I am currently creating a planet with an atmosphere rich in noble gasses, especially neon, xenon, and argon. It's composition is roughly about this:
- Nitrogen (N2) - 61.5%
- Oxygen (O2) - 21%
- Neon (Ne) - 15.5%
- Xenon (Xe) - 1%
- Water Vapor (H2O) - 0.5%
- Argon (Ar) - 0.479%
- Carbon Dioxide (CO2) - 0.02%
- Trace - the rest
The atmospheric pressure is 0.98 atm. Most of the elements here are quite easy to get (nitrogen, water, CO2, and oxygen are common in the universe) and argon comes from the radioactive decay of potassium in the crust. However, neon and xenon are a problem.
My solution to get neon in the atmosphere was the planet to originally be a gas giant with some trace amounts of neon. The star then went into its T-Tauri phase and blew off most of the hydrogen and helium from the gas giant early in the planet's history. Neon, being heavier stayed in the gas giant. Then, a major collision moved the planet into the habitable zone and the rest of the hydrogen and helium was lost. Eventually, the rock and water from the formed a core, mantle and crust.
This works for neon since it is quite common in the universe. However, xenon is much much rarer, so how could xenon accumulate in the planet in the first place?