Here is a procedure I've made to suit me. It is a portion used for the habitable zone.
Maybe it could be helpful.
https://sites.google.com/site/moukotiger/files/uwp_article.txt
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The Physical World; UWP
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I find that this hinders the suspension of disbelief for me and I have worked to create a variant that suits me better. While I would be among the first to deny that my variant is true to real world planetology, I feel that it produces worlds that follow known science more closely than the old procedure. It shows a better correlation between world size, atmospheric pressure, hydrographic percentage and average temperature than present methods.
I do not use ‘taint’ in my methods. As the only significant in-game effect of taint being a determiner for characters to wear filter/respirator masks when going outside, I feel that ‘taint’ is little more than background fluff that should be added by the worldbuilder to support the background. I prefer to see the atmosphere number as being a direct indicator of surface pressure and not of composition. Atmospheric composition is beyond the scope of this simple writing. Traveller generally treats atmosphere as being a “standard oxygen/nitrogen mix” anyways. One place where ‘taint’ can have a direct impact is by adding to the greenhouse factor of a world. However, that can be simulated easily after the worldbuilding process is complete by multiplying the temperatures by 1 plus the small amount that the greenhouse is increased. To increase the greenhouse factor by .08, CO2 released by volcanic action for example, multiply the calculated temperature by 1.08.
The UWP process described in MegaTraveller and DPG’s ‘World Builder’s Handbook’ specifically designates atmospheres as being nitrogen/oxygen mixes with possible taints for all atmospheres except vacuum, exotic, corrosive, and insidious atmosphere types. It also specifically lists the hydrographics percentage as being liquid water except for exotic, corrosive, and insidious atmospheres. The extended system generation also expects the main world to be placed in the habitable zone.
I’ve tried to avoid tables and special-case dm’s so that it should be very easy to use in a spreadsheet.
Step 1
Determine system presence, star type and size. Use your preferred method here.
The luminosity of the star(s) and the main world orbital distance are needed to calculate the ‘effective luminosity’ for the world.
Lum_effective = Lum_star/(dist_AU)^2
To be in the habitable zone, the effective luminosity should be between 1.5 and .5
Alternatively, you can just pick an effective luminosity within that range, closer to 1.5 for warmer and closer to .5 for cooler, and determine the orbital distance after picking your star, then placing the world there. Earth’s effective luminosity is ‘1‘.
From this, calculate the world’s temperature multiplier. The temperature multiplier is related to the world’s average surface temp which can be determined in later steps.
Temp_multiplier = Lum_effective^.25
This number will be used in determining the atm and hydro% UWP values
Example; A G6V star has a luminosity of .792 and the world orbits at .95 AU for an effective luminosity of .878 ( .792/(.95^2))
The temperature multiplier is .968 (.878^.25)
For comparison, Earth’s Lum_effective = 1 and Earth’s Temp_multiplier = 1
Step 2
For the basic size of the world, roll 2d6-2 for diameter in miles. If you further refine the value, keep it as a decimal number. For example, if you decide the world is 7,846 miles in diameter, then the value used in later steps would be 7.846
Decide on the density of the world in Earth_densities. This can be determined randomly as in the ‘World Builder’s Handbook or just pick a likely value. 1 Earth_density = 5.519 tonnes/m^3. For worlds in the habitable zone, this can be left as 1.
Now calculate the surface gravity. This value will be used in later steps.
Grav_surface = density * size/8
For example, this world has a surface gravity of .98 g’s ( 1 * 7.846/8 )
Step 3
For the amount of atmosphere the world has accumulated, roll 2d6-7+size. During system formation, larger bodies will accrete more material than smaller bodies. Use this, gravity and temperature to find the UWP value. Keep decimal places for more accuracy. This value will then be used to find surface pressure.
atmosphere = material_accumulated * Grav_surface/Temp_multiplier
Pressure_surface = (atmosphere^2)/49
I chose to use gravity and temperature in this manner because gravity prevents the loss of the atmosphere to space, therefore, stronger gravity means there is more gases retained. Also, surface pressure is proportional to gravity. High temperatures tend to allow gases to escape the atmosphere more easily, so the amount of gas in the atmosphere is inversely proportional to temperature and the temp_multiplier relates to the world’s blackbody temperature.
The determination of surface pressure simply gives a curve that matches ( somewhat ) the pressures given in ‘World Builder’s Handbook’ for each UWP step. A UWP value of ‘7' is equal to 1 atm of pressure.
For example; our world accreted 5.846 ( rolled 5- 7 + 7.846 )
This gives an ‘atmosphere’ value of 5.92 ( 5.846 * .98 / .968 )
For a surface pressure of ~.71 atm. ( ( 5.92^2)/49 )
Step 4
For the amount of water the world has accumulated, roll 2d6-7+size. During system formation, larger bodies will accrete more material than smaller bodies. Use this, Atm_pressure and temperature to find the UWP value. Keep decimal places for more accuracy.
hydrographic% = material_accumulated * (Atm_pressure^.5)/Temp_multiplier
I chose to use Atm_pressure and temperature in this manner because Atm_pressure prevents the evaporation of water into the atmosphere. Therefore, greater Atm_pressure means there is less water evaporated and more remaining in the oceans. High temperatures tend to allow water vapor to evaporate into the atmosphere more easily, so the amount of water remaining in the hydrosphere is inversely proportional to temperature and the temp_multiplier relates to the world’s blackbody temperature.
For example; our world accreted 8.846 ( rolled 8 - 7 + 7.846 )
This gives an ‘Hydro%’ value of ~7.70 ( 8.846 * .84 / .968 )
Water covers ~ 77% of the surface
Step 5
Determine average surface temperature.
Knowing the atmosphere value and hydrographic % can allow us to approximate the world’s albedo and greenhouse values. With this information, we can estimate the average surface temperature.
Temp_surface = 286 * (1-albedo)^.25 * Temp_multiplier * greenhouse value
Hydrographic % can be used to estimate a world’s albedo.
cloud% ~ hydro% * .7 cloud_albedo ~ .6
land% ~ 1-hydro% land_albedo ~ .15
sea% ~ hydro% sea_albedo ~ .05
albedo_world ~ (cloud% * .6) + (( 1-cloud% ) * ( land% * .15 + sea% * .05 ))
Our sample world has a cloud% of .77 * .7, or 54%, a land% of 23% and a sea% of 77%
Its albedo ~ .324 + ( .46 * ( .0345 + .0385 ), or ~ .356
Atmospheric pressure and hydrographics % can be used to estimate the base
greenhouse factor.
greenhouse_world ~ 1 + ((Atm_pressure^.5) * .05 ) + ( hydro% * .1)
Our sample world has a greenhouse factor of 1 + .042 + .077, or ~ 1.119
Our sample world’s average surface temperature is
286 * .968 * (( 1 - .356 )^.25) * 1.119 or ~278K
x-868xxx-x
very much like Earth, but with a slightly thinner atmosphere
There are a few things to keep in mind about this procedure.
This procedure rapids breaks down when used outside of the habitable zone and can give ‘odd’ results if used in the inner, middle or outer zones. This is because of the importance of liquid water in determining the composition of a world’s atmosphere.
Liquid water is important for removing CO2 from the atmosphere as part of the carbonate-silicate cycle where the CO2 is locked into the crust. If the CO2 that is locked in the crust of Earth were released, Earth would have a CO2 atmosphere with a pressure in excess of 60 atm. Because water is kept close to the surface by the ‘cold trap’ where temperatures force water vapor to condense in the troposphere, relatively little water is lost by UV disassociation. This disassociation allows for the hydrogen to escape the atmosphere. The height of the ‘cold trap’ can be estimated by use the adiabatic lapse rate of the atmosphere.
Venus’ ‘cold trap’ is at a very high altitude which allowed for the hydrogen to be lost after the water is broken down by UV rays. The oxygen recombined with other elements. With the water being lost, the carbonate-silicate cycle was broken and the CO2 remained in the atmosphere. Runaway greenhouse pushed the cold trap higher.
On Mars, in the middle zone, the carbonate-silicate cycle was broken by the fact that water is frozen into the crust. Liquid water does not exist to remove the CO2 from the atmosphere, thus CO2 is the main component of the atmosphere of Mars.
Type ‘M’ stars might not have a habitable zone, and for those that do, the world will be tidally locked. A habitable zone around a type ‘M’ star will be extremely close to the star and being tidally locked will mean that the magnetosphere will be very weak if it exists at all; the atmosphere will be eroded away by the solar wind.... and then there are the flares.
These red dwarfs will have huge convection zones which will give them very active with strong magnetic fields. This will cause them to flare more often and more violently than the Sun.
For type ‘K’ stars, any world in the habitable zone will most likely be tidally locked.
