Name of Planet: ʿUr, aka the World of Darkness (Derived from Mandaeism, a sect of Gnosticism.)
Distance from Earth: 125 light years
Radius: 5410 KM
Mass: 0.75 Earths
Density: ???
Atmospheric composition: Higher levels of oxygen, nitrogen, and carbon dioxide dominate it, along with a large amount of water vapor as a result of the warm temperatures this planet is currently going through.
Day-night length: 36 hours
Moons: 3. Of note is that all 3 originated from further out, in the great asteroid belt system, but were captured. Two are dwarf planets, one is around the size of Ceres, the other is around the size of Pluto, and one is a relatively small asteroid, roughly the size of Hygiea.
Life Chemical Composition: Carbon-centric, but quadruplicate DNA, strontium sulfate shells and bones, and prevalent usage of hemocyanin.
Known hazards: Heavy storms, strong seasonal changes due to an elliptical orbit, low visibility in some areas due to extreme fog, and massive earthquakes + major volcanic events becoming very prevalent as the continents are beginning to merge into one. In addition, the ratio of gases in the atmosphere is toxic to humans.
Notes: Hosted two civilizations at various points in time. The most recent one only went extinct because of a cometary impact that also marked the end of a geological era, due to the mass destruction it unleashed on the world, as all of their artifacts indicate they were in an industrial era of development. The older one is harder to gleam info on, but evidence suggests they started exploring further into the solar system as a whole, and had some capacity to alter it to some extent, with stuff like a towed asteroid found around one planet, and evidence of asteroid mining. Why they went extinct is unclear, but some of the youngest artifacts relating to them found indicate a mass plague of some kind broke out on their homeworld, that also triggered a fairly minor extinction event consisting of other species related to them in their clade, and animals + plants that they had domesticated. The few that remained in space may have attempted to return, but the population was too small to bounce back, and died off. Perhaps if this plague never happened, they would've taken over the entire system, and possibly nearby ones as well. Humanity is still disturbed by the discovery of finding two dead civilizations here.