Xenosulia

Xenosulia

Xenosulia is the fourth planet in orbit around a small red dwarf star around 16 light years from Earth. Its formal designation, originally given to it by Earth based astronomers in the 38th Century, is Zhimuchua 23 d; the name “Xenosulia” wasn’t used until after settlement of the planet had begun. The planet has oceans of liquid water and abundant life. 

Habitability to humans

The planet was known to lie in the habitable zone and possess large amounts of atmospheric oxygen since the 21st Century, although this knowledge was lost until more recently. It was for this reason that it was chosen as a prime target for colonisation; this is in addition to more advanced observation methods leading to almost conclusive proof of the existence of life on this planet. 

The planet isn’t perfectly suited for human habitation, however; small amounts of sulphur dioxide and hydrogen sulphide exist in the atmosphere, in high enough concentrations to cause problems over long periods of time. Carbon monoxide levels are also high enough to cause mild carbon monoxide poisoning, and even the high oxygen concentration can prove to be an issue. Other than the atmospheric composition, temperatures and pressures are comfortable to humans, and solar flares are relatively rare compared to other red dwarf stars. 

The atmospheric problems can be fairly easily overcome with the technology that was available on Earth when the first colony ships were sent out. The high gravity is another barrier to human settlement, but it poses little difficulty to people born on the planet. 

Tidal effects 

Because of its proximity to its parent star, Xenosulia is tidally locked, so the sun appears virtually frozen in the sky. While for some planets this would lead to a much greater temperature difference between the day and night sides, deep oceans – covering most of the surface – mitigate this effect. The Neopacific is especially important in regulating the planet’s temperature, as it provides a wide area for ocean currents to transfer heat between the day and night side, uninterrupted by continental masses. However, this is far from necessary in maintaining habitability, and due to tectonic movements there have been points in the planet’s history where such a large ocean didn’t exist. 

In fact, observation of other tidally locked planets has shown that atmospheric circulation alone is usually enough to prevent run-away ocean boiling near the substellar point, as long as the air isn’t too thin. While Xenosulia does have an atmosphere somewhat thinner than Earth’s, it is thick enough that, when coupled with the planet’s oceans, the entirety of the day side is hospitable – as well as most of the night side. While the furthest parts of the planet’s night side are too cold for humans to live comfortably without heavy reliance on technology, it isn’t that much worse than Antarctica on Earth was before the 22nd Century. 

Still, the temperature gradient between the day and night sides is greater than the temperature variation on Earth, so the planet experiences stronger winds in an attempt to equalise this. These winds are exacerbated by the tidal influences of nearby planets – much more closely packed than in the Solar System – in addition to the presence of large oceans that allow for the build-up of wind speed uninterrupted by land. 

The tidal effects of Xenosulia’s parent star also lead it to be more volcanically active, which is one of the primary sources of the planet’s atmospheric sulphur dioxide and hydrogen sulphide. 

Life

The life found on Xenosulia is as varied as the life on Earth, with a wide range of different taxa exploiting almost every possible niche the planet offers. At the most fundamental level, the life on Xenosulia is quite similar to life on Earth, being carbon based and using liquid water as a solvent. While all life encountered by humans so far has used carbon based chemistry, the use of water as a solvent has proven to be far from universal. 

Looking deeper, however, reveals a number of differences. They don’t have DNA, instead using a different macromolecule previously unknown to humans known as PDP (polyanelous dimorphic polythioethers). One noteworthy difference of this DNA analogue is that it has a like-attracts-like system, as opposed to the complimentary pairs of DNA. PDP has little preference for the production of either right handed or left handed molecules, although it uses molecules of different chirality for different purposes. 

Life on Xenosulia incorporates sulphur far more in their chemistry, and phosphorus is less important than it is in Earth life. The other main elements used are the same; nitrogen, hydrogen, and oxygen. In spite of their radically different means of encoding genetic information, they have independently developed many of the same proteins, sugars and other biomolecules as life on Earth. 

Because of differences in biochemistry between Earth life and Xenosulian life, most organisms on the planet provide very little nutrition. Many of the amino acids used to build proteins on Xenosulia aren’t found on Earth,  but because there’s some overlap it’s possible to find food that can be safely eaten. However, humans have had to mostly rely on growing food introduced from Earth. 
Effects of orbiting a variable star

The red dwarf Xenosulia orbits is less active than many other red dwarfs, with flares not being much of an issue. In fact, this was one of the reasons Xenosulia was selected for colonisation. Nevertheless, it is a variable star, with frequent changes in stellar output over long spans of time. This leads to long “winter” periods where the planet is far colder than usual, which occur seemingly randomly. 

Physical properties 

Mass: 2.17 Earth masses
Radius: 7735 km
Density: 6.687 g/cm3
Surface gravity: 1.47g

Motion

Semi-major axis: 5.960 million km
Orbital period: 6.915 Earth days 
Rotational period: Synchronous 
Axial tilt: Negligible
Eccentricity: 0.0058
Orbital inclination: 14 degrees

Atmosphere 

Dry composition
Oxygen: 82.8%
Carbon dioxide: 14.9 %
Nitrogen: 1.9%
Argon: 0.4%
Methane: 0.035%
Carbon monoxide: 0.023% 
Hydrogen: 0.006%
Sulphur dioxide: 0.002%
Hydrogen sulphide: 0.001%

Atmospheric pressure: 0.64 bar

Surface temperature: 40 degrees Celsius at sub-stellar point, -50 degrees at counter-stellar point, around 18 degrees at twilight. This varies from time to time depending on the quantity of sunspots on Zhimuchua 23, with frequent “winters”. 

World Map of Xenosulia 
note; the circular lines represent separation from the sub-stellar point in 15 degree intervals. The day side is to the left of the central line, and the night side is to the right.