In this step, we will work out the implications of several of the preceding steps for the composition of the world’s atmosphere. In most cases, this will depend solely on the atmospheric class (from Step Twenty-Three). However, in the case of a Class III (Earth-type) atmosphere, we will also use quantities developed in Steps Twenty-Five and Twenty-Six to determine some of the details. This will be useful in determining whether a given Earth-like world’s atmosphere is comfortably breathable for humans – or for invented aliens.

Procedure

Apply the appropriate case from the following.

Class I Atmosphere

A Venus-type atmosphere will be composed almost entirely of carbon dioxide, very hot and at immense pressures. It will also contain a small portion of nitrogen, and traces of other compounds such as sulfur dioxide. Exposure to such an atmosphere will be almost instantly fatal to human life.

Class II Atmosphere

A Titan-type atmosphere will be composed almost entirely of nitrogen, with a small portion of methane and traces of other compounds, such as carbon dioxide and various hydrocarbons. The atmosphere itself is likely to be suffocating to human life, but not actively toxic.

Class III Atmosphere

We will deduce the details of an Earth-type atmosphere from other elements of its surface conditions.

Begin by making a note of the total atmospheric mass as determined in Step Twenty-Three. Then, after determining the partial atmospheric mass taken up by each of the following components, subtract that quantity from the total atmospheric mass, so as to keep a running tally of how much of the atmospheric mass remains to be accounted for.

Also, make a note of the total greenhouse effect as determined in Step Twenty-Five. After determining how much of this greenhouse effect can be attributed to each atmospheric component, subtract that quantity from the total greenhouse effect, keeping a running tally of how much of the greenhouse effect remains to be accounted for.

Water Vapor

A world’s atmosphere will contain a significant amount of water vapor (H₂O) if it has Moderate, Extensive, or Massive prevalence of water, and its average surface temperature is at least 251 K. If either of these conditions fails to hold, skip this component.

To determine the atmospheric mass taken up by water vapor, refer to the following table:

Water Vapor Table
Average Surface Temperature	Multiplier
251-260 K	0.001
261-270 K	0.002
271-280 K	0.003
281-285 K	0.004
286-290 K	0.005
291-295 K	0.008
296-300 K	0.010
301-305 K	0.014
306-310 K	0.019
311-315 K	0.025
316-320 K	0.035
321 K or higher	0.050

Multiply the total atmospheric mass by the multiplier from the table for the world’s average surface temperature. Then multiply the result by 0.8 (for Extensive prevalence of water) or by 0.3 (for Moderate prevalence of water). The result is the partial atmospheric mass of water vapor for the world.

To compute the greenhouse effect caused by this water vapor, evaluate the following:

Here, G is the greenhouse effect due to water vapor in kelvins, and M is the partial atmospheric mass of water vapor as determined above. Round G to the nearest kelvin (minimum 0).

At this point, round M to the nearest thousandth, then subtract M and G from the running tally of atmospheric mass and greenhouse effect before moving on to the next component.

Methane

A world’s Class III atmosphere may contain methane (CH₄) in trace amounts if it has life. Even very primitive life forms generate some methane, in tiny amounts compared to the whole volume of a world’s atmosphere. However, methane is a very effective greenhouse gas, so even traces of it can cause measurable warming.

A world’s atmosphere will contain a significant amount of methane if it has undergone abiogenesis, either in deep hydrothermal vents or on the surface (see Step Twenty-Six). If this condition fails to hold, skip this component.

To estimate the greenhouse effect caused by methane, evaluate the following:

Here, G is the greenhouse effect due to methane in kelvins, and M is the total atmospheric mass for the world. Round G to the nearest kelvin (minimum 0).

At this point, subtract G from the running tally of greenhouse effect before moving on to the next component. The actual atmospheric mass of methane will always be insignificant.

Ozone

A world’s Class III atmosphere will contain ozone (O₃) in trace amounts if it has significant free oxygen. Ozone is formed in the upper atmosphere when molecular oxygen (O₂) is exposed to the primary star’s ultraviolet radiation. Ironically, the “ozone layer” then blocks much of that ultraviolet from reaching the world’s surface. The presence of an ozone layer may be a requirement before multicellular life can safely colonize exposed land.  As with methane, ozone is a very effective greenhouse gas, so even traces of it can cause measurable warming.

A world’s atmosphere will contain a significant amount of ozone if it has undergone an Oxygen Catastrophe (see Step Twenty-Six). If this condition fails to hold, skip this component.

To estimate the greenhouse effect caused by ozone, evaluate the following:

Here, G is the greenhouse effect due to ozone in kelvins, and M is the total atmospheric mass for the world. Round G to the nearest kelvin (minimum 0).

At this point, subtract G from the running tally of greenhouse effect before moving on to the next component. The actual atmospheric mass of ozone will always be insignificant.

Carbon Dioxide

All of the remaining greenhouse effect on this world will be attributable to carbon dioxide (CO₂) in the atmosphere. Carbon dioxide is a common component of planetary atmospheres, generated through volcanic activity or the oxidation of organic matter. Carbon dioxide is not a very strong greenhouse gas, but it often exists at much higher concentrations than either methane or ozone, so it is usually the largest or second-largest contributor to a world’s greenhouse effect.

To estimate the partial atmospheric mass of carbon dioxide, evaluate the following:

Here, M is the partial atmospheric mass of carbon dioxide, and G is the remaining greenhouse effect in kelvins after the contributions from water vapor, methane, and ozone have been accounted for. Round M off to the nearest thousandth.

At this point, all of the world’s greenhouse effect has been accounted for. Subtract M from the running tally of atmospheric mass before moving on to the next component.

Molecular Oxygen

If the world has photosynthetic life, then a portion of the atmosphere will be made up of free molecular oxygen (O₂).

If the world has photosynthetic life, but has not gone through the Oxygen Catastrophe, then estimate the amount of molecular oxygen by rolling 3d6, multiplying by 0.002, and then multiplying by the total atmospheric mass. Round the result off to the nearest thousandth.

If the world has gone through the Oxygen Catastrophe, then estimate the amount of molecular oxygen by rolling 3d6+15, multiplying by 0.01, and then multiplying by the total atmospheric mass. Round the result off to the nearest thousandth.

In either case, the result will be the partial atmospheric mass of molecular oxygen. Subtract that figure from the running tally of atmospheric mass before moving on to the next component.

Helium

A world whose M-number (computed in Step Nineteen) is 4 or less will have helium (He) in its atmosphere, left over from the world’s original formation. To estimate the amount of helium, roll 3d6, multiply by 0.025, and then multiply by the total atmospheric mass. Round the result off to the nearest thousandth.

The result will be the partial atmospheric mass of helium. Subtract that figure from the running tally of atmospheric mass before moving on to the next component.

Argon

Almost every world with a Class III atmosphere will have some argon (Ar) in its atmosphere, almost all of it generated by the decay of radioactive isotopes in the lithosphere. To estimate the amount of argon, roll 3d6, multiply by 0.001, and then multiply by the total atmospheric mass. Round the result off to the nearest thousandth.

The result will be the partial atmospheric mass of argon. Subtract that figure from the running tally of atmospheric mass before moving on to the final component.

Molecular Nitrogen

The remainder of the world’s atmosphere will be made up of molecular nitrogen (N₂), possibly with a few traces of other, more exotic non-greenhouse gases. Make a note of the remaining atmospheric mass, rounding off to the nearest thousandth. The result will be the partial atmospheric mass of molecular nitrogen.

Feel free to list all of the atmospheric components determined above, in order by partial atmospheric mass. This list will give you or your readers a quick way to determine how congenial the atmosphere is for human use.

Converting Partial Atmospheric Masses to Partial Pressures

To determine the partial pressure (in atmospheres) for any atmospheric component at a world’s surface, multiply the partial atmospheric mass for that component by the world’s surface gravity.

Evaluating Human Breathability

In order for humans to comfortably breathe an atmosphere, it must meet all of the following criteria:

• Molecular oxygen at a partial pressure of 0.190 to 0.240 atmospheres. Below this range, a respirator is required. Above the range, a filter or “reducing respirator” will be needed.
• Carbon dioxide at a partial pressure no higher than 0.015 atmospheres. Above this range, humans will suffer respiratory difficulties and may need a filter.
• Molecular nitrogen at a partial pressure no higher than about 4 atmospheres. Above this range, humans will suffer from nitrogen narcosis and eventually lose consciousness. A filter or reducing respirator will be required.

The other likely components of a Class III atmosphere are unlikely to have any toxic or suffocating effect, as long as the above criteria are all met.

Class IV Atmosphere

A Mars-type atmosphere will be composed almost entirely of carbon dioxide, with traces of nitrogen, argon, and other compounds. The atmosphere itself will be extremely thin and suffocating to human life, almost as bad as high vacuum, but it will not actively toxic.

Class V Atmosphere

A Luna-type atmosphere is vanishingly thin and may be composed of whatever particles of the interplanetary medium happen to be captured in the world’s gravity well at the moment. It will be an effective vacuum, quickly fatal to exposed human life.

Examples

Both Arcadia IV and Arcadia V have Class III atmospheres, so Alice prepares to work through the process for each of them. She is hoping that the atmosphere of Arcadia IV will turn out to be human-breathable. The atmosphere of Arcadia V will almost certainly not be, since the planet has no native life, but she will see what results occur for the sake of completeness.

Arcadia IV

This planet has a total atmospheric mass of 0.900, and a total greenhouse effect of 32 kelvins. Alice begins to work through the list of potential atmospheric components.

• Water Vapor: Alice refers to the Water Vapor table and determines that the multiplier will be 0.005. She multiplies the total atmospheric mass of 0.9 by 0.005 and then by 0.8 (for the world’s Extensive water) for a partial atmospheric mass of 0.0036. Using the equation to determine the greenhouse effect due to water vapor, she finds that it comes to 21 kelvins (rounded off). She rounds the partial atmospheric mass of water vapor off to the nearest thousandth and corrects her running tallies. She now has atmospheric mass of 0.896 and greenhouse effect of 11 kelvins to account for.
• Methane: Arcadia IV definitely has life, so there will be traces of methane in the atmosphere. Using the equation, she estimates that 2 kelvins of the greenhouse effect will be due to this greenhouse gas. She corrects her running tallies, and now has atmospheric mass of 0.896 and greenhouse effect of 9 kelvins to account for.
• Ozone: Arcadia IV has gone through the Oxygen Catastrophe, so there will be traces of ozone in the atmosphere. Using the equation, she estimates that 1 kelvin of the greenhouse effect will be due to this greenhouse gas. She corrects her running tallies, and now has atmospheric mass of 0.896 and greenhouse effect of 8 kelvins to account for.
• Carbon Dioxide: The 8 kelvins remaining of greenhouse effect implies a partial atmospheric mass of 0.004 of carbon dioxide, about the same amount by mass as found in Earth’s atmosphere. Alice updates her tally of atmospheric mass, finding that she still has 0.892 to account for.
• Molecular Oxygen:As already noted, Arcadia IV has already gone through the Oxygen Catastrophe. Alice rolls 3d6+15 for a total of 28, multiplying this by 0.01 and then 0.9 to get a partial atmospheric mass of 0.252. She updates her tally of atmospheric mass, finding that she still has 0.640 to account for.
• Helium: The planet’s M-number is 5, so there will be no significant helium in the atmosphere. Alice skips this component.
• Argon: Alice rolls 3d6 for a total of 16, multiplies that by 0.001 and then 0.9, and rounds the result off to 0.014 for the partial atmospheric mass of argon. She updates her tally of atmospheric mass and still has 0.626 to account for.
• Molecular Nitrogen: The last 0.626 of atmospheric mass will be composed of molecular nitrogen and traces of other non-greenhouse gases.

Alice multiplies all of these partial atmospheric masses by the planet’s surface gravity of 1.05, and arranges them in order from largest to smallest:

• Molecular Nitrogen: 0.657 atmospheres
• Molecular Oxygen: 0.264 atmospheres
• Argon: 0.015 atmospheres
• Carbon Dioxide: 0.004 atmospheres
• Water Vapor: 0.004 atmospheres

The atmosphere of Arcadia IV looks reasonably close to human-breathable, although it actually has a little too much oxygen in the mix. Humans living there may be able to breathe without assistance for a while, and may even find the air exhilarating, but they are likely to suffer various forms of oxygen toxicity over the long term. The oxygen-rich atmosphere might also encourage fires.

Arcadia V

This planet has a total atmospheric mass of 0.700, and a total greenhouse effect of 17 kelvins. Alice begins to work through the list of potential atmospheric components.

• Water Vapor: The planet’s average surface temperature is only 221 K, well below the minimum to have any significant water vapor in its atmosphere. Alice skips to the next component.
• Methane: Arcadia V has no native life, and so no significant amount of methane.
• Ozone: Arcadia V has no photosynthetic life and so no ozone.
• Carbon Dioxide: Apparently the planet’s 17 kelvins of greenhouse effect are entirely due to carbon dioxide in the atmosphere. That amount implies a partial atmospheric mass of 0.033. Alice updates her tally of atmospheric mass, finding that she still has 0.667 to account for.
• Molecular Oxygen:No photosynthetic life, and so no free oxygen in the atmosphere.
• Helium: The planet’s M-number is 6, so there will be no significant helium in the atmosphere.
• Argon: Alice rolls 3d6 for a total of 9, multiplies that by 0.001 and then 0.7, and rounds the result off to 0.006 for the partial atmospheric mass of argon. She updates her tally of atmospheric mass and still has 0.661 to account for.
• Molecular Nitrogen: The last 0.661 of atmospheric mass will be composed of molecular nitrogen and traces of other non-greenhouse gases.

Alice multiplies all of these partial atmospheric masses by the planet’s surface gravity of 0.82, and arranges them in order from largest to smallest:

• Molecular Nitrogen: 0.542 atmospheres
• Carbon Dioxide: 0.027 atmospheres
• Argon: 0.005 atmospheres

As expected, the atmosphere of Arcadia V is completely unbreathable, and in fact the high partial pressure of carbon dioxide would be mildly toxic to any human who made the attempt. Visiting humans will need full respirators.

In this step, we will determine whether the world has native life, the nature of that life, and how complex it has become at the present time.

The evolution of complex life forms takes time, and also requires that local conditions be congenial. We will estimate the amount of time it requires for seven specific evolutionary steps to take place, affected by the world’s other properties. In the process, we will sketch out the world’s evolutionary history.

Procedure

Begin by noting the current age of the star system, as established in Step Four. For any of the seven following events, if the total time to the event is greater than the star system’s age, then that event has not yet taken place on the world under development. Any further events that depend upon that one can be ignored.

Abiogenesis (Deep Hydrothermal Vents)

If all of the following conditions are true for the world under development:

• Class II, Class III, Class IV, or Class V atmosphere
• Moderate, Extensive, or Massive prevalence of water
• Molten, Soft, Early Plate, Mature Plate, or Ancient Plate lithosphere
• World does not have Fixed Plate Tectonics

Then roll 3d6, multiply by 30 million years, and make a note of the result as the time to deep abiogenesis.

If the age of the star system is greater than this, then the world has native life which derives energy and nutrients from volcanic vents at the bottom of its oceans.

Note that the world does not require exposed liquid-water oceans in order for this form of life to appear. For example, there is speculation that life may exist on Jupiter’s moon Europa, in a large liquid-water ocean beneath the icy surface.

Abiogenesis (Surface Refugia)

If all of the following conditions are true for the world under development:

• Class III or Class IV atmosphere
• Moderate or Extensive prevalence of water
• Average surface temperature is at least 265 K
• Lithosphere is Molten, Soft, Early Plate, Mature Plate, or Ancient Plate
• World does not have Fixed Plate Tectonics

Then roll 3d6, multiply by 100 million years, and make a note of the result. Also, if the world has undergone deep abiogenesis (see above), roll 3d6, multiply by 75 million years, add the time to deep abiogenesis, and make a note of that final result as well. Take the lesser of these two times and make note of it as the time to life in surface refugia.

If the age of the star system is greater than this, then the world has native life which derives energy and nutrients from sources in shallow water or on exposed land. Very early on, this life will require special conditions that sometimes occur in shallow oceans, on beaches, or in fresh-water ponds. These refugia form the basis for all later expansion of life across a world’s surface.

Special Case: Lifeless Worlds

Note that all of the following evolutionary events are dependent on the presence of life in either the deep oceans or in surface refugia. If a world doesn’t meet the criteria for either of the above events, then it will be barren and effectively lifeless. It’s possible that some hardy form of bacterial life clings to existence, but it will be difficult to detect and will have no significant effect on the world’s environment.

Multicellular Life

If the world has undergone deep abiogenesis (see above), then roll 3d6, multiply by 75 million years, and then add the time to deep abiogenesis. Make a note of the final result as the time to multicellular life.

If the age of the star system is greater than this, then the world’s native life has developed the ability to form multicellular organisms. These organisms will likely be simple and primitive for a very long time, but they will form the basis for all future complex ecosystems based on plant and animal life.

Photosynthesis

Photosynthesis is critical to the development of a complex surface ecology for a world, such as exists on Earth. Early forms of photosynthesis are likely to appear very soon after the development of surface refugia. However, the form of photosynthesis with which we are most familiar – the so-called oxygenic photosynthesis that releases free oxygen as a by-product – takes much longer to evolve.

In any case, the difficulty of developing photosynthesis is strongly dependent on the color of light that a world receives from its primary star.

Stars similar to Sol produce most of their radiation in the visible-light range, which passes easily through the transparent atmosphere of an Earth-like world. Smaller and cooler stars (K-class and especially M-class) produce more of their radiation in the infrared bands, which have more trouble penetrating an atmosphere. Furthermore, the chemical pathways involved in photosynthesis are most effective at certain visible light wavelengths. A world with a small, cool primary star receives less such light, and may take much longer to develop photosynthetic life.

If the world has life in surface refugia (see above), and its primary star is not a brown dwarf, then refer to the following table:

Photosynthesis Table
Primary Star Class	Photosynthesis Development Timescale
A, F, or G0-G7	100 million years
G8-G9	105 million years
K0	110 million years
K1	115 million years
K2	120 million years
K3	130 million years
K4	145 million years
K5	160 million years
K6	180 million years
K7	210 million years
K8	240 million years
K9	270 million years
M0	300 million years
M1	360 million years
M2	480 million years
M3	600 million years
M4	800 million years
M5-M9	1 billion years

Roll 3d6, multiply by the Photosynthesis Development Timescale entry for the spectral class of the world’s primary star, and then add the time to life in surface refugia. Make a note of the final result as the time to photosynthesis.

If the age of the star system is greater than this, then the world’s native life has developed the ability to synthesize food by using sunlight to drive the necessary chemical reactions. Organisms which develop this trait will establish all of the world’s families of plant life.

Oxygen Catastrophe

Although the development of oxygenic photosynthesis opens many new opportunities for a world’s ecology, it can also be immensely harmful for existing life. Life forms which arose in the absence of oxygen will often find it to be actively poisonous. The period of Earth’s history during which free oxygen became prevalent is sometimes called the Oxygen Catastrophe.

After oxygenic photosynthesis develops, oxygen will be released by photosynthetic organisms, but at first that oxygen will tend to be dissolved in the oceans, or it will combine with exposed materials on land. It may take a long time for photosynthesis to build up significant free oxygen in a world’s atmosphere.

If a world has developed photosynthesis (see above), roll 3d6, multiply by 1.5 and by the appropriate entry in the Photosynthesis Table, and then add the time to photosynthesis. Make a note of the final result as the time to the Oxygen Catastrophe.

If the age of the star system is greater than this, then the world’s oceans, land surfaces, and atmosphere have all become saturated with free oxygen and oxygen compounds. Many earlier forms of anaerobic life will likely have been driven into extinction, but new aerobic forms will have arisen which can tolerate or even take advantage of the available oxygen.

Animal Life

Although multicellular life may appear quite early in a world’s history, the development of animals – complex multicellular organisms that are mobile and survive by eating organic matter – may take a very long time. The appearance of free oxygen in the environment can promote the development of animals, but it is theoretically possible for anaerobic animals to evolve as well.

If a world has developed multicellular life (see above), roll 3d6, multiply by 300 million years, and then add the time to multicellular life. If the result is longer than the time to the Oxygen Catastrophe, then subtract half of the difference. Make a note of the final result as the time to animal life.

If the age of the star system is greater than this, then the world will have given rise to complex animal life. Equivalents on Earth would be life forms that first appeared late in the Precambrian period, leading up through the so-called Cambrian Explosion about 540 million years ago. These ancestral forms eventually gave rise to all of the animal phyla that exist today.

Pre-Sentient Life

The earliest animals are driven purely by biological programming, the tropisms and instincts that they evolve to govern their behavior. Eventually, however, animal life will develop more elaborate systems for gathering and processing sensory data. Some animal species will even develop the first potential for self-aware, tool- and language-using minds. We will use the term pre-sentient for such species.

If a world has developed animal life (see above), roll 3d6. Multiply the result by 50 million years if the world has hydrographic coverage of less than 100% (from Step Twenty-Four), or by 100 million years otherwise. Add the time to animal life to compute the final result. Make a note of the final result as the time to pre-sentient life.

If the age of the star system is greater than this, then the world will have given rise to one or more classes of animal life that have advanced nervous systems and may be capable of complex learned behavior. Equivalents on Earth would be the behaviorally complex animals that first appeared late in the Triassic period, about 300 million years ago.

Examples

The Arcadia star system is 5.6 billion years old, so Alice makes a note of this as the endpoint for her generation of the evolutionary history of both Arcadia IV and Arcadia V.

Arcadia IV

Arcadia IV has a Class III atmosphere, Extensive water, and a Mature Plate Lithosphere with Mobile Plate Tectonics. It meets the criteria for deep abiogenesis. Alice rolls 3d6 for a result of 12, multiplying that by 30 million years, and she makes a note that the time to deep abiogenesis is 360 million years.

Arcadia IV also has an average surface temperature of 286 K, so the planet has liquid-water oceans and meets the criteria for surface refugia. Alice rolls 3d6 for a result of 10, multiplying that by 100 million years, and she makes a note that the first result is a full 1 billion years. She also rolls 3d6 for a result of 8, multiplying that by 75 million years and adding the time to deep abiogenesis of 360 million years, for a second result of 960 million years. The time to life in surface refugia is the lesser of the two results, or 960 million years. Apparently, Arcadia IV’s later surface life derives from forms that first evolved near the deep hydrothermal vents.

Meanwhile, Alice rolls 3d6 for a result of 10, multiplying that by 75 million years and adding the time to deep abiogenesis of 360 million years. She makes a note that the time to multicellular life is also about 960 million years. Even while some of the deep-vent life forms were colonizing the planet’s surface, other organisms were beginning to experiment with multicellularism.

Arcadia IV has life in surface refugia, so it is likely to develop photosynthesis. The primary star of the system has spectral class of K2. Alice rolls 3d6 for a result of 14, multiplies that by 120 million years, and adds the time to life in surface refugia of 960 million years. She makes a note that the time to photosynthesis is about 2.64 billion years.

From there, Alice moves on to estimate the time to the Oxygen Catastrophe. She rolls 3d6 for a result of 14, multiplies that by 1.5 and then by 120 million years, and adds the time to photosynthesis. She makes a note that the time to the Oxygen Catastrophe is 5.16 billion years. The development of an oxygen atmosphere on Arcadia IV apparently took noticeably longer than it did on Earth!

To estimate the time of appearance of animal life, Alice rolls 3d6 for a result of 14, multiplies that by 300 million years, and adds the time to multicellular life of 960 million years. Her first estimate for the time to animal life is (by an odd coincidence) 5.16 billion years, or almost exactly the same time as the Oxygen Catastrophe. She makes a note of that as well.

Arcadia IV has hydrographic coverage of about 70%. Alice rolls 3d6 for a result of 9, multiplies that by 50 million years, and adds the time to animal life of 5.16 billion years. She makes note of the result: 5.61 billion years, or slightly more than the total age of the star system. Alice decides that Arcadia IV does have complex ecosystems of plants and animals, but that the first pre-sentient species are just now beginning to appear. The planet has lots of equivalents to fish, insects, amphibians, and small reptiles, but is only starting to develop the equivalent of dinosaurs or primitive mammals.

Arcadia V

Arcadia V has a Class III atmosphere, Moderate water, and a Mature Plate Lithosphere with Fixed Plate Tectonics. Since it has Fixed Plate Tectonics, it does not meet the criteria for deep abiogenesis.

Arcadia V has an average surface temperature of 221 K, which does not meet the criteria for abiogenesis in surface refugia.

Since all of the evolutionary events that follow are dependent on abiogenesis taking place in at least one of these two locations, Alice quickly concludes that Arcadia V is effectively lifeless. There may be some form of hardy bacterial life, lurking in water deposits under the surface, but this isn’t sufficient to have any significant effect on the planet’s environment.

In this step, we will determine the world’s average surface temperature. This quantity is strongly dependent on the world’s blackbody temperature, established in Step Nineteen. However, worlds are not perfect thermal blackbodies, so this estimate will need to be adjusted.

In particular, some of the primary star’s energy input will be reflected away from the world’s surface, making no contribution to its heat budget. This factor is related to the world’s albedo, a measure of its reflectivity. Meanwhile, a world with an atmosphere containing certain gaseous components (the so-called greenhouse gases) will tend to retain some heat, warming the surface in a phenomenon called the greenhouse effect. These two factors are critical to any estimate of a world’s average surface temperature.

Unfortunately, a world’s albedo and greenhouse effect are dependent on a swarm of factors, many of which are poorly understood or beyond the scope of this design sequence. In fact, the values of these factors can change drastically over time on a single world. We will therefore determine a world’s albedo and greenhouse effect at random, or by designer choice within a range of plausible results, and then determine some of the consequences of those random choices in later steps.

Procedure

To compute a world’s average surface temperature, determine its albedo and greenhouse effect, then use these two factors to modify the blackbody temperature.

Albedo

To determine a world’s albedo at random, begin by referring to the following table.

Worlds with Class V atmospheres are a special case. The albedo of these worlds is strongly dependent on whether their surfaces are subject to volcanic activity. Even worlds with Massive presence of water, covered with thick layers of ice, may have cryovolcanoes which constantly refresh the icy surface. Check to see whether any such world falls into any of the following cases.

• If the world has a Molten or Soft lithosphere, add 0.5 to the base albedo.
• If the world has an Early or Mature Plate lithosphere, add 0.3 to the base albedo.
• If the world has an Ancient Plate lithosphere with Mobile plate tectonics, add 0.3 to the base albedo.
• If the world has an Ancient Plate lithosphere with Fixed plate tectonics, or it has a Solid lithosphere, and its blackbody temperature is lower than 80 K, add 0.3 to the base albedo.

Finally, roll 3d6, multiply the result by 0.01, and add it to the base albedo. The final result is the world’s actual albedo.

Greenhouse Effect

The greenhouse effect for a given world is measured in kelvins. The procedure for estimating the greenhouse effect depends on its atmosphere type. Apply the appropriate case from the following.

Class I Atmosphere

To determine the greenhouse effect for a Venus-type atmosphere, compute the following:

Here, M is the atmospheric mass of the world, and G is the world’s greenhouse effect in kelvins. Round the result to the nearest integer.

Class II Atmosphere

To determine the greenhouse effect for a Titan-type atmosphere, compute the following:

Here, M is the atmospheric mass of the world, and G is the world’s greenhouse effect in kelvins. Round the result to the nearest integer.

Class III Atmosphere

To determine the greenhouse effect for an Earth-type atmosphere, compute the following:

Here, M is the atmospheric mass of the world, and G is the world’s greenhouse effect in kelvins. Feel free to adjust the result by up to 1.5 times the atmospheric mass in either direction. Round the result to the nearest integer.

Class IV Atmosphere

To determine the greenhouse effect for a Mars-type atmosphere, roll 1d6-4 (minimum 0). The result is the world’s greenhouse effect in kelvins.

Class V Atmosphere

A Class-V (Luna-type) atmosphere is far too thin to create a significant greenhouse effect. The greenhouse effect in this case is always 0 kelvins.

Average Surface Temperature

With the world’s albedo and greenhouse effect established, the average surface temperature can be computed. Evaluate the following:

Here, T is the average surface temperature in kelvins, B is the world’s blackbody temperature, A is the world’s albedo, and G is the world’s greenhouse effect in kelvins.

Examples

Arcadia IV has a blackbody temperature of 281 K, a Class III atmosphere with atmospheric mass of 0.9, and Extensive water.

Alice begins by estimating the planet’s albedo. She rolls 3d6, gets a result of 11, multiplies that by 0.01 and adds it to the base albedo of 0.22. Arcadia IV has an albedo of 0.33, and so is slightly more reflective than Earth. This is most likely due to high-altitude clouds, covering a slightly greater portion of the planet’s surface than on Earth.

Alice then estimates the planet’s greenhouse effect. She rolls 3d6, gets a result of 12, multiplies that by 3 and then 0.9, and gets a final result of 32.4. She decides not to adjust this result and rounds it off to 32 kelvins, indicating a slightly weaker greenhouse effect than that of present-day Earth (a little over 33 kelvins).

Computing the planet’s average surface temperature, Alice gets:

This result is almost identical to Earth’s average surface temperature in the present day (about 287 K).

Arcadia V has a blackbody temperature of 226 K, a Class III atmosphere with an atmospheric mass of 0.7, and only has Moderate water.

To estimate the planet’s albedo, Alice rolls 3d6, gets a result of 14, multiplies that by 0.01 and adds it to the base albedo of 0.19. By an odd coincidence, Arcadia V also has an albedo of 0.33, although this is likely due to extensive sheets of ice and snow on the surface rather than high-altitude clouds.

Alice then moves on to the planet’s greenhouse effect. She rolls 3d6, gets a result of 8, multiplies that by 3 and then 0.7, and gets a final result of 16.8. Again, she decides not to adjust this figure and rounds it up to 17 kelvins. Arcadia V has a noticeably weaker greenhouse effect than Earth.

Computing the planet’s average surface temperature, Alice gets:

Arcadia V is bitterly cold, with surface temperatures averaging 221 K (about -52° C), comparable to winter temperatures in Antarctica on Earth. The planet is a little warmer than Mars, however (average surface temperature about 210 K).