Rynomed is an Earth-like planet with a very tropical climate, not too different from Earth during the Jurassic or Cretaeceous period, meaning even the poles have temperate to subtropical temperatures. The planet has 1.7 Earth masses and a gravity of around 1.28g. The atmosphere is 1.8 bar at sea level with 28% Oxygen, which is a toxic amount of Oxygen for humans (Yes, oxygen toxicity is a real thing). Rynomed is orbited by two moons named Sinus and Vector, which also lead to very difficult to predict tides that can be either weak or very destructive.
While the planet hasn't been very relevant in Galactic politics since it is mostly uninhabited due to the atmosphere, it has gained more and more relevance. In my setting there are only 20 Earth-like planets with biospheres in the entire Galaxy and around half of them are dying due to war, industry and rogue bioweapons, which makes Rynomed an increasingly interesting location for multiple factions.
The history of this world has many ups and downs, most importantly it was one of the first habitable exoplanets discovered by humans and millions of settlers rushed to it in order to escape the struggling planets of Earth and Mars. This is why most landmarks are named after historically significant humans such as scientists or phylosophers. Unfortunately many of these people came very unprepared. Cities grew fast, the largest one having been Newton on the southeastern coast of the Socrates continent (The rift was later named after the city). However most people underestimated the danger of the oxygen levels. Blindness, misscarriages and deaths piled up over the years until the first alien diseases cought on and started killing people like wild fire. Most were evacuated and most of those who stayed died off before a sufficient amount of vaccines was able to be produced and distributed.
However some people who either got lucky or had a random mutaiton to be immune against the worst diseases, survived. These people travelled up the mountains to higher elevation where the oxygen is more managable due to the decrease in pressure. The giant, flat and dry plateau along the Socrates mountains is where 90% of humans live on Rynomed. The area is approximately the size of China. The total number of a permanent population is just short of 300.000. At the peak of colonization the population was at around 16 million. All original cities now lie abandoned, the surviving people were originally thought of as dead, and the planet has been left alone for decades before a united Earth returned to it. Due to the human population, the State of Terra (main human faction) claims the entirety of Rynomed for themselves, however their actual military control is limited to the orbit and the moons.
The biosphere of Rynomed is ancient. Reptile-like animals walked on land before multicellular life even kicked off on Earth. Most life forms on this world resemble reptilians or early synapsids with scaly hide, four limbs and primarily egg-laying. The biochemistry shares a lot of similarities to Earth: It is carbon and water based and is made out of levo (left-handed)-amino acids. The main difference is that life on Rynomed has a slightly different set of amino acids. Earth life has 20, Rynomed lfie has 22 with four of them not being found naturally on Earth. This makes local food sources (especially proteins) very difficult to digest for humans, not necissarily toxic, but of low nutritional value without proper gut bacteria.
Over the hundred years of isolation, the local human population adopted native bacteria into their gut, which helps them break down the alien proteins, allowing them to gain more nutritients out of native food sources. However the majority of their diet still consists of remnants of what their ancestors brought to this planet: Potatoes. Yes, the potato is a very important food source here since it can easily grow in the poor soil of the mountainous regions where people live. In fact it has become an invasive species and multiple parts of the planet where humans no longer live faced mutliple extinctions since potatoes outcompeted a few native plant species.
The high oxygen content in the atmosphere is excellent for forest fires, and the regions that receive lower than moderate amount of rain have long since burned down completely. The regions with a more humid climate are home to thousands of different plant species specialized to grow and reproduce fast, with very deep roots made to endure long periods of time. With all the oxygen, fire can spread rapidly throughout a rainforest even during a rain storm, so many plants simply let the fire eat through the top layers, while the roots remain secure beneath the ground, allowing them to quickly grow anew once the fire subsided. Due to heavy volcanic activity along many tropical regions, the soil is extremely fertile, allowing forests that burned down to recover noticably in mere months.
The fire also affects the fauna. Many animals are either cave dwellers, burrowers or semi-aquatic, seeking shelter in rivers and lakes during forest fires. Trying to outrun the fast spreading fire in a dense rain forest is nearly impossible, making it a non-optimal survival strategy outside of open plains.
The oceans are home to many many different lineages of aquatic reptile-like animals, but also to many massive fish species. The warm to moderate temperatures along with high amounts of oxygen being diffused into the water despite the temperature, allows gill-breathers to maintain a high metabolism, while still being able to breathe perfectly fine, making the oceans extremely competitive across the globe. Invertebrates also grow to increadible sizes, mostly in water. Due to the gravity on land, there are very few species capable of flight and those that are are very small relative to Earth analogues.
The novel I mention in the title will follow a soldier from Earth who, due to an accident, got stranded along the western side of the continent of Curia just south of the Rankine peninsular. The story is about his struggle for survival, exploring an alien biosphere, being hunted by angry lizards, finding remannts of the colonial past, all while slowly being killed by the very air he needs to breathe, while other factors slowly eat away his mental state. I don't want to reveal too much, especially since I am not that far in writing it, but I do have many plans. I should also note that the world does not exclusively exist for the novel, the novel is just a small part of the larger setting.
I would be happy to read some of your guys' thoughts. Feedback is appreciated.