This article was brought to my attention a while back: “How small is the smallest habitable exoplanet?” (EarthSky, October 2019). The basic takeaway was kind of eye-catching.

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.