Eiar is the 8th and last planet orbiting Castro, in the Castro-Lux system. It is a T-class world warmed up by tidal heating from its moon, Ver. What is interesting is that Ver is actually nearly as massive as Eiar. Ver is 99.5% the mass of Eiar, making the two a binary planet. They orbit Castro every 6.9 years, and Castro from that distance is almost 1600 times dimmer than Sol is from Earth. This is why Eiar is so fricking dark when viewed from almost anywhere. Unlike other T-class worlds, Eiar also has liquid ammonia on its surface, which colours the oceans black-ish.
Eiar has an atmosphere which is exclusively made of hydrogen. We don't know why that is yet.
Eiar is only 14.3 million years old, so it formed relatively recently. Here is a brief timeline:
14169000 BCE: Eiar forms as a little asteroid orbiting the then-coalescing Castro.
13900000 BCE: Eiar grows into a small planetesimal. Right now it forms at 3.5 AU from Castro.
13396000 BCE: Eiar reaches 1/100th of an Earth mass.
13050000 BCE: Due to Lux quickly gaining mass, Eiar is forced inward and is now orbiting at around 1 AU. This makes it share orbits with another small planet, the prehistoric Ver.
11990000 BCE: Eiar collides with this planet, forming a small synestia.
11956000 BCE: This synestia rapidly cools and forms two similar-mass bodies, the new Eiar and Ver. Coincidentally, most of Eiar's mass today originally was the prehistoric Eiar, and the same can be said for Ver (most of present-day Ver is from prehistoric Ver).
10570000 BCE: Eiar has cooled enough for the first oceans to form, initially of water.
8787000 BCE: Due to even more cooling, Eiar's oceans now froze, and hydrocarbons such as methane and other molecules such as ammonia rain down, replacing the water oceans. Ver is now completely analogous to how it looks today.
2645 CE: For the first time in its life, Eiar is visited by something from a sentient species (Humans).
2902 CE: The first human steps foot on Eiar.
3627 CE: The first domed city is built on Eiar.
Due to being in a binary planet, Eiar has some very interesting geological features.
Sometimes looking at Eiar, you can see what looks like claw markings. These are actually pools of moving methane centered on elongated valleys caused by tidal squeezing from Ver, and they are slightly eroded, making the pools harder and harder to drain out. They are still like this to this day.
There are many patchy areas around the planet. It is not exactly known why these exist, but it is thought to have been the interaction between the ground and the exclusively-hydrogen atmosphere.