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.

Some links to useful references:

Innes, H. et al. (2023). “The runaway greenhouse effect on hycean worlds.” The Astrophysical Journal, 953:2.

Pierrehumbert, P. and E. Gaidos (2011). “Hydrogen greenhouse planets beyond the habitable zone.” The Astrophysical Letters, 734:L13.