Moderately Believable Planetary Systems, baked fresh daily.
Port of the StarGen Stellar System Generator from C to Python3. Focus on enhancing readability and extensibility. The name is an Anagram of the original.
Garnets directly owes its existence to an earlier program, Jim Burrows' StarGen, which itself has a complicated lineage.
To quote the author:
StarGen is ... a program for creating moderately believable planetary systems around stars other than our own. The most recent version runs on Macintosh and Unix machines and produces HTML files as output.
There are a couple reasons.
It outputs HTML, which is not really ideal as an interchange format. It's also set up as a stand-alone program, and it's not very easy to adapt into a library.
As Burrows mentions, StarGen was at least 15 years old in 2004, and borrowed code and concepts from as early as the 1970s.
Taken together, StarGen has a lot of code baggage, and there are places where it shows. There are (to me at least) arcane tuning variables, there are a lot of weird unit conversions and choices (some parts use cgs, others stick to AUs, others km).
- Hot Jupiters
- Collisions between bodies can produce important results.
- Venus spins the wrong way
- Earth's moon is massive compared to expectations -- belived to be the result of a collision between two rocky proto-planets.
Some StarGen code is included as a reference while I'm doing the initial porting. Since StarGen is MIT Licensed, this in on the up and up. :)
Pretty self explanatory -- garnets should do what Stargen does.
By nature of the way it's organized, StarGen is not very modular. I want to break out, for instance, the atomspheric simulation into it's own module, and the orbit calculation into another.
Right now, the units are a right mess, and I'm not confident everything is working right. I intend ot use natu to clean up the units and ensure that there are no more messy conversion factors hanging around.
There are multiple places in our own solar system where we see the effects of gravitational interactions. In particular:
- Jupiter has strong gravitational effects on asteroids and comets in the inner solar system, acting to "vaccum" eccentric objects away.
- Jupiter and other planets shape where smaller bodies like asteriods congregate using their lagrange points and resonances (see Kirkwood gaps).
- Jupiter likely prevented the formation of a planet from the materials in the asteroid belt.
- Three of Jupiter's Galilean moons are locked into resonant orbits, which stabilizes them.
- Hot Jupiters exist -- our current understanding is that these planets started their lives further out, and through interactions with other planets, they fell inward towards their stars.
I'd like to replace the current proto-planet subroutine with a quick simulation of the protoplanets actually encountering each other, perhaps by using rebound.
In addition, I want to add some postprocessing to allow the system to develop stable resonances and weed out unstable ones.
Support Planetary migration.
This will, I hope, help to address the lack of moons around large planets. We know from our own solar system that gas giants tend to support numerous small rocky and icy moons, but StarGen tends not to have the same result because these planets are not generally numerous enough to be captured.
I won't know until it's implemented, but I think this is likely because of how StarGen handles moon formation as a capturing process, rather than letting large protoplanets form their own accertion disks.
Right now, the roche limit is handled by ensuring that moons that are captured do not end up with orbits inside the Roche Limit. Sensible, but it means that there's no way for planets to form saturn-like rings!
Support Chthonian Planets
In the time since StarGen was written, we've gotten a ton of actionable data about solar system formation in the form of exoplanets. That includes some really cood data about gas giant colors, and a prevalence of Hot Jupiters.
There's a lot to digest.
Author of StarGen (last updated c. 2008), which Garnets is most closely based on.
Author of Starform / Accrete (c. 1988)
Forked and updated Starform
Forked and updated Starform
Wrote an early version of this concept and published a paper on it. This paper appears to have directly inspired Starform.
Shamlessly stolen from StarGen docs.
"The Internal Constitution of the Planets" D. S. Kothari, Ph.D. , Mon. Not. Roy. Astr. Soc. Vol 96, pp. 833 - 843, 1936
"Habitable Planets for Man" S. H. Dole, Blaisdell Publishing Company, NY, 1964.
"Q in the Solar System" P. Goldreich and S. Soter, Icarus, Vol 5, pp. 375 - 389, 1966
"Formation of Planetary Systems by Aggregation: A Computer Simulation" S. H. Dole, RAND paper no. P-4226, 1969
"Computer Simulation of the Formation of Planetary Systems" Dole, S. Icarus, vol 13, pp 494-508, 1970.
"Computer Simulation of Planetary Accretion Dynamics: Sensitivity to Initial Conditions" Isaacman, Richard & Sagan, Carl Icarus, vol 31, p 510, 1977.
"The Evolution of the Atmosphere of the Earth" Michael H. Hart, Icarus, Vol 33, pp. 23 - 39, 1978
"Extra-Solar Planetary Systems: A Microcomputer Simulation" Fogg, Martyn J. Journal of the British Interplanetary Society, vol 38, 501-514, 1985.