You may be aware that I’m in the process of picking up a second undergraduate degree, this time in the natural sciences with a focus on astronomy and planetary science, from the Open University of the United Kingdom. Mostly this is just to round out everything I’ve learned on the subject informally across a long lifetime. I’ve also got some notion of teaching the subject myself at the undergraduate level after I retire. I expect to finish my second BSc about 2028 or so, and maybe move on to pick up an MSc if I have the resources and the world hasn’t gone utterly insane by then.
So far I’m about a third of the way through my undergraduate work. At first, a lot of the course-work was nothing but review, time-consuming but not much of a challenge.
This year’s course on “Planetary science and the search for life,” on the other hand, has decidedly not been all review. It digs into details of planetary science and astrobiology that I’ve never picked up before. I’m already picking up bits and pieces that might (for example) make their way into a second edition of Architect of Worlds.
In particular, I’m becoming quite fond of the course’s first textbook, written by a trio of Open University instructors: An Introduction to the Solar System (Third Edition), from Cambridge University Press. It’s still at the undergraduate level, but it’s very meaty. Highly recommended for anyone else who is interested in picking up a solid grounding in planetary science.
Watch this space – I may have some more recommended texts as I work through this process.
An interesting result in the current issue of Earth and Planetary Science Letters, suggesting that Earth may have had a significant ring system lasting up to 40 million years during the Ordovician period, about 466 million years ago:
The mechanism is particularly interesting, and has implications for Architect of Worlds. At present, the design sequence simply will not produce rings around a terrestrial planet comparable to Earth. In this case, the hypothesis is that a largish asteroidal body had a near-miss encounter with Earth, within the planet’s Roche limit, and broke up to form rings. Which suggests that any terrestrial planet in a system that includes at least one planetoid belt might have a temporary ring system at any given time.
I’ll have to think about this some more, but there might be some additional guidelines forthcoming to cover this case. Not to mention that a ring system would cut back on insolation and have a profound effect on planetary climate . . .
I’m currently in the process of a final editorial and layout pass on Architect of Worlds before the book gets released. For an idea of how that’s going, I’m up to page 62 out of 188, and as long as I can wrangle an hour or two in a given evening, that usually gets pushed another 10-12 pages forward.
I hadn’t planned on doing extensive rewrites of any of the existing text as part of this final pass – just polishing typos and stylistic inconsistencies, and preparing the layout for both e-book and print releases. However, I’ve recently come across some research that really asks for some revisions of the current model. (Thanks to patron Thanasias Kinias for putting me on this particular trail.)
The subject is what Architect calls Class 2 or “Dulcinea-type” worlds. These are super-Earths that have thick atmospheres dominated by primordial hydrogen and helium, and in the Architect model they almost invariably have lots of water as well. In astronomical circles, these are starting to be called hycean worlds (“hycean” coming from “HYdrogen” and “oCEAN”). It’s been one of my secret pleasures that the models used in Architect allowed for such worlds before they became a common hypothesis in real-world astronomy.
Some of my recent reading, though, tells me that Architect is probably dead wrong about some of the surface conditions of such worlds.
For one thing, astronomers modeling such worlds have suggested that they need more than just plenty of mass to hold onto that primordial hydrogen and helium. The issue isn’t simple Jeans or thermal escape (which Architect does model), but the fact that a world too close to its primary star will likely have that primordial envelope blasted away by its ultraviolet and X-ray output and stellar wind. Once the primordial atmosphere is gone, it’s not likely to be replaced by vulcanism and outgassing, so the eventual atmosphere will more closely resemble the nitrogen-carbon dioxide mix typical of a smaller world.
On the other hand, I’ve assumed all along that the primordial hydrogen and helium in the dense atmosphere of such a world wouldn’t generate any greenhouse effect. Molecular hydrogen and helium aren’t polar, so by themselves they don’t tend to be opaque to infrared light the way (e.g.) carbon dioxide or water vapor can be. Unfortunately, there is a way that a dense hydrogen atmosphere can generate a pretty significant greenhouse effect – I don’t entirely understand the physics of it yet, but in the papers I’ve been reading the effect is described as pretty pronounced.
Normally I wouldn’t be too worried about any of this, but both Architect and real-world astronomy suggest there there are a lot of super-Earths out there. Any plausibly realistic interstellar setting is going to have to contend with them. So I think I need to make some adjustments to the final release version of the text. I think the relevant steps in the design sequence are Twenty-Six, possibly Twenty-Eight, and Thirty.
One interesting thing about this change: not only should it model these hycean (Dulcinea-type) worlds more accurately, it may open the window to a wider variety of Earth-like planets. At the moment, Architect says that a world doesn’t have to be very much bigger than Earth before it starts retaining (at least) primordial helium. If I make the conditions for that a bit more restrictive, we may end up seeing more “just-a-little-bit-super-Earths” that have a fully Earthlike atmosphere. At least you’ll be able to land and walk around on them without sounding like Alvin and the Chipmunks.
So yeah, this is probably the last set of changes to the Architect design sequence before release. Which implies you’re all going to have to wait for said release to see the results, but at least that event is getting closer by the day.
Here’s a neat little bit of “new science” that I might be able to quickly build into Architect of Worlds while I continue editing and laying out the release draft.
The idea is that Jupiter, just after its formation, was probably much more luminous than it is today due to its heat of accretion. Its luminosity might have been as high as about 0.00001 times the current solar level. That doesn’t sound like much, but with the Galilean satellites (Io, Europa, Ganymede, Callisto) orbiting so close to the young, hot Jupiter, they would have undergone a period of extreme heating. It wouldn’t have lasted long – Jupiter would have cooled off and ceased to radiate so enthusiastically – but it seems to have been enough to drive off a lot of water ice and other volatiles.
Notice that Io, the closest to Jupiter, is almost free of water ice to this day. Which makes sense – in its first few million years, Io would have been getting over 30 times as much irradiation from Jupiter as it currently gets from the Sun. More than enough to melt and then boil water ices, and then drive the resulting water vapor into space. For Europa and Ganymede the effect wouldn’t have been as pronounced, which is why those moons still have plenty of ice today.
At present, the Architect of Worlds design sequence has a weird kludge in place to differentiate Io-like from Europa-like or Ganymede-like gas giant moons. It shouldn’t be too difficult to replace that with a rough estimate of a gas giant primary’s early luminosity, which (when taken with the moon’s orbital radius) will indicate how much irradiation the moon got early in its history. Particularly important for super-Jupiters, which we’ve already observed plenty of and for which the design sequence certainly allows.
I think I may also rearrange some text between Steps Sixteen (world density and surface gravity) and Seventeen (placing moons). Right now that’s the only place in the design sequence where you implicitly have to back up a step – after you place a moon in Seventeen, you may want to go back to Sixteen to determine its density and so on. Easy enough to move some of the pertinent text forward, so you can figure out a moon’s properties in the same step when you place it. That’ll also allow me to insert the new computation at a convenient place in the sequence.
Apparently some modeling work had been done to try to find the boundary between “planet-like” and “comet-like” water-rich objects. The distinction (in this specific context) is that “planet-like” objects can have atmosphere and liquid surface water, whereas “comet-like” objects can’t – they either retain water ice on their surface, or they lose their water entirely. The models pointed in the direction of surprisingly small objects falling into the “planet-like” domain – rocky planets or moons with as little as 2.7% of Earth’s mass could be “habitable” in this sense.
Naturally, that led me to raise an eyebrow, given that the Architect of Worlds design sequence is decidedly not going to give us worlds that small with liquid surface water. One of the reasons I wrote Architect in the first place was as a reaction against early planet-design sequences, in games like Traveller, which sometimes gave us those really implausible cases of worlds as small as Luna with Earthlike atmospheres and oceans. Had I been operating under a false assumption all along?
So I tracked down the actual paper: “Atmospheric Evolution on Low-gravity Waterworlds” (Astrophysical Journal, August 2019). If I’m reading this right, this is one of those cases where the Architect model probably doesn’t need to be adjusted to fit new science.
What the paper seems to be saying is that even some of these very low-mass worlds might be able to retain an atmosphere and liquid surface water. It looks primarily at the possibility of a runaway greenhouse, and at the mechanism of hydrodynamic escape for water. It doesn’t seem to address the possibility of simple thermal or Jeans escape, and it doesn’t take photodissociation into account at all. So it’s only looking at some of the mechanisms for atmospheric or water loss . . . and even so, these low-gravity worlds aren’t going to retain atmosphere or water indefinitely. What the authors have shown is that under ideal conditions, some of these small worlds may be able to retain liquid-water oceans for a while – up to a billion years or so. Which is interesting, but it doesn’t tell us anything about a long-term stable state, much less the possibility of the evolution of a local biosphere.
Architect generally assumes that the planetary systems you design with it are stable on several-billion-year timescales. Planets and systems of moons aren’t going to be crashing into each other, planetary surface conditions aren’t going to be in a state of rapid change. Which means the Architect model isn’t designed to look at edge cases like these, which are only likely to appear in very young star systems.
To astronomers, “habitable” means “there can be liquid water right now.” Which can include worlds that are not going to be at all comfortable for humans without environment suits and sealed habitats. It can also include worlds, as here, where the “habitable” state is more or less transient.
So in this case I’m not seeing the need to adjust my design sequences as they stand. It occurs to me that it might be worthwhile to provide some material on system or planetary states that aren’t long-term stable, so the reader can place some outliers. Planets that are likely to collide sometime in the next few thousand years, say, or tiny worlds like these with a surprising amount of free water on hand. For the moment, I think that’s going to be delayed until I write a second edition of the book.
I came across this article a few days ago, and it’s making me think I need to make a small adjustment to the Architect of Worlds planetary design sequence: “Astronomers identify a new class of habitable planet” (Astronomy.com, September 2021).
The case in question is one that we should all have been aware of for a while: super-Earths with very dense atmospheres dominated by hydrogen, with deep world-spanning liquid-water oceans. Architect would call these Class 2 (Dulcinea-type, after Mu Arae c) worlds with Massive prevalence of water.
The problem is that if these worlds are too warm, the current Architect design sequence quickly turns them into Class 1 (Venus-type) worlds: very hot due to a runaway greenhouse, but very dry because their primordial oceans have been boiled away and lost to photodissociation. But if I understand the physics correctly, this shouldn’t happen in these specific cases.
If a Dulcinea-type world has a rocky surface, it’s buried under many kilometers of ocean, and atmospheric heat isn’t going to bake carbon dioxide out of the rocks to cause a runaway greenhouse. Now, these worlds are likely to have a ton of water vapor in their atmospheres, and water vapor is itself a really effective greenhouse gas. But that doesn’t seem likely to boil the ocean itself away. With a really dense atmosphere, the boiling point of water soars and you can keep liquid-water oceans with surface temperatures well above 370 K. Meanwhile, these worlds aren’t going to lose their water due to photodissociation, because they’re massive enough to retain molecular hydrogen anyway. Any water vapor that gets into the upper atmosphere may break down due to high-energy sunlight, but the hydrogen won’t just fly off into space, it’ll stick around to recombine with oxygen again.
Fortunately I think adjusting for this will be an easy fix in the Architect draft, something I can do on the fly while I’m doing the rough layout. Basically, I’ll build an exception into the sequence for Dulcinea-types, forbidding them to make the usual transition to a runaway greenhouse somewhere just above a blackbody temperature of 300 K. I may need to add a provision in the procedure to compute surface temperature for these worlds – if they’re already hot, they’re going to have a fierce greenhouse due to water vapor in the atmosphere and yet will still keep their liquid-water envelope.
These strike me as odd worlds to call “habitable,” although in the scientific literature astronomers generally use that word to just mean “probably has liquid water.” You could theoretically land on one of these, but it wouldn’t be a remotely shirt-sleeve environment for humans.
The core of the Architect of Worlds design sequence is the series of steps in which the user places planets in orbit around a given star. Right now, that’s Steps Nine through Twelve:
Step Nine: Structure of Protoplanetary Disk
Step Ten: Outer Planetary System
Step Eleven: Inner Planetary System
Step Twelve: Eccentricity of Planetary Orbits
This is the section of the design sequence that’s been rewritten the most times, largely driven by the discovery of new exoplanets or new planetary systems in formation over the past few years. It works . . . but it doesn’t work well. Frankly, it’s a mess, it requires a lot of complicated and fiddly special cases, and I’m told it’s a bear to try to automate.
I’ve been thinking about doing yet another rewrite, as part of the process of producing a fully integrated draft of the book for the first time.
Now, as often happens, this gets into a peculiarity of my design process. There are times when I go for days or even weeks without writing a single word on a given project, because I’m chewing on some thorny problem. In a novel, it might be a bit of plot or character development that isn’t coming clear. In a game design, it’s a mechanic or subsystem that doesn’t want to work the way I would like. In either case, I do a convincing imitation of a writer who’s creatively blocked – but that’s not really the case. What’s really happening is that my brain is mulling over the problem with every spare cycle. Eventually, usually at the subconscious level, some inspiration comes along and I see a way forward.
I’m not quite at that point with this piece of Architect, but I think I’m getting close.
The way the system works now, you start by sketching out the mass and structure of the protoplanetary disk. Then you place planets roughly in the order in which they form – gas giants due to disk instability first, then gas giants due to rapid accretion, then rocky terrestrial worlds in the inner system. The results of each step can affect the parameters of the next, of course. That means lots of special cases where you have to put constraints on a mechanic, or where you have to fiddle with the outcome to make it fit.
This gets particularly annoying when the mechanics for planetary migration (i.e., movement inward or outward across the disk during formation) interact with the final placement of planetary orbits. Easy to get a case where you’re placing planets later in the process and you get an arrangement that interferes with planets you placed earlier on. Annoying.
So it occurred to me, possibly some night recently while I was drifting off to sleep, that I could just turn the whole process on its head. Instead of placing the young planets and then using a bunch of rules to shift them around due to disk migration and other factors, why not just do something like the following:
Determine with a few random numbers and table lookups how many planets survive the formation process in each of the three categories (disk-instability gas giants, rapid-accretion gas giants, rocky terrestrials). Assume these three categories of planets always fall in that order, outer orbits to inner.
Determine the orbital radii of the innermost planet and the outermost planet.
Space all the other planetary orbits more or less evenly in between, using a procedure that won’t generate impossible cases that have to be fixed.
Then, and only then, generate the masses of each planet.
One of the neat features of a system like this is that it can take into account things like disk migration and a Grand Tack for the system’s largest gas giant, without having to explicitly recapitulate all that evolution. If there aren’t any rocky terrestrials, that must mean that your innermost rapid-accretion gas giant migrated inward and stayed close to its primary, a “hot Jupiter.” If there are several rocky terrestrials, then that gas giant either didn’t migrate in very far, or it got pulled back outward by a Grand Tack. Done – no need to work through a several-step process, full of exceptions and special cases, to capture all the possibilities.
Hopefully this will be quite a bit easier to use. Ought to be a lot easier to automate, too. I can already hear K. Nakamura cheering, off in the distance.
I’m not quite ready to start rewriting this section of the sequence – I still need to work through some of the implications in my head first – but I might start taking a crack at it within a few days. If it works out, that will be a big step toward having a complete version 1.0 draft of the whole book that I’d be willing to share with my beta readers and patrons. Stay tuned.
Protoplanetary disk of the young star AB Aurigae. Notice the distinctive spiral-arm-like structures. One of these appears to be associated with the formation of a massive gas giant planet, at an unusually large distance from the protostar.
I’ve spent the first few days of April working with thecurrent version of Architect of Worlds, building planetary systems for nearby stars. Almost immediately, I’ve run into an issue which may be connected to recent scientific results.
It’s ironic that the process of writing Architect has been a little like doing original scientific research. The book’s main design sequence, when you get right down to it, is a big elaborate model that I hope will have predictive value, in that it will generate planetary systems that resemble what we’re seeing in the real universe. The goal is fictional plausibility, not true explanatory power, but the process of development is often the same. If I start comparing the model to the real universe (that is, to known exoplanetary systems) and the model seems unable to mimic the visible results, then there’s a problem and I need to adjust the model.
The immediate issue is that the current (v0.8) draft of the Architect design sequence assumes the core accretion model for planetary formation. That is to say, we assume that planets form in certain regions of the protoplanetary disk, when solid particles clump together and form protoplanets massive enough to start quickly accreting more material. We expect smaller rocky planets to form inside the “snow line,” in a region where water ice isn’t available. We expect gas giant planets to form outside, with the largest gas giant preferentially forming close to the line. We also play with planetary migration and the so-called “Grand Tack” model, so that the largest gas giant may move inward or outward from that initial position, but only within reasonable limits.
Our own planetary system seems to fit that model reasonably well, as do many of the other exoplanetary systems we’re aware of. There’s a catch, though. In some cases, we find what appears to be the largest gas giant forming far outside the snow line. Much further than the core-accretion model can account for, even with a generous “Grand Tack” hypothesis thrown in. Here are some examples I’ve pulled together over the past few days:
Star
Predicted Snow Line
Innermost Gas Giant
Ratio
Wolf 359
0.15 AU
1.85 AU
12.3
Proxima Centauri
0.17 AU
1.49 AU
8.8
Lalande 21185
0.56 AU
2.85 AU
5.1
Groombridge 34
0.59 AU
5.40 AU
9.2
Gliese 832
0.75 AU
3.46 AU
4.6
Epsilon Indi
1.75 AU
11.55 AU
6.6
HR 8799
8.10 AU
16.25 AU
2.0
AB Aurigae
23.30 AU
93.00 AU
4.0
Of all these cases, only HR 8799 is one that the current version of Architect could easily handle, and even that planetary system is problematic – because we know of four exoplanets there, and the one on this table is only the innermost of the four. Most of these gas giants are much further out than my current “Grand Tack” procedures could possibly account for.
Meanwhile, the masses of most of these exoplanets are a lot higher than we would normally expect for their primary stars. For example, several of these stars are low-mass red dwarfs – we wouldn’t normally expect them to generate gas giant planets at all. Some of the others have planets several times as massive as Jupiter, approaching masses more typical of brown dwarfs.
Notice the first few rows on this table are several of the stars closest to Sol. If I’m running into difficulty this quickly, that means I’m not seeing rare special cases here. There’s some way in which planetary formation just isn’t (always) working as I expect. Not the first time this has happened during the development of Architect of Worlds, and it won’t be the last.
Fortunately, there’s a new model that seems to help. That’s the so-called disk instability model for the formation of gas giant planets. Apparently, at least in some cases, gas giants don’t form close to the snow line via a well-behaved process of core accretion. Instead, especially if the protoplanetary disk is unusually dense, or if gravitational interaction from nearby stars stirs things up, the disk becomes unstable. Simulations of the process show that much of the disk can form “spiral arms” rather like those of a galaxy . . . and the result can be the rapid formation of unusually massive planets much further out from the protostar than expected.
We’ve actually imaged an example of this happening, as some very recent results show. The very young star AB Aurigae appears to be in the process of forming a massive gas giant, over 90 AU out from the protostar (the last line of the table above covers this case). This, along with some other observations, seems to lend some credence to the disk instability model for at least some planetary formation.
What this means for Architect of Worlds is that I’m probably going to need to add some material to the current Steps Nine and Ten, in which the structure of the protoplanetary disk and the arrangement of the outer planetary system are determined. I think I’ve already worked out some of the details, so I may be able to make the necessary revisions to my working (v0.9) draft within another day or two. Then I should be able to get back to the test run on which I had planned to spend the month of April.
All of which means that my patrons and other readers can reasonably expect a free v0.9 update to the main Architect document this month, along with anything else I produce.
Rings in the protoplanetary disk around the young star HD163296 (Image courtesy of Andrea Isella/Rice University)
The science of planetary formation has been advancing in leaps and bounds for the last decade or two, driven by the discovery of thousands of exoplanets and fine-detail imaging of other planetary systems. This has been giving us a lot of insight into not only the history of our own Solar System, but also the general case of planetary formation elsewhere.
With my Architect of Worlds project, I’ve been trying to keep abreast of the current science while designing a world-building system for use in game design and literary work. The current state of the system is pretty good, I think, but it’s a bit complicated. I’ve built a model that tracks the formation of a system’s primary gas giant (if any), follows that planet as it migrates inward (and possibly outward), and uses the results of that evolution to determine the mass and placement of the rest of the planets. Lots of moving parts there, and a few of the steps are kind of unwieldy.
The idea is that it wasn’t specifically the migrations of Jupiter that brought about the architecture we see of the inner Solar System. Instead, the protoplanetary disk probably had several “pressure bumps,” places where infalling particles released gases due to the increasing temperature close to the embryonic Sun. These pressure bumps tended to accumulate dust particles, and created an environment where planetesimals could form and coalesce, without continuing to spiral into the Sun. The authors of the paper predict the presence of three such “pressure bumps,” which ended up giving rise to the rocky inner planets, the gas giants, and the Kuiper Belt objects respectively.
The idea makes a lot of sense, especially since we’ve started to get fine-detail images of young stars and their protoplanetary disks, and we sometimes see exactly the system of “rings” that the model would predict. Take the image that leads the Rice University article, which I’ve included above.
Scientifically, speaking, the neat thing about this new model is that it explains several things that previous models (which assumed a more uniform disk and relied on Jupiter-migrations to make things work out) had trouble with – especially the specific isotopic composition of inner-system as opposed to outer-system material. The new model also doesn’t have any trouble producing a small Mercury or Mars, or a planetoid belt (with mixed composition) between Mars and Jupiter.
From my perspective, it may mean that I can simplify the model on which Architect of Worlds is built, making the whole thing much easier for people to use. I’m going to be reading the literature on this, and thinking about the implications.
We’ve assumed for a while that the planets of red dwarf stars are poor candidates for habitability, for a couple of reasons.
The main problem is that any planet close enough to a small, cool red dwarf star to bear liquid water is going to find itself seriously sandblasted during the star’s energetic “flare star” era. Without a strong magnetic field – itself unlikely if the planet rotates slowly because it’s tide-locked – it’s going to have a hard time retaining any atmosphere. If there’s plenty of geological and volcanic activity, an atmosphere may reconstruct itself once the primary star settles down.
The more subtle problem is that red dwarf starlight is lacking in the shorter visible-light frequencies driving the kind of photosynthesis we’re familiar with. A red dwarf may produce most of its energy output in the near infrared, which doesn’t do much for green plants. If photosynthesis has a hard time taking off, you’re not likely to get a breathable atmosphere with plenty of free oxygen in it.
The current draft of Architect of Worlds addresses both of these factors, in such a way that it’s actually quite difficult to generate an Earthlike world circling any but the most massive red dwarf stars (maybe M0 V or M1 V, at most).
The paper linked above, though, seems to indicate that this is too conservative. The authors worked with certain kinds of extremophile photosynthetic bacteria found on Earth. They subjected them to simulated red dwarf sunlight . . . and found that the bacteria carried on photosynthesis quite well. Even some of the more common bacteria they tested were able to carry on some photosynthetic activity under simulated red dwarf starlight.
This may be one of those cases where we need to account for the possibility of “life not quite as we know it” being able to exploit a niche we wouldn’t expect. Assuming a planet can retain (or rebuild) its atmosphere after the primary’s flare-star era, photosynthesis that leaves it with plenty of free oxygen in the air may not be as unlikely as we thought. I think one thing I’m going to do this month is to adjust parts of the Architect of Worlds design sequence to allow for this possibility.
