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: 0.765 Earths
Density: ???
Axial Tilt: ???
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.
Other notes on life here: Sequential hermaphroditism is the prominent form of sex determination on this planet. However, compared to Earth, it is far more extreme, and individuals of a single species are known to flip between female and male sexes rapidly, based on various criteria, be it seasonality, individuals in the area, social cues, etc. Disease can also disrupt these systems, however, and cause events such as individuals changing sex out of a specific season, in species that change sex based on season. These organisms are also polypoid, and have 4 pairs of all chromosomes, making them tetraploids.
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.

The first civilization emerged during the time of the mid Jurassic on Earth, the second in the mid Eocene on Earth.

Major clades:

"Floaters" - Massive colonial organisms that spend most of their time in the air, or in gigantic, pulsing masses on the ground, they share a common ancestor with the organisms considered analogous to animals on Earth, but this ancestor was before both became multicellular. They appear to be mixotrophic, with each colony having various members dedicated to doing specific tasks. The ones that constitute the topmost layers of their body can photosynthesize, and the ones at the bottom can either inflate the organism's hollow interior over time with gases from the surrounding area, eventually allowing it to gain lift, due to the fairly dense atmosphere, or they can be repurposed into mouths to feed on other floaters and animals. The insides are filled with cells that can easily form a makeshift stomach extremely quickly, with flagella-like structures that can keep a prey item in place while the other cells drain it dry. They do not fossilize well, due to having no hard parts, but evidence suggests they are extremely ancient, and have survived out many, many mass extinctions, even ones that would put an organism like this in danger. Unlike their animal relatives, they do not reproduce sexually, but instead, grow buds that form off into new individuals. While they can't have actual sex, they can exchange cells between colonies, in a process analogous to horizontal gene transfer. As they are fairly slow, even in the air, they are often preyed on by animals, but some have unique relations with them, such as some floater species having symbiotic relationships for protection. Aquatic species exist as well, and it is believed the current line of species that all exist are descended from an aquatic ancestor, due to evidence of a gamma ray burst during the Cambrian of Earth that hit this planet. It also seems they have genes that show this, with excessive amounts of antioncogenes being common in these organisms, as well as extremely convoluted mechanisms to destroy diseased or cancerous cells.

??? - Animal analogues. At least two species, in the past, have become sophonts of varying degrees. Like floaters, they have likely repopulated land and sky multiple time as a result of extreme mass extinction events being more common simply due to the planet probably being able to remain far more viable for life than many others, as a result of their host stars. While not as resilient against cancer like their weird floater distant cousins, they too, seem to have excessive amounts of antioncogenes, and various other mechanisms for handling cancer far more aggressively. However, as a result, autoimmune diseases are also not unknown here and there.

??? - One clade of plant analogues