Eiar is the 8th and last planet orbiting Castro, in the Castro-Lux system. It is a hydrocarbonia 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.
Eiar is far too cold for anything to live on the surface without support, but it has a strangely large amount of radioactive material in its core, which allows a safer temperature to be reached in the deep caves. Life always finds a way, and eventually the Areial species evolved. The Areial have since constructed vast cities in the deep pockets beneath the surface, and they were recently contacted by the United Nations of the Flower Galaxy, allowing them to expand outwards, colonizing the entire Castro-Lux System.
1416900000 BCE: Eiar forms as a little asteroid orbiting the then-coalescing Castro.
1390000000 BCE: Eiar grows into a small planetesimal.
1305000000 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.
1199000000 BCE: Eiar collides with this planet, forming a small synestia.
1195600000 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).
1057000000 BCE: Eiar has cooled enough for the first oceans to form, initially of water.
878700000 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.
513300 BCE: the Areial begin to evolve
144000 BCE: the Areial reach their present state
56452 CE: For the first time in its life, Eiar is visited by something from a non-native species (a Marakat scout ship)
63224 CE: A drilling probe expedition breaks into a previously unknown great city on Eiar, inhabited by the Areial.
66271 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.