Planet Characteristics of the Milky Way

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The Milky Way Galaxy is full of Stars, and around many of those stars are planets, that form solar systems. Here are a few aspect of planets that you may have not considered.

  • NOTE: Most of the habitable planets that have write-ups are shown in the Solar Systems section.

Explanation of Planet Types

Planets are a varied bunch. They come in all kinds of sizes and chemical makeup, from dusty balls of dry rock, through aqueous worlds brimming with life, to massive spheres of gas heavy enough to outweigh all other planets in their host systems. Planets can be divided into two broad classes, terrestrial (or rocky) and gaseous (or jovian).

Gaseous Planets are generally referred to as “gas giants”. They generally have no solid surface, and are dominated by hydrogen, helium, ammonia, methane and water. The atmosphere becomes steadily denser the deeper one goes until it finally segues seamlessly into a liquid layer, with no sharp boundary. Deep at the very heart of the planet, beneath many thousands of kilometers depth of liquid gas, is a massive rock/metal core.

Gaseous planets come in several kinds, though not as many as the rocky terrestrial planets. Gaseous planets are generally grouped by mass and temperature, since their makeup is relatively similar and only the effects of temperature make a difference to the overall chemistry. The nomenclature works by using the prefix type and then the mass range. For instance, Jupiter is a Eugaseous, while Uranus is a Cryosubgaseous, and the extrasolar planet Osiris is an Osirigaseous.

Subgaseous Planets

Mass Range: 0.03 – 0.1MjPlanetSubGaseous.jpg

Subgaseous planets have a smaller core, though it makes up a greater proportion of the overall mass. Hydrogen and helium make up far less of the atmosphere. In the Frigid and Superfrigid zones, the atmosphere tends to be dominated by methane and other hydrocarbons. This gives these planets a greenish or bluish tinge. In the temperate zone, subgaseous planets evolve into aquarians. In the torrid and supertorrid zones, water vapour and sulphur compounds become a major constituent, and the planet turns first white, then brownish.

Gaseous Planets

Mass range: 0.3 – 3MjPlanetGaseous.jpg

Gaseous planets range from the mass of Saturn to several times that of Jupiter. They have rock/ice cores of several Earth-masses, hidden beneath a massive layer of liquid-metallic hydrogen. These planets are characterized by extremely powerful magnetic fields, creating lethal radiation belts.

Transgaseous Planets

Mass range: 10MjPlanetTransGaseous.jpg

Transgaseous planets are dominated by hydrogen, with traces of other materials. The difference between a transgaseous and a brown dwarf is subtle and not always easy to see. Essentially, brown dwarfs are either fusing deuterium or have done so in the past, whereas transgaseous never had enough mass to do so. Only the osiritype and the eutype are present amongst transgaseous; Eutransgaseous are generally referred to simply as transgaseous.

Explanations of prefixes used to describe planets

I don’t bother generally specifying the classification of each planet in the write-ups, but if someone were to ‘science the hell’ out of a planet, here’s some of the information they could get.

Prefix Definition
Osiri This type are generally dark in color, ranging from brownish to black. They are extremely close to their stars, and are surrounded by an extended envelope, or thermosphere, of ionised gas torn from them by the star. Subgaseous are generally reddish-brown. The osiritype is named after Osiris, the ancient Egyptian god.
Calo The calotype are not as hot as the osiritype, with fewer metal ions in the atmosphere and little if anything in the way of particulate clouds. Generally a deep azure in color, cloud banding is often not obvious, even where the planet is not tidally locked to its star.
Eu The eutype are characterized by banded cloud layers of icy crystals, usually of water and ammonia ice as well as hydrocarbon droplets. In both the Gaseous and Subgaseous classes, weather is violent and spectacular in form. Gaseous planets range from whitish, through yellow-ivory to reddish-brown or orange, with high bands of white cloud creating an agate-like appearance. Subgaseous examples retain the white zones of cloud, but the underlying deck is usually greenish or bluish from the rich concentration of atmospheric methane.
Cryo The cryotype are those planets scattered into the deep ranges of the system, beyond the normal planetary zones. The deep chill creates thick atmospheres with little structure, since there is little energy to generate weather. Cirrus-like clouds may form for weeks or months at a time, but the impressive banding is either absent or extremely subtle. Terrestrial planets are by far the most common. They range from objects barely the size of the Moon or Europa to bodies that border on the mass of a subjovian/subgaseous. Mass, chemistry and location play much greater role in the evolution of terrestrial planets, and they display greater variety than the gas giants.

Selenian Planets

Mass range: 0.01 to 0.1MePlanetSelenian.jpg

A selenian planet is composed of rock and metal, sometimes with organic materials but too small to hang onto any appreciable atmosphere. Mercury, Mars and the Moon look like classic selenians, but the Moon is an unusual example. Most selenians have more metal in their cores. Geological activity is pretty much nonexistent, and often restricted to the odd fault-induced tremor and the last gasps of near-extinct volcanism. Landforms are ancient and often dominated by impact scars and volcanoes, sometimes large shield volcanoes powered by internal hotspots. Some of the largest selenians have the beginnings of large-scale tectonic features powered by internal forces, but the active processes rarely last beyond the system’s Young phase.

Glacian Planets

Mass range: 0.01 to 0.3MePlanetGlacian.jpg

A glacian planet is similar to a selenian or terran, but forms in the star’s cold zone. Such a planet attracts a lot of water, which forms an icy overmantle (which is a rad band name OVERMANTLE!!!) over the rocky crust, anything from hundreds to over a thousand kilometers thick. The surface of such planets are often very ancient, though tidally-heated examples can produce enough volcanism to renew the surface on a regular basis. In the Solar system, all of the glacian planets are represented by Jupiter’s Galilean moons and Saturn’s moon Titan. Atmospheres on glacian planets are generally much thicker than on their selenian or terran counterparts, due to the lower temperature. Earth-mass glacians attract so much atmospheric gas that they become low-mass subgaseous planets.

Terran Planets

Mass range: 0.3 to 1MePlanetTerranCor.jpg

Not to be mistaken for specifically the planets associated with the Terran system, but Terran-style planets in the context of planet types are are massive rocky terrestrials, like Venus in our Solar System. Unlike their smaller selenian relatives, Terran planets still have strong heat sources within them that power cyclical catastrophic and hotspot geological processes. Water is often hard to find on Terran planets, although many do have standing bodies of water that cover up to a quarter of their surfaces. Unlike the Nerean (earth-like) planets, however, water does not play a vital role in its geological processes, apart from weathering and the formation of sedimentary rocks. Plate tectonics are not possible as there is insufficient water, so the crust tends to be thick. Life is possible on terran planets that do have reserves of water, although the cyclic geological process can make life very hard to maintain.

Nerean (Earth-like) Planets

Mass range: 0.3 to 1MePlanetTerran.jpg

Nereans are high-mass terrestrial planets with enough water to form global oceans. Our own Earth is an excellent example. Such oceans cover from 40% to 100% of the planet, and the water’s deep penetration into the basal rock lubricates them enough for crust subduction to be a major force. The crust is relatively thin, and broken into basalt plates of varying sizes. This constant subduction and recycling of the planetary crust creates a very young surface that is often no more than a few hundred million years old. In some cases, continental masses of lighter rock such as granite float atop the crust plates, preserving older structures and encouraging the formation of various kinds of rock.

Aquarian Planets

Mass range: 3MePlanetAquarian.jpg

Aquarians are massive planets, often twice the diameter of the Earth and ranging from three to ten Earth-masses. The inner part of the planet is composed of rock and metal, generally up to two Earth-masses’ worth, while above this lies a deep layer of water often thousands of kilometers thick. The unrelenting pressure turns most of this water into ice, but in the temperate zone (the only place where aquarian planets are found) the surface remains liquid, giving rise to an ocean 100km thick. In most cases, Aquarians are formed from Cryosubgaseous that stray into the Temperate zone.

Hadean Planets

Mass range: 3MePlanetHadean.jpg

Rocky planets can form that mass much more than the Earth or even the rocky core of an aquarian planet. As the mass of a rocky planet increases, so does the proportion of heavy elements in its core. Larger planets contain more heat, and are more tectonically active. Beyond 3 Earth-masses, all pretense of Earthlike behaviour is lost. The crust becomes very thin and the planet roils with its barely-contained internal heat. Volcanism is constant, and episodes of megavolcanism are common events. A thick, heavy atmosphere forms, dominated by carbon dioxide, water vapor and sulphur dioxide. These hellish, heavy worlds are the hadean planets (Hadean, like Hades). What water these worlds contain is usually in the form of vapor, although some hadeans can host shallow seas in which primitive, extremely resilient life ekes out an extreme existence. Anything that can survive on planets like this are usually things you never, ever want to meet, according to the Lazoni.

Vestan Planets

Mass range: 10MePlanetVestan.jpg

Vestans are the largest and most massive of terrestrial planets. Between ten and fourteen times the mass of the Earth and generally twice as large, the paper-thin crust boils and blisters with uncontained heat from the planet’s innards. Massive global volcanism is a daily event and there are often standing bodies of liquid rock that could be called oceans. The atmosphere is thick and deadly, and the temperature, irrespective of the planet’s position, is like that of a blast furnace. Maps of vestan planets are more like weather reports; the planet can completely resurface itself in a matter of years. No known life can survive on a vestan planet, not even the magmacutaceous Lazoni, due to the extremes of heat and tectonic upheaval.

Asteroid Belt/Cluster

Mass range: 0.01 to 0.1MePlanetAsteroidBelt.jpg

An asteroid belt may be formed by one of two processes. In many cases, a region of the star’s protoplanetary disc fails to coalesce into a planet. Other bits of leftover rubble are shepherded from elsewhere in the system into the ring of orbiting debris, forming an asteroid belt. Sometimes, a planet is torn apart by collision with another planet, or is torn apart by tides from a massive planet. In either case, what’s left is an asteroid belt, like that of the Terran Solar System.

Asteroids naturally survive elsewhere, but this belt is a major object in the system it inhabits, and comprises a mass that can be considered equivalent to a planet. Asteroid belts contain asteroids and even the odd planetoid (see below).

Large planets of Gaseous mass or greater sometimes shepherd large amounts of asteroidal matter into ellipsoidal groupings centered on their L4 and L5 points. Jupiter does this in the Terran Solar System, creating the Trojan asteroids. Like a belt, an asteroid cluster only shows up as an important body if its total mass is equivalent to a planet; for instance, the L4 Trojans (a bunch of rocks and planetoids and comets clustered together) are equivalent in mass to the planet Mars. There is a chance that an asteroid cluster may contain planetoids.

Transplanetary Belt

Mass range: 10 to 100 MePlanetTransBelt.jpg

The transplanetary belt in Earth’s solar system is a realm of pristine material almost untouched since the system first formed. It lies beyond the orbit of the outermost planet and is composed of about 50:50 ice and rock. Most of the system’s comets originate from here; they can be perturbed by the close approach of a planet that distorts their orbit into a long ellipse, sending the comet into the inner system where it develops a coma and a tail. The transplanetary belt contains a lot of material (the Kuiper Belt possesses a whopping thirty-plus Earth-masses of rock and ice), but it is extremely widely spread, which is why it never formed into a planet. The transplanetary belt may be home to dozens or even hundreds of icy planetoids

Chthonian Planets?

PlanetCthonian.jpgChthonian planets were originally thought to be the remnants of gas giant planets that have wandered too close to their host star. A chthonian in such a case is the denuded, rocky-metallic core of the planet after all of its atmosphere has been scoured away. However, it can take tens of billions of years to scour away so much gas (Osiris, for instance, has lost at most only 7% of its atmosphere over the last four billion years), and stars long-lived enough to achieve this are the lowest end K-class stars or red dwarfs. They’re not bright enough to strip away a planetary atmosphere like this. As a result, it seems that true chthonians are rare; in fact, highly unlikely to exist at all. Even if they do exist, they are already represented by the selenian and terran classes.

Planets, Planetoids and Pluto

PlanetPluto.jpgNotice that a planetary type covering Pluto is conspicuous by its absence. There are several reasons for this, (some game-related, some astronomical). Pluto is a very light body, only two-thousandths the mass of Earth and barely a fifth the mass of the Moon. This is far below the lowest mass class of 0.01Me. It is also very small, less than two-thirds the diameter of the Moon. Furthermore, there are a lot of similar bodies to Pluto in the Kuiper Belt, possibly hundreds, and the discovery of Eris, which is larger than Pluto, finally settled the issue. In a landmark move in August 2006, the International Astronomical Union determined that after the Chaldathan war decimated the planet, Pluto was simply a large Kuiper Belt Object, and not a planet. However, it also created a new class of body, the “dwarf planet”, of which there are already a few members, and more will inevitably follow. In our game we call these objects Planetoids, and Pluto is a shattered half-planetoid due to the results of the Chaldathan invasion several years earlier. Other famous planetoids include Ceres, Europa and Triton.

Planetoids can orbit their stars as scattered objects or, more often, as part of a belt or cluster of asteroids. I don’t generate detailed eliptical orbits for these bodies, but do discuss some of their characteristics. (like the AU distance from the home star of a system). The mass of a planetoid is highly variable, and ranges from 0.0006 to 0.009 Earth-masses. A planetoid is described simply as a body that falls below the 0.01Me mass-class but still has enough self-gravitation to hold a spheroidal shape. The limiting diameter in this case is 1000km (because it’s a nice, round number – in reality it’s closer to 800km, but who is counting?) Such bodies have miniscule surface gravity, no magnetic field to speak of, trace atmospheres or no atmosphere at all.

Planetoids in Asteroid Belts and Clusters

PlanetToids.jpgBelts and clusters of asteroids, comets and the like can add up to a significant amount of material. By way of example, the Terran Asteroid Belt adds up to slightly more than the mass of the Moon. The L4 Trojan cluster of Jupiter contains ten times as much material, closer to the mass of Mars. The mass of a cluster or belt also determines the number of planetoids that may exist within it.

Orbital Characteristics

In ancient times on First Earth, it was believed that there was a sharp discontinuity between the Earth and the rest of the Cosmos. Below the Moon, existence was changeable, corrupt and imperfect. Above the Moon was the heavenly realm of pristine, unchanging perfection. Part of this belief was the idea that the planets moved in perfect circles. Eventually, long after the Posthumans left for the stars, during the time of the First Republic taking the majority of the precursor race of Primmortal’s knowledge with them, the astronomer Johannes Kepler developed his laws of planetary motion that are still used to this day, and helped Newton to develop his theory of Universal Gravitation. One of the great revelations of Kepler’s discovery was the fact that the planets don’t orbit in perfect circles.


Planets, moons, asteroids, comets etc orbit in ellipses, with the centre of their orbit at one of the ellipse foci. The other focus is empty.

The upshot for us is that planets have a near and far point in their orbits. On Earth, these points are called Perihelion when closest to the Sun (which happens the first week January), and Aphelion when furthest away (which happens in early July). With some planets, like Earth, this is no more than a couple of million kilometers either way, while Mars and Mercury have much more elliptical orbits. Some extrasolar planets have extremely eccentric orbits.

This “stretching” of the orbit away from a circle is called a planet’s eccentricity. A planet essentially dominates the orbital range between perihelion to aphelion, and this range is off-limits to other planetary orbits. If two planets’ orbits cross, the lesser planet will be lost (either cast out of the system, hurled into the sun, or consumed by the larger planet).

Orbital eccentricity can have a profound effect on a solar system. Systems dominated by giant planets with high eccentricities generally have fewer planets, since the wandering giants use up more of the protoplanetary disc, and their gravitational interactions can either capture planets or throw them out of the system entirely.

Orbital Period

How long is a year on your world? The orbital period of a planet, the length of its year, is determined simply by its distance from the star and the mass of that star. Using the list below, we can determine orbital periods at that good ’ol standard distance of 1AU:

Orbital Periods

Giant Stars: Major Stars: Minor Stars:
B: 122 days A: 259.33 days M: 820 days
G: 310 days F: 310 days L: 1550 days
K: 350 days G: 386.5 days
M: 290 days K: 438.33 days
Neutron Star: 122 days D: 374 days

For distances other than 1AU, the orbital period is worked out by Kepler’s pretty simple math:
p2 = d3, where p is the period (as in the list above), and d the distance in AU.

Naturally, these simple figures don’t give every single possibility for stars. Some stars are a touch heavier or lighter, and there are oddballs such as the aging, subgiant stars, but who cares, this is for fun, and most of this we won’t even use.

Shared Orbits and Lagrange Points

Lagrange.jpgLaGrange Points (or L-Points) are actually a pretty cool phenomenon. It goes like this: In any orbital system involving a primary and secondary (planet and star, moon and planet), there are regions where the gravitational attractions between the two bodies cancel out and nullify each other. These points are called libration points or, more often, LaGrange points, after the mathematician who discovered them on First Earth. Of particular interest are points L4 and L5, since these are the strongest and most stable. These are located 60 degrees to either side of the planet at the same distance from the sun. Bear in mind that unless the orbit is near-circular, this does not mean quite the same thing as being ahead of and behind the planet in its orbit. Matter that falls into these areas remains stable there, and can form asteroid fields or perhaps even planets. It’s possible for starships to use these points to travel without fuel in front of or behind the orbit of a given body as well. Though such travel is slow by hyperflight standards, for a starship that has run out of energy or needs to conserve it, these ‘gravitational bus stops’ can be the difference between life and death for a crew on the drift. (It does require a Mind + Physical Sciences Astronomy check to tho.)

Binary Planets

Binary planets are a very rare expression of an extreme shared orbit. The two planets actually form in their binary partnership; capture events are not possible, since any wandering planet will be moving too fast. Binary planets form within two mass classes of each other, and are often the same mass class.

Axial Tilt

AxialTilt.jpgAxial tilt refers to the planet’s alignment with its orbit. For example, Earth has an axial tilt of just over 23 degrees. Because of this tilt, we have seasons, tropics where the Sun can appear directly overhead at certain times of year, and arctic zones where the Sun can disappear below the horizon for weeks on end. If Earth had little or no tilt at all, like Jupiter, the Sun would never seem to climb and fall through the sky from season to season. More to the point there point there would be no seasons to speak of, since any part of the globe would receive the same sunlight from one day to the next. At the other extreme are planets like Uranus. Uranus has an axial tilt of just over 90 degrees, which means it effectively spins on its side. Seasons on Uranus are about as extreme as you can get, with one hemisphere pointed at the Sun for half the year, and the other in darkness for half.

Rotational Period

When a planet forms from the protoplanetary disc, it spins up. This is a natural result of the conservation of momentum, rather like a skater speeding up a spin by drawing in his arms. Over the ages, these spins are affected by tidal tugs from other planets, moons and the primary star or stars. Sometimes they can be affected by major impacts, as well. Within a minimum distance, the star’s tidal effects force the planet to orbit like a moon, with a locked rotation, keeping one face always pointed at the star. Further out, the orbit isn’t totally locked, but there is a strong resonance between the planet’s rotation and its orbit, so that one rotation takes 2/3 of a year. An interesting upshot of this is that because the rotation is such a long part of the year, a solar day (sunrise to sunrise) takes two full orbits to achieve. Mercury rotates in this way. Beyond this region rotations become more chaotic, as they’re subject to other forces besides solar tides.

Physical Characteristics

Planetary Mass

Planetary masses range from tiny, Moon-like selenian planets massing only 1% that of Earth, to monsters weighing in at 13 times the mass of Jupiter. The two main kinds of planet, gaseous (jovian) and terrestrial, have a small amount of crossover. The smallest gaseous planets are about ten times the mass of Earth, like Uranus. The uppermost theoretical limit for a rocky terrestrial planet is about fourteen times the mass of the Earth. To make things simpler, these masses can be grouped into mass classes.

Mass Type Description
Me Rocky terrestrial planets are classed in Earth-masses, or Me for short.
Mj Gaseous planets are noted in Jupiter-masses, or Mj for short. For the sake of convenience, a Jupiter-mass is simplified to 300Me, (300 x Earth) rather than its actual value of 318Me.

These classes are arranged scale below, along with some real-world example planets out there in the galaxy. You can look up any of these planets online if you want to try to create a planet just like a certain type you’ve read about above – or you can choose not to care and I can assign attributes to the planets based on your general description. It’s a sci-fi game, and this hard science stuff is really just here for flavor and guidelines, or if a character really wants to dig into the nitty-gritty of the science related to a given planet.

Planetary Mass Classes

Terrestrial Planets:

0.01 Me 0.03 Me 0.1 Me 0.3 Me 1 Me 3 Me 10 Me
Luna Mercury Mars Venus Earth Mu Arae d Gliese 876 d

Gaseous (Jovian) Planets:

0.03 Mj 0.1 Mj 0.3 Mj 1 Mj 3 Mj 10 Mj
Uranus Epsilon Eridani c Saturn Jupiter Tau Bootis b HD161020b
Neptune 55 Cancri c 55 Cancri b 16 Cygni b 55 Cancri d HD39091b

Planetary Geology

Planets with solid surfaces have geology; landforms shaped by sudden catastrophic events and slow, but implacable, forces. There are four kinds of geology: passive, sporadic, cyclic and active.

Geology Type Description
Passive Passive geology is just that; passive. Planets with passive geology have lost, or never had, a source of internal energy capable of resurfacing the planet. Landforms under such circumstances are dominated by the relics of past episodes of activity, impact history and weathering, if applicable. Terrain formed by passive geology is marked by impact craters and low, rolling mountains, as well as faulting and scarps. The terrain will generally be very ancient, measurable in billions of years.
Sporadic Sporadic geology is very similar to passive, but in this case there is just a feeble ember still glowing at the planet’s core, enough to power occasional tectonic shifts and highly sporadic volcanism. This will tend to add trace gases such as methane or sulphur dioxide to an otherwise inert atmosphere, but will have little effect on the overall terrain, which will tend to look like passive geology. Often only a geologist, or an unfortunate explorer who could’ve sworn the volcano was extinct, would be able to tell the difference.
Cyclic Cyclic geology is generated by an active core that’s blocked by an overly thick crust. This is often the case of low to middle-mass terran planets. The geological cycle often undergoes quiescent periods for tens or even hundreds of millions of years until the trapped heat overwhelms certain fracture points in the crust. Then the situation changes dramatically, and global megavolcanism takes place, sometimes capable of resurfacing the entire planet within a few million years. The atmosphere often thickens and becomes highly toxic, making life very difficult for complex forms. Eventually the pressure wanes and the surface becomes quiescent again.
Active Active geology take place constantly, with earthquakes and eruptions every year or so. It comes in two distinct flavours: hotspot and plateotectonic.
Active: Hotspot Hotspot geology also occurs on terran planets, as well as occasional low-mass nerean worlds. Plumes of hotter material from deep in the mantle near to the core create zones of high pressure and excessive heating under the crust. Eventually this material breaks out in a region of high volcanism. Because the crust is immobile, unlike plateotectonic worlds, these hotspot regions give rise to massive shield volcanoes hundreds of kilometers across, sitting on extensive highland regions. On some worlds, they can grow over a thousand kilometers across, and rise high enough to poke out of the planet’s stratosphere.
Active: Plateotectonic Plateotectonic geology is common on nerean (Earth-like) planets, where there is enough water to fuel the process. Unlike normal planetary crusts, plated crusts are broken into a number of regions, or plates. Water from global oceans permeates the rock, lubricating it and allowing the plates to slide under each other. This causes tension stress on the far side of the plate, and new material is dragged up from below to fill in the gap. This often forms a jagged suture line, most commonly under the oceans themselves, and hence named a mid-oceanic ridge; a line of volcanic mountains often thousands of kilometers long. The subducted edges of the plates are easily melted since they carry water as an impurity, triggering volcanism on the surface above the subduction zone. This complex and active process creates very young crust, often only a few tens to hundreds of millions of years old, and is dependent on large quantities of water, hence it only appears on nerean planets. Sitting on the basalt crust lie regions of lighter granites. They are carried along like rafts, sometimes merging, sometimes being broken up by the activity below them. These form the continents on such planets.

Geology and Weathering

Planets with atmospheric pressures a single Earth-atmosphere and above, and extensive hydrography, can give rise to weathering effects. This is the erosion we’re familiar with on Earth, which breaks down all but the largest features and wipes out primal features such as impact craters. It breaks mountains, submerges continents and fills in canyons in tens or hundreds of millions of years. Planets with no atmospheres have little to no weathering, mostly in the form of the occasional frost and impact weathering from micrometeorites. These processes have minimal impact, even over billions of years.


Some planets have a layer of liquid, either just below a solid, icy crust or sitting above a rocky one. These layers may range from isolated little seas to vast global oceans up to a hundred kilometers deep. Most planets have little or no surface liquid, but even they may play host to subterranean aquifers or permafrost on colder planets. Sometimes an entire ocean may be concealed below a shell of ice many kilometers thick.

Planets with seas or oceans are often the most sought-after by spacefaring civilisations, since it is these that have the greatest chance of supporting and harbouring complex life of their own.


alien-ocean.jpgMost oceans in the Universe are composed of water. Water is one of the commonest substances in existence, and it has a broad range of temperatures and pressures at which it is liquid. Water may be quite pure, or more often carries a number of impurities. Depending on the planetary conditions, oceans may be saline, or even somewhat acidic or alkaline. Extreme concentrations of impurities are more common on smaller bodies of water. Second to water, liquid hydrocarbons like methane and ethane can sometimes form seas and oceans under conditions of sufficient atmospheric pressure and low temperatures, but such planets are rarer than aqueous ones, and confined to the Superfrigid zone of a system.


WaterArea.jpgMany planets have no surface water at all, but larger ones are generally wetter. Some larger planets only bear small numbers of contained seas, dotted about the surface. Others can be partially or totally covered by oceans. Aquarian planets host global oceans a hundred kilometers deep. As well as liquid oceans, planets may also develop mantles of ice near their poles, called polar caps. If you have something specific in mind for a planet you’re building, then choose from the options below, based on the planet type. Or you can roll on the table if you’re a nerd like me:

Terran/Hadean Nerean

1-4 Desiccated planet. No surface water exists, although permafrost or subsurface aquifers are possible on some planets. Life on the surface is probably non-existent, but if it’s there must be extremely tough to survive.
5-8 Arid planet. Water covers approximately 20%. Seas are few and shallow, and most surface water is in the form of lakes and seasonal rivers.
9-12 Semi-aqueous planet. Water covers approximately 40%. Planet has many large and noticeable bodies of water, but the overall geography is still dominated by deserts. There is sufficient water on these planets to saturate the crust rocks and generate plateotectonic geology.
13-16 Aqueous planet. Water covers 60 – 80%. Planet is Earthlike, dominated by oceans, with isolated land-masses. Plateotectonic geology is inevitable under these conditions.
17+ Oceanic planet. Water covers of 100%; only scattered atolls and small volcanic islands are visible above the surface, at most.

-3 modifier for planets in systems dominated by gas giants with eccentric orbits.

Subsurface hydrography for Glacian planets

2-4 Frozen planet. The overmantle is solid ice, with no liquid water.
5-12 Semiliquid subsurface. The overmantle contains a region of slushy ice that is capable of flowing, but is not truly liquid water.
13-16 Discontinuous subsurface ocean. A subsurface ocean exists in tidal stress zones (see Moons) or volcanic hotspots warmed from beneath. Overall hydrography 40 – 60%.
17+ Global subsurface ocean. The subsurface ocean is contiguous, with little or no interruption. Overall hydrography 80 – 100%.

+4 modifier for Glacian moons orbiting in the High Tide zone of a gas giant.

Magnetism & Radiation

Some planets, such as First-Earth or Jupiter, are capable of generating substantial and homogenous magnetic fields. These fields are generated by a dynamo effect within the planet’s interior, by means of the planet’s rotation and turbulence within a conducting fluid inside the planet. On terrestrial planets, this is usually generated at the boundary of the solid metal inner core and liquid metal outer core. On jovian planets, this is generated within the liquid-metallic or plain liquid hydrogen mantle. As a result, terrestrial planets with strong magnetic fields usually have a dipole field closely aligned with the planet’s rotation axis, while that of jovian planets can exhibit a strong deviation from the rotation axis. Jovian planets sometimes even exhibit quadripolar fields.

Not all planets meet these conditions, of course. Venus, for example, has almost no field strength, but then it has almost no rotation, either. The Moon rotates more quickly than Venus, but its small size means its core has long since cooled to a solid or semi-solid state. Mars rotates as quickly as Earth, but again its small size means that it has run our of internal energy to keep a fluid core. Such magnetically-inactive planets still have magnetic fields, everything does, but they’re weak and limited, with micro-poles scattered across the surface rather than as a single, powerful field.

Even when a planet has a global magnetic field, its strength and orientation can change. Earth’s field, for instance, has regularly undergone reversals of north-to-south, and is currently weakening in preparation for another reversal.

A planet’s magnetic field affects the local radiation environment, both on the planet’s surface and in the space around it. Magnetic fields trap charged particles from stellar winds and similar space weather. These particles form toroidal bands around the planet, like the Van Allen belts of Earth. These bands form high-radiation environments that can be hazardous to spacecraft, as well as living beings. However, by trapping these particles they also keep them away from the planet’s surface, providing a protected environment for life that may exist there. Jovian planets like Jupiter can give rise to powerful fields of well over a thousand Gauss, stronger than the magnet in an MRI scanner. Such fields are often the size of a main sequence star like the Sun, and contain radiation belts capable of killing a person in minutes.

Planets without such protection are almost defenceless before the charged particle winds of their stars, and the radiation environment will match that of local space with lethally high peaks during stellar storms. The presence of an atmosphere, even a slight one, can mitigate this somewhat, dropping the ambient surface radiation to a lower, survivable level, though it will provide no protection against stellar storms. Heavy atmospheres can provide complete protection, and even generate their own magnetic field when under assault from high stellar winds, preventing the atmosphere from being lost.

Magnetism and Atmosphere

Planets with weak or surface-localised magnetic fields have another problem when dealing with solar radiation. These charged particles aren’t stopped by the field and trapped in radiation belts. Instead, they hit the atmosphere directly.

In low gravity planets, those of less than 0.3Me, solar wind particles often strip away the atmosphere in a matter of a few million years. This is what happened to Mars. On heavier planets, charged particles are absorbed by the atmosphere and break apart hydrogen bonds, breaking down water into hydrogen and oxygen. The hydrogen, being so light, is lost to space and stripped away, leaving the planet parched for water, but the planet’s gravity can still hold onto the heavier gases. This is what happened to Venus. From these examples it’s easy to see that the protection of a powerful magnetic field is vital for any planet that harbors complex surface life.

Planetary magnetic fields can be divided into six groups:

Mag level Density Description
Mag 1: Weak, localised Weak, localised fields are like that of the Moon and Mars. There is no global field at all, and small concentrations of polarity are scattered at random across the surface.
Mag 2: Weak, global Weak, global fields are similar to that of Mercury. They range between 0.01 and 0.1 Gauss in strength. They can be used for navigation but little else.
Mag 3: Strong, global A strong, global field is like that of Earth. They range in strength from 0.1 to 10 Gauss (Earth’s field is 1 Gauss on average), and provide effective protection from most space radiation.
Mag 4: Powerful, global A powerful, global field ranges in strength from 10 to 100 Gauss. It’s very rare to see such a field generated by a terrestrial planet, but this strength is common amongst subjovian worlds.
Mag 5: Jovian, global Jovian fields range on average from 100 to 10,000 Gauss. They are the most powerful fields commonly found. At the highest ranges, even outside the radiation belts, fields of this strength are dangerous to living things and lengthy exposure can even be fatal.
Mag 6: Lethal Lethal fields are in the 10,000+ Gauss regime. They pose an immediate danger to living things, since the field is strong enough to interfere with physical and chemical processes. Such fields are only commonly found near starspots.

Beyond a million Gauss, atoms themselves are distorted into elliptical shapes, aligned with the field. Chemical bonds are overwhelmed and molecules cease to exist, all matter loses its structure and becomes what’s referred to as “magnetic soup”, composed of isolated atoms. Fields of this strength only exist near the surface of certain rare neutron stars, called “magnetars”.

  • Planets of mass 0.01 Me are restricted to Mag 1.
  • Planets of mass 0.03 to 0.1 Me may be Mag 1 or 2.
  • Planets of mass 0.3 to 3 Me may be mag 2 or 3, depending on their rotation; less than 500 hours rotation will in almost all cases generate a Mag 3 field, while slower rotation will generate a Mag 2.
  • Vestan planets may generate a Mag 4 field if they rotate in less than 100 hours, otherwise they generate a Mag 3.
  • Subjovian planets generate a Mag 4 field.
  • Jovian planets generate a Mag 5 field.

Planetary Atmosphere

AlienAtmosphere.jpgPlanets attract gaseous envelopes during their formation, and more gases are added to the envelope from within the planet as well, via volcanoes. This gas is referred to as an atmosphere, and on First Earth there exists a rather a nice one. Atmospheres are described by: pressure, chemistry and temperature.

Chemical Makeup

Below is a list of the most common naturally-occurring gases known, listed in order of molecular mass. It’s there to give you an idea as to how to mix up an atmosphere for your planet. Note that the lightest gas Earth can hang onto for any great length of time is methane, while Jupiter and Saturn have immense atmospheres of molecular hydrogen.

Gas Molecular Mass Chemical Symbol
Atomic Hydrogen 1.0 H
Molecular Hydrogen 2.0 H2
Helium 4.0 He
Atomic Nitrogen 14.0 N
Atomic Oxygen 16.0 O
Methane 16.0 CH4
Ammonia 17.0 NH3
Water Vapor 18.0 H2O
Neon 20.2 Ne
Molecular Nitrogen 28.0 N2
Carbon Monoxide 28.0 CO
Nitric Oxide 30.0 NO
Molecular Oxygen 32.0 O2
Hydrogen Sulphide 34.1 H2S
Argon 39.9 Ar
Carbon Dioxide 44.0 CO2
Nitrous Oxide 44.0 N2O
Nitrogen Dioxide 46.0 NO2
Ozone 48.0 O3
Sulphur Dioxide 64.1 SO2
Sulphur Trioxide 80.1 SO3
Krypton 83.8 Kr
Xenon 131.3 Xe


AlienAtmosphere2.jpgHalogens like fluorine, chlorine and to a lesser extent bromine are corrosive planetary atmospheres. However, these gases are highly reactive (which is why they’re corrosive), and even in the absence of oxygen can react strongly with other gases. They’re also very rare in the Universe at large, which is why they don’t show on the chart above. They only show up in any concentration in the gases surrounding a major volcanic eruption. In short, they’re not the best choice. For a corrosive atmosphere, sulphurous and other acidic compounds are just as good, and more stable to boot. Venus does very nicely with an atmosphere of sulfur dioxide and carbon dioxide.

Halogens in noticeable concentrations would only be present on planets dominated by global megavolcanism, like Vestan planets. Some other gases are also highly reactive, and need to be constantly replenished. Good examples are oxygen and methane. Both of these react strongly with other substances, one reason why we breathe oxygen in the first place. However, it also means that planets can run our of these gases quite quickly (a few thousands or millions of years). If planets have these atmospheric gases then they’re being replenished, perhaps by volcanism or maybe life. Inert and lifeless planets won’t have these kinds of gases in any great quantity.


Pressure varies from planet to planet, but its main contributing factors are the mass and temperature of the planet. When gas molecules are given energy (by sunlight, for example) they spring off in high ballistic trajectories, until they either land or hit something else, like another gas molecule. Given enough energy, a gas molecule can reach escape velocity and fly off into space. As a result, only heavier planets can hang onto the lighter elements, such as hydrogen and helium. The following table lists the range for atmospheric pressure, depending on where the planet is in its system.

Atmospheric Pressure 0.01 Me 0.03 Me 0.1 Me 0.3 Me 1 Me 3+Me
Supertorrid 0 0 0 0 – 1 0 – 2 5
Torrid 0 0 0 2 – 5 2 – 5 5
Temperate 0 0 0 2 – 5 2 – 5 5
Frigid 0 0 – 1 1 – 2 3 – 5 3 – 5 5
Superfrigid 1 – 3 2 – 3 3 – 4 4 – 5 4 – 5 5


The surface temperature of a planet can vary, depending on its various sources of heat. The three most common sources are sunlight, internal geothermal heat and latent heat trapped by oceans and atmosphere. Sunlight, depending on its intensity and proximity, will heat a planetary body according to the simple scale laid out below:

Zone Day Night
Supertorrid Zone H5 H1
Torrid Zone H4 H1
Temperate Zone H3 H1
Frigid Zone H1 H0
Superfrigid Zone H0 H0

Temperature varies wildly when depending on sunlight alone. However, for very massive planets, internal heat sources can provide an extra level of warmth, as seen below:

Hadean H+1
Vestan H4 – 5

Temperature and Atmospheres

AlienAtmosphere3.jpgThe presence of atmospheres and oceans serves to hold onto heat gathered in the daytime and release it at night. It also distributes heat between the dayside and nightside of a planet via winds and weather systems, ironing out the most severe differences between day and night. An atmosphere of two atmospheres and above confines temperatures in most cases to a single Heat value between the extremes. In the Torrid and Supertorrid zones, the upper heat value remains the same, but the night-time temperature raises to meet it. In the temperate zone, the temperature settles into an intermediate value between H3 and H2. In the Frigid and Superfrigid zones, the temperature range is already so low that the atmosphere has no noticeable insulating effect.


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