Yin is the fifth planet from the start Epsilon Indi. This hot O-Class planet is located just 12 light-years away from Sol. Yin is a binary planet with the hot D-Class world of Yang. Yin and Yang were discovered in 2360 by a joint FTL exploratory mission from NASA and the CNSA. Due the obvious contrary difference in the two worlds officials from both NASA and CNSA both elected to call the binary twins Yin and Yang in honor of an ancient Chinese philosophy. There is evidence that Yang had a much thicker and wetter atmosphere in the past and many marine fossils have been found in ancient seabeds. All that remains today however is a dry desert world.
Yin's high temperature and the incredibly large amounts of oxygen in the atmosphere make Yin unsuitable for prolonged Human life. Though it is a good terraforming candidate, lack of any valuable resources on the planet have made this very unlikely. In 2372 a small scientific outpost manned by 50 individuals was placed on Yin, the outpost was abandoned five years later in 2377 due to insufficient funding.
Yang has had a slightly more prosperous history than her sister Yin. In 2372 a scientific outpost was placed in an equatorial basin. Geological surveys of the planet found it to contain large quantities of Platinum and Rhodium. A gold rush headed by SepMin plc ensued in the year 2373. The mining was mostly robotic, but Yang's population peaked at 13,000 miners in 2375. By the year 2402, advances in robotic mining and exhaustion of most easily reachable resources caused a massive decline in population. However, heavy robotic mining is still done on Yang to this day.
Yin is a hot world. Though it lacks any continents, large seamounts cause chaotic currents. As a result steaming monsoons and hurricanes rage unchecked in the winter time. Yin's atmosphere is 92 percent oxygen. Yin's atmosphere is breathable by Humans for a short while, but will result in Acute Oxygen Poisoning after 3 to 4 days. During the winter time large swaths of the atmosphere ignite into fiery infernos when the storms rage. Yin is a candidate for terraforming, yet due to the harsh weather and the shift of attention onto her twin terraforming never occurred.
Yang is also a hot world. It has a thin atmosphere of CO2. Due to the thin atmosphere and lack of water currents storms on Yang are rare. It has the occasional dust storm but nothing dangerous compared to her sister Yin. Yang is much easier to live on than Yin. Interest never arose for terraforming, and the SepMin Plc never considered terraforming Yang. Instead SepMen Plc elected to use robotic mining. Yang's atmosphere is toxic to humans, and it is very hot. Exposure on the surface will result in a Human's blood boiling, and asphyxiation.