Here is an attempt to briefly sketch Thalassa’s solar system. Such a description can be used to flesh out other civilizations and cultures living in nearby celestial bodies or to give some astrological basis and references to Thalassa’s cultures.
The mundane plane is basically made of Thalassa's solar system (from Naryan, the central sun, to a cloud of comets extending up to a light year from it) and a surrounding background of stars, nebulae and galaxies. It is unclear whether such a background is real or it is just an illusion taken from some other place of the multiverse and projected on the the mundane plane boundaries to give just the impression of space.
Thalassa's Solar System:
It is made of a central sun (Naryan) mostly analogous to real-Earth sun, and 13 main planetary bodies orbiting around it. More than 200 moons and other satellites, four asteroids' belts, comets and other wandering rocks complete the system. Thalassa is the fourth planet from the sun, orbiting at about the same distance as the real Earth orbits from the Sun.
The tenth planet from the Naryan is actually a failed star (a brown dwarf), a huge gas giant about 11 times more massive than Jupiter whit a large court of satellites. Such a planet generates enough internal heat that a couple of its satellites are suitable for life.
Four planets (including Thalassa) and four satellites (including the two cited above) are suitable for life as we know it. Other very odd and alien forms of life inhabit more extreme habitats (extremely hot or cold or gaseous environments, etc.) scattered here and there in the solar system.
One of the main distinctive features of Thalassa's solar system is that the 13 planets all orbit on the same plane and their orbital periods are multiple. They align themselves along the same row once every 3600 years. Such an alignment start a burst of extraplanar interaction between the Edge pocket Plane and whatever Plane of existence is closest to it. For a few hours, until the planetary alignment is over, the number and the size of the extraplanar gates throughout the mundane plane (and maybe involving also the esoteric plane) is increased a hundredfold and whole towns or populations can be transferred through a single gate. The planetary alignment is the main way to gather large number of homogeneous people to Thalassa: such groups formed the nucleus of many past and current races, populations and countries established across Thalassa and other worlds.
Sages and scientists still debate if such a regular disposition of planets is a natural feature of Thalassa's solar system or if some ancient, powerful and long forgotten race arranged the planets' orbits in order to obtain the planetary alignment phenomena.
A sketch of the solar system of Thalassa is shown below, with a table summarizing the most interesting features of the various planets.
A full resolution image can be downloaded here
Planets and other major features of Thalassa's solar system (listed from increasing distance from the sun) are briefly described below. Names for most planets are not given (suggestions are welcome!)
the central sun, similar in radius (700000km) and temperature (5800K) to real Earth's sun. It emits magical particles which are captured by planets possessing a strong magnetic field (Thalassa is among them).
a demi-molten small planet of lava with a surface temperature of 750°C (1400°F) on the day side (the planet is tidally locked: it shows always the same face to the sun). It has a poor atmosphere of mostly rock and metal vapors, but its powerful (but unpredictable) magnetic field and its proximity to the sun enable it to be showered by a powerful flux of magic particles.
a small tidally locked planet of rock, mud and shallow seas, with a thick, steamy atmosphere and an average temperature of 57°C (135°F) which is almost constant day and night, at all latitudes. A jungle of giant fungi, lichens, molds and other alien plants covers most of the lands, and a fauna of slimes, worms and invertebrate creatures populates the jungles. The impressing towering trees thrive on this planet, reaching heights up to 80-100km. This is the only place in Thalassa's solar system where such trees are able to spread their seeds to the outer space. Some of them seldom manage to reach a suitable land on other worlds and to root there. T-greenhouse has no magnetic field and it capture a very small stream of magic particles.
a tidally locked world similar in size to the real planet Venus, but farther to the sun and with a far milder tropical climate (20.5°C or 68.9°F on average). It is a mostly jungle world, inhabited by many civilized races. Its environment is very similar to early science-fiction pulp novels about Venus. At least a dozen of towering trees with a maximum height around 50km are scattered on T-venus almost anytime. Its magnetic field, though weaker than Thalassa's one, enables it to capture a small amount of magic particles from the sun.
an Earth-like world, slightly larger and more massive than ours but with the same gravity and climate, home to many life forms and intelligent races. It has a large moon, slightly smaller than ours but barely habitable due to a terraforming effort performed by some unknown race a few million years ago (the moon will lose all its water and atmosphere in a few million years). Thalassa has a more or less permanent ring stabilized by two moonlets orbiting above the equator, and the planet's magnetic field is able to entrap magical particles from the sun which are then showered to the poles, where high magic civilizations thrive. A couple of asteroids float on geostationary orbits above the equator: one of them is just above the middle of a large feature in the ocean called "the edge". The edge is some kind of unknown force field which keeps most water away from a circular region of some thousand km in radius: the border is a vertical step of water going down to the ocean floor. Once every 60000 years on average a towering tree seed manages to root on Thalassa: they reach 20-30km in height and usually live far shorter than their brothers in the inner planes (their average lifespan on Thalassa is some 30000 years nevertheless).
a world similar in size and features to the real planet Mars, but closer to the sun and more habitable (with an average temperature
of 1.5°C or 34.7°F and a far thicker, breathable atmosphere), but without a significant magnetic field. It is a cold, mostly deserted world inhabited by hardened life forms and intelligent species. It is very similar to the pulp description of the real Mars in early science-fiction novels. T-mars has a single moon, about half the size of Thalassa's moon, which was terraformed by some unknown race in the past (maybe the same creatures who altered Thalassa's moon environment) and it is still barely habitable (lichens and other die-hard plants make for most of the vegetation).
5b) Asteroid belt #1:
a relatively narrow asteroid belt between the orbits of T-mars and T-ceres. On the inner side there are mostly rocky bodies, while on the outer side there is a mix of rocky/icy bodies. The largest asteroid is about 200km in size. Solar radiation on the inner side is strong enough that a pressurized habitat would have a temperature around -40°C (-40°F) so it would be possible to build a comfortable environment there by using some natural or magical heating source.
a large asteroid similar in size to the real dwarf planet Ceres, with an average temperature of -104°C (-155°F); about half of this celestial body is made of ice. It has no magnetic field.
a gas giant analogous to Neptune. Its average temperature is -102°C (-151°F). It has 18 moons of various size but no rings. The planet's powerful but displaced magnetic field traps more magic particles above the southern pole, and less in the northern one.
a large gas giant analogous to Saturn. Its average temperature is -162°C (-260°F). It has 47 moons of various size (a likely T-Titan or T-enceladus might be inhabited by some alien form of life) and it shows an impressive set of rings. The planet's powerful magnetic field traps many magic particles above its poles. T-saturn system is interesting also because it hosts two other small icy planets moving along the same orbit of T-saturn, 60° ahead and 60° behind the largest planet (the so-called Trojan orbits: to help identify the two planets we could provisionally call them T-athens and T-troy). Both icy planets, while small, are quite reflective and visible in Thalassa's night sky: the triad of T-saturn, T-athens and T-troy are believed in many Thalassan cultures to represent the triad of Intuition, Meditation and Knowledge (which also have a similarity in Sorcerer, Cleric and Wizard spellcasting methods).
8b) Asteroid belt #2:
a large asteroid belt between the orbits of T-saturn and T-uranus. It is mostly made of icy bodies with a maximum size of about 100km. A few, displaced mixed rock/ice small asteroids can be found here as well.
a gas giant analogous to Uranus. Its average temperature is -190°C (-310°F). It has 9 moons of various size and a set of rings. this planet and its moon system are tilted on its side. The planet has a strong but displaced magnetic field which traps most magic particles above the south pole. One of its moons, T-Titania, has a thicker nitrogen atmosphere and its temperature range is so that it possess large seas of liquid nitrogen on the night side. such seas evaporate every 20 years, when they are lit by Naryan's light, to condensate again in the other emisphere, now in the dark. The coldest region of T-Titania seldom experience solid nitrogen formation as well. This moon might be home of a whole alien ecology where nitrogen has the same place as water on warmer planets.
9b) Kuiper belt #1:
A very narrow asteroid belt between the orbits of T-uranus and T-jupiter, made of icy bodies not larger than 100km. It is believed that at least a few of the shortest-period Grand Comets come from this belt.
10) T-jupiter, or the brown dwarf:
This is the failed star described above. It appears as a huge and hot gas giant, with a radius almost triple than the real planet Jupiter and an external temperature of 830°C (1526°F). When high in Thalassa's sky this planet appears about 3 times larger than real world Venus, with a luminosity about 1/4 of that of real world full moon and it is visible even on many daily hours far from midday. T-jupiter is even brighter than this on infrared, making it a celestial body of particular significance for those races who possess infravision. This planet produces a very powerful magnetic field which traps a huge amount of magic particles (it is believed that T-jupiter also generates a relatively small amount magical particles on its own). Moreover, due to interaction with its inner large moon T-io (see below) such magic particles are scattered throughout the inner moon system, enhancing magical powers on all recipients living there. T-jupiter owns 140 moons of various composition and size (some of them as large as small planets): among them T-europa and T-antarctica are able to host life, while T-io is important as it diffuse magical particles all around. All these three satellites are tidally locked to T-jupiter.
T-europa is a large moon covered with a thick atmosphere, a permanent cloud coverage and a powerful magnetic field which shield the satellite of most harmful radiation produced by T-jupiter. The average temperature of T-europa is 15°C (60°F) allowing a 40% water coverage and temperate habitats. It is home of many life forms and civilized races: towering trees seldom manage to root on the surface (with about the same chances they have on Thalassa). The only towering tree standing nowadays on T-europa (on the lit side) has an height of 25km; it bypasses most of the cloud coverage and lets T-europa creatures living on it to receive an increased amount of magic particle and to see the impressive view of T-jupiter dominating the sky.
T-antarctica is a smaller and farther moon than T-europa, with a similar cloudy atmosphere and a strong magnetic field. Its average temperature is -6°C (21°F); 30% of surface is covered by saltwater seas where algae and other organisms produce breathable air, while most lands are made of uninterrupted snow fields dotted by rocky formations covered in ice and lichens. While T-antarctica shows an harsher environment than T-europa, it is nevertheless home to many cold-accustomed creatures and civilizations.
T-io is a warmer version of the real Jovian moon Io (the average temperature on T-io is 66°C or 151°F) with a stronger volcanic activity and a permanent magnetic field of its own. Volcanoes eject gases and vapours in the outer space, which are then irradiated by T-jupiter magical particles and then scattered throughout the internal moon system. T-io's atmosphere is not breathable, there is almost no liquid water and the surface is so heavily irradiated that it would kill an unshielded life form within a few hours.
10b) Kuiper belt #2:
A vast and sparse asteroid belt between the orbits of T-jupiter and T-eris, encompassing the whole orbit of T-pluto. It is almost entirely made of icy bodies and it is believed that most of the Grand Comets originated in this area.
a small world similar in size and composition to the real dwarf planet Pluto. The average temperature is -228°C (-378°F). It has two small icy satellites and no magnetic field.
a small world similar in size and composition to the real dwarf planet Eris. the average temperature is -240°C (-400°F) It has a small icy satellite and no magnetic field.
13) T-second earth:
a large world, the farthest from the central sun Naryan, a rocky planet about as large as real Earth. It has three small moons and a magnetic field slightly weaker than Thalassa's one. T-second earth average temperature is a chilling -248°C (-414°F), low enough for liquid hydrogen to exist on the planet’s surface, but it is also possible that some natural or magical heating process allows hidden patches of life or even colonies of intelligent beings.
13b) Cometary nebula:
a huge and very diffuse nebula of comets extending from outside T-second earth orbit until half a light year away from Narya. it is believed that the longest period Grand Comets originated here.
13c) The Slow Void:
over the course of millennia the most advanced spacefaring civilizations of the mundane plane have always attempted to venture outside Thalassa's solar system and explore nearby stars. Shortly after the external boundaries of the cometary nebula (more than half-light year away from the central sun, a challenging distance for most advanced civilizations at their full potential) space properties begin to change, slowing any kind of magical or natural propulsion (including teleportation and the like) moving outward from the sun. An effective comparison is as if an invisible rubber wall is opposing with stronger and stronger force to any object trying to bypass it, until the object outward velocity stops. Turning back in the opposite direction is far easier and the object does not suffer any resistance. The fastest the spaceship/object the greatest the distance traveled before stopping. The farthest distances from the central sun (about 1 light-year from Narya, in the middle of the interstellar void) are reached when moving at the speed of light (or faster) or via magical teleportation. Nobody has managed to go farther, so far.