Hey, something I actually know a thing about.
The way the procgen for planet conditions works is that each time it's called to do something it looks at a table and 'draws' what are essentially 'cards' from a deck composed of number in that table.
For most conditions, the majority of these 'cards' are blank - ie; the planet gets no condition for that particular trait/resource etc.
Inimical Biosphere is one of the simplest conditions. It's binary. It's either present or not.
It also doesn't have any exclusions, so it can appear anywhere as long as it's specified in the table.
The first planet type in the list is Gas Giant.
For I. Biosphere the gaint category has listed under it:
biosphere_no_pick: 500
inimical_biosphere: 1
This means that whenever a Gas Giant is created by the procgen, it has a 'deck' of 501 cards, and only 1 of those is I. Bioshpere. All the rest are blank.
So a gas giant has a chance of 1/501 =
0.002% 0.2% of having Inimical Biosphere.
Chance for Inimical Biosphere, all planet catagories (and exceptions) are:
Gas Giant: 1/501 =
0.002% 0.2%
Barren: 1/1001 =
0.001% 0.1%
Rocky Unstable: 0/0 = 0% (Blank entries - effectively zero)
Rocky Ice: 0/0 = 0% (Blank entries - effectively zero)
Barren Desert: 1/101 =
0.01% 1%
Volcanic: 1/1001 =
0.001% 0.1%
Frozen: 1/101 =
0.01% 1%
Cryovolcanic: 1/101 =
0.01% 1%
Irradiated: 1/10001 =
0.0001% 0.01%
Toxic: 1/1001 =
0.001 0.1%
Terran: 0/0 = 0% (Blank entries - effectively zero)
Terran Ecc.: 1/11 =
0.1% 9%
Arid: 1/11 =
0.1% 9%
Water: 1/11 =
0.1% 9%
Tundra: 1/11 =
0.1% 9%
Jungle: 10/20 = 50% (This is the only specific planet type which has an override/exception to its category for this condition.)
Desert: 1/101 =
0.01% 1%
So your answer is; quite rare.
When you generated your sector, that specific planet was a 1-in-a-thousand event. The second rarest possible.
And this is all vanilla. Unknown Skies doesn't alter existing conditions, it only adds them to it's new planet types.
[e]
I left all the dumb stuff in because lol.
