In the Solar Forge the random 5 and 6 player maps (single star) each include 4 random_rich_planet fillings which should make it possible to spawn a desert planet according to the galaxy_generator.uniforms file:
--------------------------------------------
"name": "random_rich_planet",
"filling":
{
"items": [
{
"name": "terran_planet",
"probability": 3
},
{
"name": "city_planet",
"probability": 1
},
{
"name": "desert_planet",
"probability": 3
},
{
"name": "magnetic_planet",
"probability": 4
},
{
"name": "oceanic_planet",
"probability": 5
},
{
"name": "crystalline_planet",
"probability": 5
},
{
"name": "ferrous_planet",
"probability": 5
},
{
"name": "swamp_planet",
"probability": 5
},
{
"name": "gas_giant_planet",
"probability": 2
}]
}
---------------------------------------------------------------------
They are fairly low probability but I would think after 30-40 games you would have seen at least one.