Mneme World Generator/Definitions

Definitions

With Regards to Definitions

Earth Centric Definitions

A peculiarity of an Earth-centric world view is that it doesn’t account for the state of material Elements and Compounds outside of Earth as not having the same “default” or “natural” state as they would on Earth. When we use the term “Ice” here, it doesn’t mean water (H2O), but frozen solid volatiles (elements and compounds that readily vaporize). Ocean refers to any large, continuous body of liquid that dominates a planet, regardless of chemical composition. The terms “gas”, “Terrestrial Planet”, and others might not refer to the same things they would in casual conversation here on planet Earth. Our word use here will be close to scientific Astronomical terminology, but with a fair number of notable differences.

Mneme Definitions

Mneme is the name given to these Variant Rules for the Cepheus Engine. The terms and definitions used here are only for Mneme and may not align with terms for other variant rules.

Proper Names of Objects

Use of the terms here in this book were done in a way meant to differentiate between what a type of thing is called and the actual proper name of the thing. For example: a Star or Sun is a type of thing, which may have its own name, such as Sol, Alpha Centauri, Bernard’s Star, Proxima Centauri, etc.

Sol Solar System Based Measurements

In Mneme, Gas giants are compared to Jupiter, Planets to Earth, Dwarf Planets to Earth’s moon, Ice Giants to Neptune, and Asteroids to Ceres. This form of comparison gives players a useful set of thresholds and benchmarks with which to gauge celestial object sizes. This Mneme convention is not the one used by the scientific community.

Definitions

Star

A star is a luminous spheroid of plasma held together by its own gravity. The nearest Star to planet Earth is Sol, which is also called the Sun. In the Mneme system, stars are measured relative to Sol using Sol/Solar mass and Sol/Solar Luminosity as units of measurement.

Companion Star

A companion star is a star in a solar system that orbits the system’s central star. About a third of stars at the center of solar systems have a companion star. A few of these have 2 or more.

Giant Stars

Giant Stars are stars 10 times brighter than Sol. They are much larger and rarer than main-sequence stars and are not covered by the current Mneme world generation rules because of how rare they are. Use of these in a Traveller game and details regarding what sort of objects orbit them are up to the referee.

Sol

In Cepheus Engine and Traveller, the name of the solar system that the planet Earth belongs to is the “Sol System”. Sol has a mass of 1.989 x 1030. Other stars are measured against Sol in mass (SM) and luminosity (SL). Other objects may be measured against this size.

Solar Luminosity

Solar Luminosity is how bright and hot a star is. The greater the Solar or Stellar Luminosity, the farther out its Infernal, Hot, Habitable Zone and the Frostline extend to. During Stellar Generation, the Stellar Class and Mass Table is used to determine the Mass and Luminosity of a Star. A sun’s Luminosity has an effect on the distances of various zones within its solar system; as indicated in the Habitability Table beneath the Habitable Zone entry under Solar System. In Mneme a sun’s Solar Luminosity = Solar Mass * Stellar Class Modifier. L☉ = SCM * SM

Stellar Classification

Mneme primarily uses the Harvard spectral classification system of stars, based on strength of spectral lines. The types are assigned letters: OBAFGKM - which uses the mnemonic “Oh Be A Fine Girl/Guy, Kiss Me.” Type O and B Stars are the hottest and brightest, while K and M are the dimmest. Stars progress from Hot and Bright to Dim before dying (OB to KM). The Mneme world building system focuses on AFGKM, and Luminosity Class V Stars. While there is another commonly used classification system based on spectral lines sensitive to stellar temperature and surface gravity; called the Yerkes spectral classification system, Morgan–Keenan system, or MK system; this will only be touched on and will not be a big part of the star generation system of this book.

Stellar Classification Table

Class	Color	Mass	Luminosity
O	blue	≥ 16	≥ 6.6
B	deep blue white	2.1 - 16	1.8 - 6.6
A	blue white	1.4 - 2.1	1.4 - 1.8
F	white	1.04 - 1.4	1.15 - 1.4
G	yellowish white	0.8 - 1.04	0.96 - 1.15
K	pale yellow orange	0.45 - 0.8	0.7 - 0.96
M	light orange red	0.08 - 0.45	≤ 0.7

Measurements are based on Solar Mass and Luminosity

Sun

This term refers to the Primary Star of a Solar System and not necessarily to our Earth’s Sun.

Solar System

A solar system is the system of celestial bodies that form around a star. A system may have more than one star (see Companion Star). In this case the Primary Star is the one with the greatest mass. Sol is the name of both the sun and the solar system humans from earth reside in. Most solar systems have a Planetary System, while others have only the trace remnants of a planetary system.

Binary Star System

A solar system with 2 stars is considered a Binary Star System. The star in that system with the most mass would be its primary star. The other star would be its Companion star.

Circumstellar disc

These are discs of gas, dust, and planetoids orbiting a star. They are part of its Planetary System. A young Solar System would form Circumstellar Disks before it would have a chance to form planets. The Asteroid Belt and Kuiper Belt in Sol are circumstellar disks.

Frost Line

This is the distance from a sun beyond which water will mostly freeze and remain frozen in worlds there that do not have a greenhouse type atmosphere. It is the boundary that separates “Inner Solar System” from “Outer Solar system”. It is often the outer limit of optimistic estimates of solar system Habitable Zones, beyond which, life is not expected to be able to develop naturally.

Hot Zone

The Hot zone is the area in a solar system extending from the outer limit of its sun’s Infernal Zone to a distance of the Square-Root of its Luminosity x 0.8 AU. It is sometimes called the Greenhouse Zone as it is the area where gases can still be found on terrestrial planets (as opposed to being stripped away by solar wind) but where the planets tend to suffer from runaway greenhouse conditions.

Infernal Zone

This is the zone within a solar system that is too close to the star for any gaseous compounds and elements to be held by a world’s gravity, due to these being stripped away by solar winds and radiation. The Infernal zone extends from the sun’s surface to a distance computed by the Square-Root of the star’s Luminosity x 0.4 AU.

Tidally Locked

Worlds that are in a system’s Infernal Zone have a high chance of being Tidally Locked (They are tidally locked on a roll greater than 4 with a 2D6).

Infernal Limit

This is the term for the outer border of the Infernal Zone. It is estimated to be a distance of the Square-Root of a star’s Luminosity x 0.4 AU from the star.

Inner Solar System

This is the inner part of a solar system, characterized by having worlds where volatile elements mostly do not remain frozen. It extends from the star itself to the solar system’s Frost Line.

Habitable Zone

This is the area around a star where conditions allow for the presence of life to develop naturally. There is no actual standard for Habitable Zone estimation and many argue that life may actually develop and exist outside of these areas.

Conservative Habitable Zone

This is the part of a solar system with temperatures and conditions much like those found near Earth’s orbit. It is estimated to be the zone in the area of a distance of around 0.8 AU to 1.2 AU from a star.

Optimistic Habitable Zone

This is the area between where atmospheres are stripped from planets by the sun and the area in which water remains mostly in a frozen state. It is estimated to be the zone in the area of a distance of around 0.4 AU to 4.85 AU from a star.

Habitability Table

Zones		Distances
Sun
Sun		0AU
Infernal Zone		0AU
Infernal Zone		L☉ 0.4AU	Infernal Limit
Hot Zone	Optimistic Habitable Zone		L☉ 0.4AU	Infernal Limit
Hot Zone	Optimistic Habitable Zone		L☉ 0.8AU
Conservative Habitable Zone	Optimistic Habitable Zone		L☉ 0.8AU
Conservative Habitable Zone	Optimistic Habitable Zone		L☉ 1.2AU
Cold Zone	Optimistic Habitable Zone		L☉ 1.2AU
Cold Zone	Optimistic Habitable Zone		L☉ 4.85AU	Frost Line
Outer Solar System		L☉ 4.85AU	Frost Line
Outer Solar System

Cold Zone

This is a Mneme Term. This is the area between a solar system’s Conservative Habitable Zone and a system’s Frost Line.

Outer Solar System

This is the area beyond a solar system’s Frost Line that is still close enough to the star that objects are pulled into its orbit. It is characterized by worlds in which volatiles mostly remain frozen when not within a greenhouse type atmosphere.

Planetary System

The term refers exclusively to the collection of non-stellar celestial objects within a solar system. This means that stars of a solar system are not part of its planetary system but circumstellar disks, the rings of planets, and planets or worlds are part of a planetary system. Space dust and creatures and objects; such as astronauts, spacecraft, artificial satellites, or debris are also typically not considered part of a planetary system.

Stellar System

The terms “Stellar System” and “Multi-star System”, will be the term used exclusively to describe solar systems with 3 or more stars, and no less.

Circumstellar Disks

These are disks of material that orbit the Sun or Multi-Star systems such as Accretion Disks and Proto-planetary disks. Examples of these in Sol are the Asteroid belt and the Kuiper belt. All planets came from these Circumstellar Disks, forming early on in the system’s development period. Disks remain after the time of planetary formation of a solar system in cases where the circle that particles orbit is too great and the particles there too far apart for these to clump together. While we call them Disks, they are shaped more like tori (plural of torus).

The Asteroid Belt has mass equivalent to roughly 3% that of the mass of Luna (Earth’s Moon). They occupy the orbital space from 2.1 AU to 3.25 AU from the Sun (1.15 AU area).

Kuiper Belt has 160% the mass of Luna and around 53 times the mass of the Asteroid Belt. It is located between 30 AU from the sun and 60 AU. Most of its significant objects are in the 38 AU and 48 AU (10 AU area).

Accretion Disks

These are disks of diffuse material orbiting a star or center of mass of the solar system. They are something like the opposite of the Protoplanetary disks. For Accretion Disks, the forces exerted by the nearby planets. are causing their materials to further fragment (instead of coalesce).

Protoplanetary Disks

The opposite of Accretion disks, the material orbiting in such disks are coalescing into a single whole due to their own gravity and the gravity of nearby objects.

Rings

Rings are just like Circumstellar Disks that orbit a World instead of a star. In other words, these orbiting particles are Discs when they Orbit a Sun and Rings orbit when they Orbit Worlds. Like Disks they are actually in the shape of a spread-out Torus.

Saturn's Rings have a mass of 1.54 x 1019 kg which is 2% the mass of Ceres. Saturn's rings can be viewed by a 25x telescope

Jupiter’s Rings are estimated to be about 1011 kg, or a thousand times smaller than Saturn’s rings.

Uranus’ Rings are estimated to be 1016 kg.

Neptune’s Rings are around 4.4 x 1016 kg.

World

The term “world” is used to describe an object that is part of a solar system’s planetary system (see above); without referring to a specific Size Class. While the term refers to what people would normally call a planet, the term “planet” in Mneme will normally refer to a specific size of world in the planet size list as follows:

Size Classes

Class	Minimum Size	Maximum Size
Super Jovians	> 1 JM (318 EM)	80 JM
Giant Planet	> 10 EM	318 EM (1 JM)
Super Earth	> 5 EM	10 EM
Planet	> 0.1 EM	5 EM
Lesser Earth	< 0.1 EM	0.1 EM
Dwarf-Planet	> 0.1 LM	1LM (0.1 EM)
Minor Planet	> 0.1 CM	1 CM (0.1 LM)
Planetoid	> 1 Gigaton	0.1 CM (100000000 Gt)
Asteroid	> 1 Kiloton	1 Gigaton (1000000 Kt)
Meteor	?	≤1 Kiloton

JM = Jupiter Mass, EM = Earth Mass, LM = Lunar Mass, CM = Ceres Mass, Gt = Gigaton, Kt = Kiloton

Super Jovian

These are Gas Giants more massive than Jupiter, reaching up to 80 Jupiter Masses. At around 60 Jupiter masses a Gas World usually heats up to become a Brown Dwarf. At 80 Jupiter masses this invariably occurs.

Jovian

These are planets comparable in size to Jupiter at around 300 Earth masses or 0.000954 Solar Masses. The term is typically used to specify worlds similar to Jupiter in other ways, due to our Sol Centric view.

Giant Planet

A Giant Planet, is a world that has more than 10 Earth masses. In Mneme, such worlds will be referred to as Super Jovian if they exceed 1 Jovian Mass. Often, the term Giant is used in combination with a word describing a type of world; such as Ice Giant, Gas Giant, or Terrestrial Giant.

Super Earths

Super Earths refer to worlds with greater than 5 times the mass of earth. In Mneme, such worlds will be referred to as Giant Planets if they exceed 10 Earth Masses.

Planet (Size)

While the term planet actually refers to any non-star in a solar system in a state of Hydrostatic Equilibrium (settled by gravity into a spheroid shape) that is also Geologically Active; In Mneme, Planet refers to a category of size of a world. This ranges from 0.1 earth masses (EM) to 5 earth masses (EM). Larger worlds are called Super Earths, Giant Planets, and Super Jovians; while smaller worlds are called Dwarf Planets, Minor Planets, Planetoids, Asteroids, and Meteors. Though the more general use of the word "planet” will be avoided in this book, characters in the Traveller setting will likely call most worlds beyond the size of a planetoid a planet.

Lesser Earth

These are Worlds smaller than 0.1 Earth Mass. The size of these ranges from Dwarf Worlds to Meteors.

Dwarf Planet

This refers to a world on the scale of the Earth’s moon, Luna. Dwarf Planets are measured in Lunar Masses, based off of Earth’s moon, Luna. In this classification system Mars is the smallest planet at 1/10 in Earth masses. Anything smaller is a Minor planet.

Minor Planet

These are Planets that have mass equal to, or less than, the mass of Luna; Earth’s moon; but that still have enough mass to have Hydrostatic Equilibrium and be roughly spheroid in shape. They may be too small to be geologically active. Ceres, the largest object in the Asteroid Belt, is in the upper range of measure of Minor planets. Hygiea, at 1/10 of Ceres’ mass, is in the lower range.

Planetoid

These are worlds of 1 million tons to 0.1 Ceres Mass in size. They do not have Hydrostatic Equilibrium, and are actually a larger class of asteroid, but will be referred to here as planetoids.

Asteroid

In Mneme, an Asteroid is an object in space that is part of a planetary system and that is larger than a meteor but not massive enough for gravity to shape it into a sphere. Asteroids range from a thousand to a million tons in mass. In Mneme, objects smaller than a thousand metric tons are considered meteors.

Meteors

In Mneme, meteors are objects in space smaller than a thousand tons in mass. Meteors smaller than 2 millimeters are considered micro meteors.

Atmosphere

A world’s atmosphere is the layer of gas surrounding it, kept there by the force of the world’s gravity. An atmosphere may be described as corrosive, polluted, or toxic; depending on what harmful substances are in them. A greenhouse atmosphere is one made up of or containing enough greenhouse gases to cause the planet to retain much more heat than it otherwise would for its distance from the star. How thick or thin an atmosphere is, will typically measured against the Earth’s atmospheric pressure.

Habitability

Habitability is an abstract numeric representation of how conducive the conditions of a world are to the survival of terrestrial life.

Gas World

Gas worlds are worlds composed of gas; such as hydrogen or helium; surrounding a solid core. A gas world with 0.1 Ceres Mass could potentially form if it were too far from other gravitational forces for these to strip it of its atmosphere. While such planets would begin to resemble more familiar gas worlds at 1 Ceres Mass, they would have to exist in the further reaches of the outer solar system. At 0.1 Earth mass or more, gas worlds would tend to be more stable and less prone to atmosphere loss. Jupiter and Saturn are gas worlds; more specifically, they are gas giants. They have solid cores, of 6.38 and 1.90 earth masses respectively. Around 90% of these worlds are gaseous matter. Gas worlds start out as terrestrial planets in the early stages of solar system formation before acquiring enough matter to become a gas world. Once they reach around 1 Earth Mass in size, they form an ocean. When they reach a few times the mass of Earth, this ocean will boil and the atmosphere will grow rapidly, faster than the solid part of the planet, eventually forming a gas giant world akin to Jupiter.

Gas World Classification

Gas Worlds vary in size from Dwarf-Planet size, to Planet, to Super-Earth, to Giant, and then to Super Jovian. They are classified according to the gas that comprises the bulk of their atmosphere:

Ammonia Cloud Gas World: Class I

The most common type of Gas World, these have atmospheres made mostly of hydrogen and helium; with Ammonia forming visible clouds when directly observed.

Temperature: 150 K (−120 °C; −190 °F) to Cooler

Position: Outer Region of the Solar System or Near a Cool star. Far past the frostline.

Examples: Jupiter and Saturn are Ammonia Cloud worlds. As of 2015, 47 Ursae Majoris c and d could be Class I planets. Upsilon Andromedae e and 55 Cancri d may also be Class I planets.

Water Clouds Gas World: Class II

Water Cloud Gas Worlds, with their atmospheres primarily made up of water vapor, can form closer to a star than Ammonia Cloud Gas Worlds, and also absorb more solar radiation and heat than other gas worlds of equivalent size.

Temperature: 250 K (−23 °C; −10 °F) to 700 K (800 °F, 430 °C).

Position: From the habitable zone outward.

Examples: HD 45364 b and HD 45364 c, HD 28185 b, Gliese 876 b, Upsilon Andromedae d, 55 Cancri f, 47 Ursae Majoris b, PH2b, Kepler-90 h, HD 10180 g.

Cloudless Gas World: Class III

These are Gas Worlds that cannot form global cloud cover due to the presence of substantial quantities of compounds in them that cause cloud condensation. Rayleigh scattering makes them seem like featureless spheres when viewed by a distant observer. Methane is the primary component of the atmosphere of such worlds

Temperature: 350 K (170 °F, 80 °C) and 800 K (980 °F, 530 °C)

Position: Innermost parts of a system, roughly corresponding to where Mercury would be.

Examples: Possible Class-III planets are HD 37124 b, HD 18742 b, HD 178911 Bb, 55 Cancri c, Upsilon Andromedae c, Kepler-89e, COROT-9b and HD 205739 b. Above 700 K (800 °F, 430 °C), sulfides and chlorides might provide cirrus-like clouds.[2]

Alkali Metal Gas Worlds: Class IV

In Alkali Metal Gas Worlds, carbon monoxide is the dominant carbon carrying molecule in the atmosphere. The abundance of Alkali metals, such as sodium and potassium will be detected there by instruments. These Gas worlds form cloud decks of silicates and iron deep within their atmospheres.

Temperature: 1300 K (1000 °C) to 900 K (630 °C/1160 °F)

Position: these worlds are found in Hot Zones.

Examples: 55 Cancri b, HD 209458 b at 1300 K (1000 °C), and TrES-2b.

Silicate Cloud Gas Worlds: ClassV

The hottest Gas Worlds, they form silicate and iron cloud decks high in their atmosphere. They reflect a lot of light, but glow red in standard sensors due to their thermal radiation.

Temperature: ≥ 1400 K (2100 °F, 1100 °C)

Position: These can be found in the Hot Zone.

Examples: Kepler-7b, HAT-P-7b, or Kepler-13b, HAT-P-11b  51 Pegasi b and Upsilon Andromedae b

Geologically Active

This term is used to describe a world in which there is sufficient gravity or heat for significant change to continue to occur in the planet’s crust, core, and mantle. This means things like mountain formation, volcanic flows, earthquakes, canyon-creation, plate motions, erosion, and so on.

Hydrostatic Equilibrium

This term is used to describe a world with enough mass that its gravity causes it to collapse into a spheroid. Planetoids reaching a mass of around 1 trillion tons and those with less rigid composition would tend to become spheres.

Terrestrial Planet

This is a world made up mostly of Silica or Rock, and of heavier elements, like iron. Terrestrial Planets tend to form near a system’s star and are primarily made up of the denser elements of the solar system.

Ice World

Ice worlds are worlds mainly composed of frozen solid compounds heavier than hydrogen and helium. Most of such worlds are made up of elements and compounds lighter than silica and iron, but heavier than hydrogen and helium. Nepute and Uranus are ice words. More specifically, they are Ice Giants, having 17 and 14 earth masses, respectively. They have a solid core of about 3.2 and 2.7 earth masses, respectively, with the rest of their mass (74%) made up of Ice water. Given the right conditions, it is possible for ice worlds to become ocean worlds.

Ocean Worlds

Ocean worlds are Ice Worlds close enough to a star, or subjected to sufficient gravitic tidal forces, to be warmed and to possess significant liquid compounds or elements. Liquid methane can exist at up to the Square Root of 1 Solar Luminance x 7 AU. Ice Worlds placed in warm enough positions (such as in the habitable zone) become Ocean Worlds. Placed in warmer positions, they may be unstable; losing atmosphere and ocean to the sun's heat.

Moon

A moon is a natural satellite that orbits a much larger celestial body. The term “Moon” describes a type of relationship to another celestial body. The Moon of Earth is named Luna, Moons of Jupiter are called Io, Europa, Ganymede, and Callisto. A moon can be as big as a Gas World or Super Earth orbiting a much larger Gas Giant; or can be as small as an Asteroid like that of Phobos and Deimos. The world a moon orbits is called a Primary.

Primary

The Primary is the celestial body that a Moon Orbits. The Earth is the Primary of the Luna (the Moon), Jupiter is the Primary of Io, Ganymede, Europa, and Callisto. Saturn is the Primary of Titan.

Starship

A spaceship with a jump drive, capable of interstellar travel on its own. CE SRD p.19

Non-Starship

A spaceship without a jump drive, and thus incapable of interstellar travel on its own. CE SRD p.18