Converting from map_gen_setting:planet_size:frequency to planet radius:
planet_size ranges from 17% min, 100% default, 600% max  (0.17, 1, 6)
percent_of_max = (6 + log(percent_of_max/6, 2)) / 6
percent_of_max converts:
17% -> 14%
100% -> 50%
600% -> 100%

planet_radius = percent_of_max * max_planet_size
max_planet_size is 10000
17% -> 1431
100% -> 5000
600% -> 10000

final formulas for scale:
planet_radius = 10000 / 6 * (6 + log(planet_size/6, 2))

planet_size = 6 * 2 ^ (planet_radius / 10000 * 6 - 6)

actual impementations use frequency:
planet_radius = 10000 / 6 * (6 + log(1/planet_frequency/6, 2))

planet_frequency = 1 / 6 / 2 ^ (planet_radius * 6 / 10000 - 6)


Planets:
  Plant tile generation follows rules set by other mods
  At a certain radius the planet-size setting kicks in and tiles spawn as out-of-map
    This makes circular planets.
  Planets have altered map-gen settings for climate: temperature, moisture, aux

Space:
  The planet-size setting should be set to something extremely large so out-of-tile are unlikely to be reached
  Space tiles spawn with extreme influence were planet-size is set to a normally unreachable setting (100)
  All non-space tiles must have an upper limit on their influence less than space tiles.

  Natural space tiles are:
    Space
    Asteroid

  Artificial space tiles are:
    Space Platform Scaffold
    Space Platform Plating
    Spaceship floor (has own layer)