First off make a new entry for your planet in solararch,
[Solar
nickname = planet_earth_50000 <-- changed this
ids_name = 60203
ids_info = 60204
type = PLANET
DA_archetype = solar\planets\planet_earth\planet_earth_50000.sph <--and this
material_library = solar\planets\planet_earth.txm
material_library = solar\planets\detailmaps\detailmap_rock01.txm
material_library = solar\planets\detailmaps\detailmap_ocean.txm
material_library = solar\planets\atmosphere.txm
material_library = solar\planets\planet_earth\planet_earth.mat
mass = 1000000
solar_radius = 50000 <--- and this
shape_name = NNM_SM_MEDIUM_FOREST_MOON
Make a new sph file by copying any old sph file you have handy then use UTF editor and open this new sph file and find the node called radius, then edit the data in this node as a float array and changed it to 50000.000000.
Next up is the new entry in your relevant system ini file,
[Object
nickname = LI06_planet_1
ids_name = 524290
spin = 0.000000, 0.000000, 0.000000
rotate = 0, 0, 0
pos = 394, 0, 0
archetype = planet_earth_50000 <-- note this
atmosphere_range = 50500 <-- and this
burn_color = 255, 222, 160
ids_info = 66069
Then make a new death zone and your done