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Alone in a Crowded Milky Way

Alone in a Crowded Milky Way

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Enrico Fermi

I don’t usually post just to link to articles, but this one was particularly intriguing: Alone in a Crowded Milky Way.

Executive summary: the author and his colleagues did some modeling of the expansion of interstellar civilizations through a segment of the galaxy. They made some fairly conservative assumptions – STL travel only, how many planets would be worth colonizing, how long a given planetary civilization would be likely to survive. The result was that space-faring cultures tended to form “archipelagos” in interstellar space, leaving vast regions unvisited for many millions of years at a time. The limiting factor was imposed by stellar cartography – the arrangement of interesting planets among a population of stars that move over time.

This strikes me as a similar approach to Geoffrey Landis’ percolation theory, and yields similar results. In either case, we get a galaxy that could be full of interstellar-capable civilizations, and yet one in which many habitable worlds (like ours) might appear unvisited at any given time. It’s a sophisticated solution to the Fermi Paradox, and one which avoids nightmare scenarios.

One insight the authors of this article mention, which I hadn’t considered, is that even if Earth had been visited – or even colonized – at some point in the distant past, we might have no way of knowing that. My own models for putative galactic civilization might stand to be relaxed a little. That might go on the list for some deeper research, if and when I get back to working on space-opera projects.

Architect of Worlds: Reality Ensues

Architect of Worlds: Reality Ensues

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One element of work on the Architect of Worlds project is that real-world science in this area has been advancing by leaps and bounds for years. Not once but several times, I’ve settled on a method for modeling some feature of planetary formation, only to see some new result published that seems to demand a different model. On one occasion, while I was testing the current model by generating planetary systems for nearby stars, I literally saw the first publication of new exoplanets in a star system I had just done randomly a few days before. This can be frustrating, although I have to admit it’s also rather exciting!

Now it seems to have happened again. The current model I use for planetary system formation implements a combination of the Nice model and the Grand Tack hypothesis, which together describe how Jupiter may have formed and migrated through the early solar system before settling down in its current location. The assumption is that a planetary system’s primary gas giant will normally form near the “snow line” and then migrate inward (and possibly outward) to its final position. This is all implemented in Step 10 of the current sequence.

Now there seems to be evidence that these models aren’t telling the whole story. In a new paper, “The consequences of planetary migration on the minor bodies of the early Solar System,” computer simulation seems to suggest that the core of Jupiter must have formed much further out than the Nice model suggests. It may have formed as far out as four or five times the snow-line radius, and then migrated inward very quickly.

I need to read the Pirani paper in more detail, and see if there’s been any discussion as to how to reconcile those results with the Nice model. To be honest, the Nice model isn’t entirely consensus in the community, so a new model that fills in some of its problematic details might work.

Still, if the Pirani paper seems supportable, it may be necessary to do a significant rewrite of the second chunk of the design sequence. It’s possible that the result may actually be simpler than what I’ve got now. I’ve thought of a way to cut out some of the current sub-steps and computation, making the process a little smoother, that might work. We’ll see what develops.