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.
