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 PlanetSubGaseous.jpg

Mass Range: 0.03 – 0.1Mj

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 PlanetGaseous.jpg

Mass range: 0.3 – 3Mj

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 PlanetTransGaseous.jpg

Mass range: 10Mj

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.

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. Subgaseouss are generally reddish-brown. The osiritype is named after Osiris, the first ever such planet discovered, and the first extrasolar planet ever given a true name.

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 characterised 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 kind in the Universe. 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 PlanetsPlanetSelenian.jpg

Mass range: 0.01 to 0.1Me

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 PlanetsPlanetGlacian.jpg

Mass range: 0.01 to 0.3Me

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 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 PlanetsPlanetTerranCor.jpg

Mass range: 0.3 to 1Me

Not to be mistaken for specifically the planets associated with the Terran system, but Terran planets in the context of blanet types are are massive rocky terrestrials, like Venus in the Solar System. Unlike their smaller selenian relatives, Terran planets still have strong heat sources within them that power cycliccatastrophic 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 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) PlanetsPlanetTerran.jpg

Mass range: 0.3 to 1Me

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 PlanetsPlanetAquarian.jpg

Mass range: 3Me

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 PlanetsPlanetHadean.jpg

Mass range: 3Me

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 pretence 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. What water these worlds contain is usually in the form of vapour, although some hadeans can host shallow seas in which primitive, extremely resilient life ekes out an extreme existence.

Vestan PlanetsPlanetVestan.jpg

Mass range: 10Me

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/ClusterPlanetAsteroidBelt.jpg

Mass range: 0.01 to 0.1Me

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 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 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 are equivalent in mass to the planet Mars. There is a chance that an asteroid cluster may contain planetoids.

Transplanetary BeltPlanetTransBelt.jpg

Mass range: 10 to 100 Me

The transplanetary belt 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.jpg

Chthonian planets were originally postulated as 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 PlutoPlanetPluto.jpg

Notice 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 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 ClustersPlanetToids.jpg

Belts and clusters of asteroids, comets and the like can add up to a significant amount of material. By way of example, the Main 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, 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.


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