New Models for Gas Giant Formation
I’ve spent the first few days of April working with the current 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.