Limax System

The Limax System consists of one central red dwarf star and six terrestrial planets, the fourth of which is Xenosulia. There are no gas giants in the planetary system, and all planets are so close to the central star that tidal effects prevent them from having moons or rings. 

The first five planets exhibit orbital resonance with each other. This has a stabilising effect, but also causes some of the planets to have less circular orbits than they’d have otherwise (since planets always tug at them at certain points in their orbit). The orbital resonance is 1:2:3:6:12. 

Zhimuchua 23 A (Limax)

Formally designated Zhimuchua 23 by Earth astronomers, the star is often referred to as Limax by Xenosulia’s inhabitants or just “the Sun” (sedu in Occassian Gontanic). It is a small, relatively inactive red dwarf star located just 16 light years from Earth. 

Although flares are much rarer now, when the star was younger it was quite volatile. 
The name “Limax” comes from the Latin word for slug, limax. This is in reference to Xenosulia’s name (see the section on Xenosulia below). 

Properties

Physical
Type: Main sequence red dwarf
Stellar classification: M4.5V
Mass: 0.16 M_☉
Luminosity (bolometric): 0.0012 L_☉
Visual luminosity: 0.00018 L_☉
Temperature: 2,700 K  
Radius: 0.16 R_☉

Observational (from Xenosulia)
Angular size: 2.22 degrees
Apparent magnitude (visual): -24.5 

Zhimuchua 23 f (Vulcanus)

Vulcanus is the closest planet to the star Limax, and has an orbital period not much longer than a day. While incredibly hot, not all of the heat it receives is from stellar radiation, as it experiences a considerable amount of tidal heating. As such much of the surface is molten, and the planet is incredibly volcanically active. It is the smallest planet in the Limax System in terms of volume, and the second least massive. Because of its low gravity and exposure to stellar radiation, the atmosphere is a near-vacuum. 

Vulcanus is named after the Roman god of fire and the forge, Vulcanus, in reference to the planet’s temperature and vulcanism. 

Properties

Orbital
Semimajor axis: 1.74 million km
Orbital period: 1.15 days
Rotational period: synchronous 

Physical
Mass: 0.20 M_⊕
Radius: 0.55 R_⊕
Density: 6.5 g/cm³
Escape velocity: 6.7 km/s
Surface gravity: 0.65g

Observational
Largest apparent size: 6.0 minutes
Smallest apparent size: 3.2 minutes
Maximum apparent magnitude: -6.8 (superior conjunction) 

Zhimuchua 23 b (Hermes)

Hermes is the second planet from the star and the most massive, over five times the mass of the Earth. Although large, the planet largely consists of rock, although it does have a very dense atmosphere. Hermes is incredibly volcanically active which, in addition to its high gravity, likely contributes to the density of the atmosphere. Greenhouse gasses are so high that the surface is hot enough to glow red. This can only be seen when viewed from the planet’s surface; from space, the surface is obscured by a layer of organic haze.

Hermes is close enough to Xenosulia that it appears like a small moon when viewed from the planet’s surface, about two-thirds as big as the Moon when it’s near its closest approach during its crescent phase. 

The planet was named after the Greek god Hermes. The planet was often compared to Mercury due to its fast motion close to Limax, so it made sense to name it after the Greek equivalent.  

Properties

Orbital
Semimajor axis: 2.77 million km
Orbital period: 2.31 days
Rotational period: synchronous 

Physical
Mass: 5.4 M_⊕
Radius: 1.6 R_⊕
Density: 7.4 g/cm³
Escape velocity: 21 km/s
Surface gravity: 2.1g

Observational
Largest apparent size: 23 minutes
Smallest apparent size:  8.2 minutes
Maximum apparent magnitude: -9.6
Apparent magnitude at superior conjunction:  -9.3

Zhimuchua 23 c (Siluna)

The third planet from Limax, Siluna, is a desert-like planet slightly smaller than Earth. An initial lack of water prevented it from undergoing a similar run-away greenhouse effect as Venus; this lack of water was likely due to bombardment from UV radiation during the initial flare stage of Limax, before the star calmed down. Water in the upper atmosphere was split into hydrogen and oxygen during such flares, with hydrogen escaping into space and the oxygen combining with various minerals. 

Because it’s tidally locked, and without oceans or a dense enough atmosphere to distribute heat, the temperature gradient is far greater than on Xenosulia. Most of the carbon dioxide originally in the atmosphere has frozen on the planet’s night side, leaving behind a very thin nitrogen atmosphere.  Because this process resulted in the thinning of the atmosphere, which in turn increased the planet’s temperature gradient, there was a positive feedback effect where only a slight initial tendency for CO2 to snow down on the Siluna’s night side eventually led to the state it is in today. 

Like Hermes, Siluna is close enough to Xenosulia that it appears like a moon from the planet’s surface. From Xenosulia, the planet appears about as large as the Moon from Earth at its closest approach, where it appears as a crescent when it’s lit enough to be visible. Its apparent size varies dramatically, however, and it can appear far smaller when it’s fuller and further away from the planet. Because of Siluna’s intense winds, sandstorms are common, and global sandstorms can occasionally be seen obscuring the surface. The CO2 ice cap is never visible, since it’s always on the planet’s dark side. As an inferior planet, closer to Limax than Xenosulia, it – along with Hermes – is impossible to observe too far from the planet’s day side. 

Unlike Vulcanus and Hermes, Siluna isn’t named after a deity. The name actually comes from the Sukian phrase si luna, meaning “cheese moon”; this is a reference to its yellowish colour and the fact it looks like a moon from the surface of Xenosulia. It also alludes to myths of the Moon being made of cheese. No authority gave it this name, and it’s unknown who coined it; it’s just a term that grew in prominence in colloquial speech in the early days of human settlement, and eventually became accepted. 

Properties

Orbital
Semimajor axis: 4.42 million km
Orbital period: 4.61 days
Rotational period: synchronous 

Physical
Mass: 0.70 M_⊕
Radius: 0.91 R_⊕
Density: 5.0 g/cm³
Escape velocity: 9.8 km/s
Surface gravity: 0.83g

Observational
Largest apparent size: 30 minutes
Smallest apparent size: 3.9 minutes
Maximum apparent magnitude: -9.8
Apparent magnitude at superior conjunction: -7.5

Zhimuchua 23 d (Xenosulia)

Xenosulia is the fourth closest planet to Limax, and it is on the inner edge of the habitable zone by most 21st Century definitions. The planet is about twice the mass of Earth and has liquid water on the surface, in addition to an atmosphere high in oxygen and with enough carbon dioxide to support plant life. While Earth-like, differences from Earth include the fact it’s tidally locked, the stronger winds, greater volcanic activity, and its larger size. 

Its orbit is more circular than Siluna or Oculia; since they make their closest approach to Xenosulia at the same point in Xenosulia’s orbit, but from different sides of the planet, their effects are cancelled out over time to an extent. However, Siluna’s gravitational influence on Xenosulia is greater than Oculia’s, which is enough to keep its orbit slightly eccentric. 

Unlike Siluna, it’s likely that its oceans were saved by the presence of a thick layer of hydrocarbons in its upper atmosphere early in its history. There’s evidence that the atmosphere had higher concentrations of methane in the past, which, under the influence of UV radiation from flares, would have formed larger molecules in the upper atmosphere. This organic haze would have blocked much of the star’s ultraviolet radiation and protected water vapour from photodissociation. 

In the initial years of human settlement, there was a convention of referring to the native life as “Xenosulians”, or at least those belonging to the phylum Hydratozoa. Since it was some time before taxonomical groupings were formalised, the term was used for convenience, and “Xenosulia” came to be used to refer to the planet itself as a back-formation. The term “Xenosulian” comes from the Greek word “xenos”, meaning alien or strange, and Sukian “suli”, meaning “slug”. This is likely a reference to the resemblance some of the legless lineages have to slugs, or the “squishy” feeling that even legged animals often have, somewhat reminiscent of the texture of a slug. In fact, there’s a great deal of evidence in the casual writings of early researchers that many thought of the Xenosulian fauna as “slugs with legs”. Once people began referring to the planet as “Xenosulia” the term ended up sticking, despite other names being suggested in the past. 

Properties

Orbital
Semimajor axis: 5.96 million km
Orbital period: 6.92 days
Rotational period: synchronous 

Physical
Mass: 2.2 M_⊕
Radius: 1.2 R_⊕
Density: 6.7 g/cm³
Escape velocity: 15 km/s
Surface gravity: 1.5g

Zhimuchua 23 e (Oculia)

Oculia is the second largest planet in the Limax system, and fifth in orbit from the central star. Although it has a rocky core, a large portion of the planet’s volume is made up of ice. The surface of the planet is mostly frozen, with a small liquid sea on the sunward side, although the planet does have an enormous sub-surface ocean. Lower down, pressures are high enough that the water is compressed into exotic forms of ice. The planet’s icy crust is thin enough that there are cracks on the surface, forming a similar criss-cross pattern as those seen on some icy moons like Europa. Due to tidal influences from Limax, exacerbated by its eccentric orbit, the planet experiences frequent cryovolcanism. This keeps the surface young since it’s regularly replenished, which gives Oculia its bright reflective surface.  

 
The planet appears similar in size to Hermes during its closest approach, and can be viewed from anywhere on the planet at least some of the time. Since it’s further from Limax than Xenosulia, it always appears full or gibbous, although it still changes drastically in size. It plays a significant role for Xenosulian life, as it acts as a light source for animals on the night side when it’s close to the planet, serving a similar role as the Moon does on Earth. In the visual range of Xenosulia’s native life, it can appear as bright as the full Moon. 

The name “Oculia” comes from the Latin word oculus, meaning “eye”, with the suffix -ia, used to form feminine adjectives from nouns. This is in reference to its resemblance to a human eye – the sub stellar sea being perceived as an iris – and the fact it’s often colloquially referred to as “the eye”. 

Properties

Orbital
Semimajor axis: 9.14 million km
Orbital period: 13.8 days
Rotational period: synchronous 

Physical
Mass: 3.6 M_⊕
Radius: 1.8 R_⊕
Density: 3.4 g/cm³
Escape velocity: 16 km/s
Surface gravity: 1.1g

Observational
Largest apparent size: 23 minutes
Smallest apparent size: 8.2 minutes
Maximum apparent magnitude: -10.4
Apparent magnitude at conjunction: -7.3

Zhimuchua 23 g (Tuka)

Tuka is the furthest planet from Limax, although it would be well within the orbit of Mercury if it was in the Solar System. It sits outside a small, tenuous asteroid belt outside Oculia’s orbit. Tuka doesn’t mark the edge of the Limax System; beyond it, there are numerous comets and asteroids, although since the parent star is a red dwarf the system is still quite a small one. 

Tuka is a small, icy world with a tenuous atmosphere, almost as thin as that of Vulcanus. Its surface appears pinkish or reddish in colour due to the presence of numerous different hydrocarbons, formed from the interaction of solar radiation with smaller molecules like methane. 

It covers about two minutes of an arc when viewed from Xenosulia, which is theoretically just about big enough to make out a disk, but it’s still difficult to distinguish it from a star. 

While all other planets in the Limax System experience 1:1 tidal locking, Tuka actually has a 3:2 spin-orbit resonance, similar to Mercury. As such it is the only planet in the system to experience a day-night cycle. Since it lacks an atmosphere and days are extremely long, temperatures can vary drastically between day and night.  

The name “Tuka” comes from the Sukian word for dot. This reflects the colloquial name for the planet settlers initially used, calling it “the dot” in Gontanic or their respective languages. This is due to its small apparent size when compared to other planets. 

Properties

Orbital
Semimajor axis: 21.2 million km
Orbital period: 48.9 days
Rotational period: 32.6 days
Synodic day: 97.8 days 

Physical
Mass: 0.11 M_⊕
Radius: 0.66 R_⊕
Density: 2.0 g/cm³
Escape velocity: 4.5 km/s
Surface gravity: 0.24g

Observational
Largest apparent size: 1.9 minutes
Smallest apparent size: 1.1 minutes
Maximum apparent magnitude: -2.8
Apparent magnitude at conjunction: -1.6