architect of worlds – Page 13 – Sharrukin's Palace

Architect of Worlds – Step Twenty-Five: Determine Average Surface Temperature

December 20, 2020 Sharrukin Comments 3 comments

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:

$G=5\times M$

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:

$G=3d6\times0.1\times M$

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:

$G=3d6\times3\times M$

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:

$T=(B\times\sqrt[4]{1-A})+G$

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:

$T=(281\times\sqrt[4]{1-0.33})+32\approx286\ K$

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:

$T=(226\times\sqrt[4]{1-0.33})+17\approx221\ K$

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).

Status Report (11 December 2020)

December 11, 2020 Sharrukin Comments 0 Comment

This is turning out to be a pretty busy month. Here’s the tentative plan for the rest of December:

By 14 December, finish a partial draft of the Human Destiny sourcebook for Cortex Prime, and post that so it can be reviewed as part of the Cortex Creators workshop. (Here’s a link to the current draft in Google Docs. Feel free to have a look.)
By the end of December, have a much-closer-to-finished partial draft of the sourcebook available for my patrons. That version will probably not be a finished first draft, but it should come to 15-20 kilowords, and it should be playable. This will be my charged release for this month on Patreon.
Also by the end of December, finish another piece of short fiction for free release here and to my patrons. I have a couple of candidate stories in mind.
Probably post one or two more steps in the Architect of Worlds design sequence.
Plan one or two pieces of short fiction for an upcoming anthology. More about this later, once I’m more sure that it’s going to come to fruition.
Start working to polish up a Human Destiny novella for publication via Amazon.
Work on The Sunlit Lands with what plentiful free time remains.

There’s just not enough of me to go around at the moment, given all the projects I have underway. Although that’s not a bad problem to have.

Architect of Worlds – Step Twenty-Four: Determine Hydrographic Coverage

November 20, 2020 Sharrukin Comments 0 Comment

In this step, we will determine how much of the world’s surface is covered by water, either as liquid-water seas and oceans, or as a layer of ice. We will express this hydrographic coverage in terms of a percentage. A world with 0% hydrographic coverage has no significant surface water or ice, while a world with 100% hydrographic coverage has no exposed dry land.

This is largely determined by the prevalence of water, from Step Twenty. However, in some cases the amount of dry land surface will also depend on the world’s geophysical parameters, from Step Twenty-One. Large land masses are unlikely to form unless a world has a strong lithosphere and is geologically active, creating variation in topographical relief faster than it can be worn down by weathering and erosion. Otherwise, the world’s surface is likely to be dominated by shallow oceans or ice sheets.

Procedure

Refer to the following table, and find the row corresponding to the world’s prevalence of water and current lithosphere. To determine the hydrographic coverage at random, roll dice as shown in the third column and apply the result.

Architect of Worlds: Some Minor Revisions

November 20, 2020 Sharrukin Comments 0 Comment

As I work on the next few steps in the Architect of Worlds design sequence, I’ve realized that I can save myself a lot of hassle by tweaking a couple of the steps I’ve recently posted.

So, for those of you who are following along and experimenting with the sequence as I post it, here are two minor changes you may want to consider implementing immediately. The next few steps are going to assume these modifications are in effect.

Under Step Twenty-One, add the following right after the “Status of Lithosphere” block:

Special Case: Molten Lithospheres and Prevalence of Water

If the world has a Molten Lithosphere, and its prevalence of water is not Massive, then it cannot currently support liquid-water oceans or ice sheets. Reduce the prevalence of water to Trace. This does not constitute a runaway greenhouse event. If the world’s surface cools in the future, water may appear.

The idea here is to avoid cases where the world’s surface is covered with molten lava and yet somehow has significant liquid (or even solid) water. Of course, if the world has so much water that its surface is going to be covered hundreds of kilometers deep, even a Molten Lithosphere isn’t going to be able to evaporate all of it.

Next modification: under Step Twenty-Three, beginning with the paragraph that starts “Make a list of the atmospheric components that meet both conditions . . .” and ending with the block titled “Third Case,” replace the text with the following:

Make a list of the atmospheric components that meet both conditions, and then refer to the following three cases. In each case, the world will also be assigned an atmospheric class of I through V, which will be relevant in later steps of the design sequence.

First Case

This case holds if one or more of molecular hydrogen, helium, or molecular nitrogen meet both conditions from the table.

In this case, roll 3d6 and modify the result as follows:

+6 if the world has Massive prevalence of water
+6 if the world has undergone a runaway greenhouse event
+6 if the world has a Molten Lithosphere
+4 if the world has a Soft Lithosphere
+2 if the world has an Early Plate Lithosphere
-2 if the world has an Ancient Plate Lithosphere
-4 if the world has a Solid Lithosphere
-2 if the world has a Moderate Magnetic Field
-4 if the world has a Weak Magnetic Field
-6 if the world has no Magnetic Field

If the modified dice roll is 0 or less, then the world will have a Trace atmosphere, with an atmospheric mass of zero. Otherwise, multiply the modified dice roll by:

10 if the world has undergone a runaway greenhouse event (the world will have a Class I or Venus-type atmosphere)
1 if the world has blackbody temperature less than 125 K and Massive prevalence of water (the world will have a Class II or Titan-type atmosphere)
0.1 otherwise (the world will have a Class III or Earth-type atmosphere)

The final result is the world’s atmospheric mass. Feel free to adjust this result by up to half of the multiplier.

Second Case

This case holds if the first case does not, but carbon dioxide meets both conditions from the table.

In this case, the world will automatically have a Trace atmosphere, with an atmospheric mass of zero. This will be a Class IV or Mars-type atmosphere.

Third Case

This case holds if neither the first case nor the second case is in effect (that is, none of the volatiles listed on the table meet both conditions).

In this case, the world will automatically have no significant atmosphere, and an atmospheric mass of zero. This will be a Class V or Luna-type atmosphere.

Assigning these “atmospheric classes” at this point will make several of the steps, starting with Twenty-Five, much more concise.

That’s all for now. I may be able to post Step Twenty-Four tomorrow, and we’ll see how smoothly the next few steps after that fall together.

Architect of Worlds: State of the Project

November 7, 2020 Sharrukin Comments 0 Comment

I haven’t posted any new Architect of Worlds material for a few days, but the project is still moving forward. The main issue is that I’m being required to ramp up my day-job telework to full-time status, so I need to make some adjustments to my time management. That’s being worked out, so I should be able to make progress on several of my creative projects over the next couple of weeks.

To review the bidding: since early September, I’ve written and posted the first drafts for Steps Fifteen through Twenty-Three of the design sequence. At this point, the reader should be able to get some of the broad outlines of a generated world’s surface conditions: blackbody temperature, the prevalence of water, geology, and some information about the composition and density of the atmosphere.

What’s left? Well, we still need the world’s actual surface temperatures, the prevalence of dry land if the world has oceans, the final composition of the atmosphere, and the presence and complexity of native life.

There are some complex interdependencies between those items, not to mention a bunch of special cases. In particular, I want to build in the possibility of a robust carbon cycle for a more-or-less-Earthlike world. The hypothesis is that the climate of Earth, or any similar planet with large oceans and life, will tend to self-correct over long periods. Carbon dioxide, in particular, will move into or out of the atmosphere in such a way as to keep the world in the proper temperature range for plentiful liquid water. That suggests a feedback loop that may require some special logic in the world-design sequence.

So I’ve been doing some storyboarding (kind of like what’s going on in this post from September) and roughing out the proper sequence of steps. I think I may end up with six or seven more major steps in the sequence. Seven would make a nice round thirty steps for the complete sequence, but if I end up with a weird prime number or something I suppose I’ll have to live with that.

In any case, I hope to break through the logic here and start posting the last chunk of the design sequence later this month. The third major piece of the sequence may be ready for PDF and posting to the Architect of Worlds page before Christmas.

That doesn’t mean Architect of Worlds, the book, will be ready. There’s a lot more I want to write before I’m ready to think about a publishable draft. How to plan an interstellar setting, how to work with real-world astronomical data, how to build star maps, that kind of thing. Still, I could see a first edition of the book finally making its appearance – most likely on DriveThruRPG.com – sometime next year.

Minor Site Reorganization

November 1, 2020 Sharrukin Comments 0 Comment

I’ve just finished doing some minor shuffling of pages on this site:

Architect of Worlds is now a top-level page.
The “Building a Better Barsoom” article has been filed under Free Articles and Fiction.
The Sharrukin’s Worlds page has been eliminated. All new articles will most likely be filed under Free Articles and Fiction for the time being.

The Pages widget has been eliminated from the sidebar, but you should be able to access the major sections of the site from the menu bar at the top. I’m not sure anyone was using the widget anyway.

This process may have broken a link or two, but you should be able to reacquire without too much trouble. Hopefully, this should make navigation a bit easier in the future.

A Bit of Insight

October 22, 2020 Sharrukin Comments 8 comments

I think I may have finally gotten myself unblocked with respect to one of my long-term creative projects. The project in question is the Human Destiny setting.

The premise is that sometime in the middle of the 21st century, an interstellar civilization arrives in the Sol system and (without much effort) conquers humanity. It’s a strangely benign sort of conquest, though. The aliens don’t have any interest in us as slaves, nor are they motivated by a desire to take the solar system’s natural resources for their own benefit. Their goals seem mostly to involve . . . nannying us. Their laws are fairly strict, backed up by almost-universal surveillance, but enforcement seems to be non-violent, completely incorruptible, and even-handed. Meanwhile, all of us are provided a standard of living better than ever before, without anyone being required to work for any of it.

Naturally, a lot of humans resent all this mightily, but there seems to be nothing that can be done about it. The longer-term question is why all this has happened. What motivates the aliens?

I’ve written and published a couple of stories in this setting: “Pilgrimage” and “Guanahani.” I have two or three more stories in my development pile too. I’m fairly sure there’s a robust series, maybe even a few novels, in there. Yet, even after years of cogitation, I’ve never been able to get the idea to launch.

The main problem is that the setting does away with a lot of human agency just by its premise. Great, the aliens have come along and solved a lot of our problems, including many of the ones driven by human conflict and misbehavior. There are certainly stories left to be told, but a lot of the writer’s tools for plot and character development are set aside already.

It’s probably telling that almost all the stories I’ve written in this setting so far involve breakdowns of the alien surveillance apparatus. It’s kind of like Star Trek‘s transporters – they’re so useful for short-circuiting plots that a writer often has to justify taking them off-line before a story can happen.

There’s also the aliens’ motivation. They’re here because they want us to survive and evolve into the kind of species that actually can play a role on the galactic stage. That means human psychology needs to change. We need to learn to live with each other and tolerate the Other, we need to get better at understanding and preserving the big systems that keep us alive, we need to start thinking on much larger scales in both space and time.

So how do I write stories about that, in which the aliens demonstrate their motivations through conflict and plot rather than by simply telling the reader what’s up?

I was idly thinking about this the other day – a lot of my creative work happens in the back of my mind while I’m doing something else entirely. Then my mind made a connection with what I was doing with my hands and my forebrain at the time.

I was idly playing a game on my iPad, you see.

Terraforming Mars has been out for several years as a tabletop game, and now has a pretty good adaptation as a mobile app as well. It’s one of those wonderfully thematic board games that does such a superb job of making a complex subject playable and interesting to the layman.

Terraforming Mars assumes an era of exploration and colonization throughout the solar system, starting either late in this century or sometime in the next. The centerpiece of that era is a generations-long project to, as it says on the tin, terraform Mars – transform that planet into an at least marginally habitable world, where human beings can live freely with little or no life-support equipment.

Well. Suddenly I could see a lot of possible context for the Human Destiny setting, Suppose the aliens, aside from simply providing a decent quality of life for most humans, also opened the door for this kind of expansion into the solar system? If humans could settle on Mars, cooperate with each other in a project that might not pay off for many human lifetimes, wouldn’t that be an opportunity for some of us to demonstrate the citizen-of-the-galaxy mindset the aliens are looking for?

Right away, my brain started working on ways to get my character Aminata Ndoye – the protagonist of “Pilgrimage” and a few of the not-yet-published stories – involved in Martian terraforming and solar-system expansion. That in turn gave me a whole raft of new ideas about the Human Destiny setting as a whole.

All of which is to say that I might be turning back to that project, finally. My creative plate is rather full at the moment, between working on my Krava stories, and Architect of Worlds, and wanting to flesh out the EIDOLON game system a bit more. Still, as 2020 winds down I think I might be able to revisit the Human Destiny setting, rework the core documentation for that, and start making some of that information available. Readers of this blog, and my patrons over on Patreon, can expect to see some results from that over the next couple of months.

Architect of Worlds – Step Twenty-Three: Determine Atmospheric Mass and Pressure

October 19, 2020 Sharrukin Comments 3 comments

In this step, we determine the atmospheric mass of the world under development. The atmospheric mass is measured relative to that of Earth – a world with surface gravity of exactly 1, and atmospheric pressure of exactly 1 “atmosphere” at the surface, will have an atmospheric mass of 1.

Some worlds will have a Trace atmosphere – enough to provide climate and weather effects on the world’s surface, but not enough to support any form of complex life. Still other worlds will have no atmosphere at all (or at least no atmosphere that can be detected without sensitive instruments). In both of these cases, the atmospheric mass will effectively be zero.

Atmospheric mass depends on a large number of factors: the world’s blackbody temperature and M-number (determined in Step Nineteen), its prevalence of water (determined in Step Twenty), whether it has undergone a runaway greenhouse event (also determined in Step Twenty), the degree of ongoing vulcanism (determined in Step Twenty-One), and the presence and strength of a magnetic field (determined in Step Twenty-Two).

Once a world’s atmospheric mass has been fixed, the pressure of the atmosphere at the surface (sea level or some other convenient “datum”) can also be determined.

Procedure

Begin by building a list of the likely major components of the world’s atmosphere. Refer to the following table, which lists a number of volatile compounds which might make up a large and stable portion of an atmosphere.

Atmospheric Components Table
Possible Major Component	Maximum M-Number	Minimum Blackbody Temperature
Molecular Hydrogen (H₂)	2	20 K
Helium (He)	4	5 K
Molecular Nitrogen (N₂)	28	80 K
Carbon Dioxide (CO₂)	44	195 K

For each item on the Atmospheric Components Table, check to see whether the world under development has an M-number that is no higher than the one given in the table, and a blackbody temperature that is no lower than the one given in the table. If the M-number is too high, that potential component of the atmosphere will undergo thermal escape. If the blackbody temperature is too low, that component will tend to “freeze out” and form liquid or solid layers on the surface. Either way, that volatile will not be available to make up a substantial atmosphere.

Make a list of the atmospheric components that meet both conditions, and then refer to the following three cases.

First Case

This case holds if one or more of molecular hydrogen, helium, or molecular nitrogen meet both conditions from the table.

In this case, roll 3d6 and modify the result as follows:

+6 if the world has Massive prevalence of water
+6 if the world has undergone a runaway greenhouse event
+6 if the world has a Molten Lithosphere
+4 if the world has a Soft Lithosphere
+2 if the world has an Early Plate Lithosphere
-2 if the world has an Ancient Plate Lithosphere
-4 if the world has a Solid Lithosphere
-2 if the world has a Moderate Magnetic Field
-4 if the world has a Weak Magnetic Field
-6 if the world has no Magnetic Field

If the modified dice roll is 0 or less, then the world will have a Trace atmosphere, with an atmospheric mass of zero. Otherwise, multiply the modified dice roll by:

10 if the world has undergone a runaway greenhouse event
1 if the world has blackbody temperature less than 125 K and Massive prevalence of water
0.1 otherwise

The final result is the world’s atmospheric mass. Feel free to adjust this result by up to half of the multiplier.

Second Case

This case holds if the first case does not, but carbon dioxide meets both conditions from the table.

In this case, the world will automatically have a Trace atmosphere, with an atmospheric mass of zero.

Third Case

This case holds if neither the first case nor the second case is in effect (that is, none of the volatiles listed on the table meet both conditions).

In this case, the world will automatically have no significant atmosphere, and an atmospheric mass of zero.

Surface Atmospheric Pressure

To determine the atmospheric pressure at a world’s surface, multiply the atmospheric mass by its surface gravity.

Examples

Arcadia IV has blackbody temperature of 281 K, an M-number of 5, Extensive water with no runaway greenhouse, a Mature Plate Lithosphere, and a Strong Magnetic Field. Major components of the atmosphere will include both molecular nitrogen and carbon dioxide, so the planet falls squarely into the first case. Alice rolls an unmodified 3d6 and gets a result of 9, so Arcadia IV has an atmospheric mass of 0.9. Since the planet has surface gravity of 1.05, the atmospheric pressure at sea level is abut 0.95, very comparable to that of Earth.

Arcadia V has a blackbody temperature of 226 K, an M-number of 6, Moderate water with no runaway greenhouse, a Mature Plate Lithosphere, and no magnetic field. Major components of the atmosphere will include molecular nitrogen and carbon dioxide, so this planet also falls into the first case. Alice rolls 3d6-6 (modified due to the lack of a magnetic field) for a result of 7, so Arcadia V has an atmospheric mass of 0.7. The planet has surface gravity of 0.82, so atmospheric pressure at the surface is about 0.57.

Status Report (17 October 2020)

October 17, 2020 Sharrukin Comments 0 Comment

A quick note to assure everyone that I’m still alive. I’ve been working on a lot of things this week, with the result that none of them have been pushed through to completion. Hopefully this weekend will help change that.

About finished with the next bit of Architect of Worlds, which will start establishing a designed world’s atmosphere and climate.
Still working on reviews of the second and third books in Gordon Doherty’s Empires of Bronze series, which will bring me to my goal for this first month of reviewing other people’s self-published books.
Picking out another piece of short fiction, to post here and send as a free reward for my patrons.
Also working on promotion for The Curse of Steel, and pushing forward with the first draft of The Sunlit Lands, both of which are important but aren’t likely to be visible here. Watch that progress bar, though!

Look for some results from that list over the next few days, I think.

Architect of Worlds – Step Twenty-Two: Determine Magnetic Field

October 12, 2020 Sharrukin Comments 0 Comment

In this step, we will estimate whether the world under development has a significant magnetic field.

The possible cases for a world’s magnetic field will be sorted into four categories: None, Weak, Moderate, and Strong, defined as follows.

None: The world has no detectable magnetic field and is completely unprotected from the stellar wind. Examples: Venus, Earth’s moon, Mars, most of the gas giant planets’ major satellites.
Weak: The world has a detectable magnetic field (about 1% as strong as Earth’s), but it offers no significant protection from the stellar wind. Examples: Mercury or Ganymede.
Moderate: The world’s magnetic field is strong enough to offer limited protection against the stellar wind (about 10% as strong as Earth’s). Examples: None in our planetary system.
Strong: The world’s magnetic field is at least comparable to that of Earth, sufficient to provide adequate protection against the stellar wind. Examples: Earth, the gas giant planets.

A world’s magnetic field seems to depend on several items:

The world needs to have a hot, liquid outer core of significant mass, composed largely of iron
There must be convection taking place in that iron outer core, causing rising and falling currents
The world must rotate on its axis

If all three of these conditions hold, the iron outer core forms a dynamo which creates a significant magnetic field. This, in turn, helps protect the world’s atmosphere from being stripped away by stellar wind, and also protects the surface of the world from some harmful radiation. In our own planetary system, only Earth and the gas giant planets have strong magnetic fields.

Note that the third condition – that the world must rotate on its axis – is almost universal. Even a tide-locked world still rotates on its axis, and physical modeling seems to indicate that even slow rotation is enough to support a working dynamo. The existence of strong convective heat transfer through a world’s outer core seems to be the critical factor.

Procedure

To determine the strength of a world’s magnetic field at random, roll 3d6 modified as follows:

+4 if the world has a Soft Lithosphere
+8 if the world has Early Plate Lithosphere or Ancient Plate Lithosphere and also has Mobile Plate Tectonics
+12 if the world has Mature Plate Lithosphere and also has Mobile Plate Tectonics

Refer to the Magnetic Field Table entry for the modified roll.

Magnetic Field Table
Modified Roll (3d6)	Magnetic Field
14 or less	None
15-17	Weak
18-19	Moderate
20 or greater	Strong

Examples

Arcadia IV has a Mature Plate Lithosphere and Mobile Plate Tectonics, so Alice rolls 3d6+12 and gets a result of 25. Arcadia IV has a Strong Magnetic Field.

Arcadia V has a Mature Plate Lithosphere but has Fixed Plate Tectonics. Alice rolls an unmodified 3d6 and gets a result of 13. Arcadia V has no significant magnetic field.