Text by John M. Dollan
Worlds with little or no metallic core.
- Sensitivity - Text by M. Alan Kazlev
The tendency of a (usually chaotic) system to change dramatically with only small perturbations in its environment.
- Sideritic Type - Text by John M. Dollan in his Planet Classification List
Asteroidal Class. Classically called M-class, these are metallic bodies that were once the cores of differentiated planetesimals, or fragments of larger metallic bodies.
- SideroSilicaceous Type - Text by John M. Dollan in his Planet Classification List
Asteroidal Class. Stony-iron in composition, these were originally fragments of partially differentiated planetesimals.
- Silicaceous Type Asteroid - Text by John M. Dollan and M. Alan Kazlev
Stony asteroids typically found in the inner region of solar systems.
- Skolian Type Worlds - Text by Steve Bowers
Worlds with a large axial tilt
- Solstice - Text by M. Alan Kazlev
The date when - as seen from the surface of a planet - the sun reaches maximum distance from the celestial equator. In single star solar systems there are two solstices annually, the winter solstice in the northern planetary hemisphere being the summer solstice in the southern, and vice-versa. Binary star solar systems have four, and so on. An important festival date on many Caretakerist worlds.
- Stevensonian Class Worlds - Text by John M. Dollan and Steve Bowers
Stevensonian worlds, also known as Planetary Mass Objects (PLANEMOS), are former planets that have left their solar system of origin and reside in interstellar space. They are more numerous than planets, but are difficult to find. These worlds are prized by Hiders and Backgrounders.
- SubJovian Type - Text by John M. Dollan in his Planet Classification List
Small Jovian worlds with masses up to 0.2 that of Jupiter. Generally, a gas giant with an icy mantle (e.g. Uranus, Neptune).
- SuperJovian Type - Text by John M. Dollan in his Planet Classification List
Large Jovian worlds with masses from 8.1 to 13.0 that of Jupiter, the theoretical upper limit of planets.
- Superterrestrial Worlds - Text by Steve Bowers
Giant terrestrial worlds: 2.5 to 10 Earth Masses.
- Terrestrial Class - Text by John M. Dollan in his Planet Classification List
Size range 0.05 to 2.5 Earth masses. These are the commonly called the 'rocky planets' but this is a deceptive name as not all of these worlds are rocky. Their composition can be quite varied, as can their surface features and conditions. In general they can be defined as small dense worlds consisting mainly of elements heavier than hydrogen and helium, with a liquid or solid surface.
- Thalassic Subtype - Text by John M. Dollan in his Planet Classification List
Gaian Type world in which water covers 25 to 50% of the surface. However, unlike the Paludial Subtype, varied surface relief leads to varied climates. Life is typically quite prolific.
- Tide - Text by M. Alan Kazlev
A bulge raised in a body by the gravitational force of a nearby body.
- Titanian type world - Text by John M. Dollan
Icy worlds with thick atmospheres.
- To'ul'hian Worlds - Text by Stephen Inniss, based on the original concept by Anders Sandberg
To'ul'hian worlds are an unusual variety of world named after the most famous of them, the homeworld of the To'ul'h. They might be regarded as a cool and wet version of the Cytherean type, or as very hot high-pressure Gaian worlds.
- TundralPelagic Subtype - Text by John M. Dollan in his Planet Classification List
Largely arctic Gaian Type worlds. However, small and scattered continents will inspire great amounts of precipitation, leading to large polar caps and land glaciation. Only the equator of most worlds have a warm year around climate.
- TundralXeric Subtype - Text by John M. Dollan in his Planet Classification List
Gaian Type world with a cold climate, with an average temperature of around -50 °C. Oceanic ice, even near the equator, can be up to 1 kilometer thick or greater. With almost no precipitation the continents are barren dry wastelands. On land life is restricted to microbial forms, while in the oceans it is typically found only around hot springs and thermal vents.
- Van Allen Belts - Text by M. Alan Kazlev
Two doughnut-shaped belts of ionized gas (plasma) that circle a Gaian Type planet. The belts are are caused by the interaction between the planet's magnetic field and the solar wind. The particles (plasma) is trapped by the planet's magnetosphere. When stray particles from these belts collide with air, they emit light, causing an aurora.
- Verdurian Subtype - Text by John M. Dollan in his Planet Classification List
Gaian Type worlds in a perpetual greenhouse state. Excessive greenhouse gases over a long period of time have created a world dominated by warm temperatures, prolific precipitation, and large areas of thick vegetation. Carbonic acid rains produce some fantastically eroded land forms. The world is almost always cloud covered, and the average surface temperature is around 50 degrees Celsius.
- Vesperian type worlds - Text by John M. Dollan and Steve Bowers
Tidally locked terrestrial worlds
- Vestian Type Planetoid - Text by M. Alan Kazlev
Planetoids with evidence of geological activity
- Vitriolic Type Worlds - Text by Matthew C. Johnson
Extreme and unusual worlds, characterized by seas or oceans of nearly pure sulfuric acid. Many are life-bearing, apparently due to the actions of an ancient xenosophont race.
- Vulcanian Type Asteroid - Text by John M. Dollan
Asteroids orbiting very near to the local star
- Xeric Subtype - Text by John M. Dollan in his Planet Classification List
Gaian Type world in which water covers 10 to 25% of the surface.
- Ymirian type worlds - Text by John M. Dollan
Worlds made almost entirely of ices