Gigantica -- A Study in Maximum Habitable Planet Size
Hello Guild,
Over the last few weeks, I've been asking the question: Just how big could a planet get while still remaining habitable? To prepare for this calculation, I proceeded to set the following constraints:
1) I want a planet with comfortable gravity, no more than 2% or so greater than the Earth.
2) I want my planet to generate an adequate magnetic field that will protect the planet's atmosphere from solar wind stripping.
3) I've decided to use the basic two-constituent model: a silicate mantle and an iron core.
I used Universe Sandbox to simulate the stripping of the atmosphere, selecting a star considerably smaller than the Sun to minimize the pressure of the solar wind. I found that I needed 0.0226 gauss to marginally maintain an atmosphere with a 0.8611 solar mass star at a habitable distance of 0.7792 AU.
Next, I derived an equation to specify the strength of a planet's magnetic field, based on the mass of its iron core and the planet's rotation rate, finding that I needed about a 0.05 Earth mass iron core to generate a field of 0.0226 gauss, using a rotation rate of 19.62 hours. I realize my calculations aren't perfect by any means, but they allowed me to pack on some extra mantle, arriving at a radius of 7,359 km, while maintaining Earthlike gravity.
Now, I'm all set to depict the tectonic history. I've chosen Nikolai Lofving Hersfeldt's tutorial on Worldbuilding Pasta to do this, because I wanted a highly obsessive level of detail. Particularly, I wanted to simulate over 900 million years of tectonic history to determine the positions (and the sizes) of the mountains. Here is the end result of the GPlates simulations, showing the base map and mountain placement, not taking into account the heights of the mountains:
And here are the tectonic boundaries. (Forgive me about the sloppiness; I had rotated the final frame but haven't had time to clean up the boundaries near the former south pole.)
I must also add that my calculations were conservative, so I could likely have exceeded the 7,359 kilometer radius without necessarily endangering the planet's atmosphere.
I hope you have enjoyed this study.
Peter
This is fascinating, and something I have thought about in the past but never taken the time to calculate. Are you planning on moving forward with this to create a more detailed map of the planet?
Also, I'm curious how you like Universe Sandbox? I have been looking at that and Space Engine and have been trying to decide which to purchase.
Hello Turambar,
Thank you so much for your response.
Yes, I'm definitely intending to move forward with all aspects of worldbuilding here, at least up to the biology and culture/history of any resident civilizations. (I'm not as interested in the so-called "human realm" as opposed to the pure physics.) I plan to create a 8000 x 4000 pixel map of the topography, then I'll model the climate with Clima-Sim and Azelor's precipitation algorithm, before finally moving on to adding moons and other planets for this lower metallicity, older, solar system.
Regarding Universe Sandbox, despite some minor limitations, I absolutely love the program. When I say "limitations," I'm referring to a lack of accurate modeling to correlate magnetic field to the core size and rotation rate, for example. (Increasing rotation rate or iron core mass doesn't affect the magnetic field.) Also, the temperature modeling for satellites of brown dwarfs is unrealistic, as I discovered for my last project, Kaunis. Moreover, it's currently impossible to simulate accretion from initial parameters (of dust cloud angular momentum, protostar size, and element distributions); which is something I could certainly use. I'm sure, however, that these limitations will be addressed in future releases of the program, as users request them, so I recommend making the 40-something dollar investment for now.
As for a comparison between Universe Sandbox and Space Engine, if you want to simulate the "destruction" of planets by collisions with asteroids, other planets, black holes, and the like, then Universe Sandbox is for you. (I personally don't care too much for this aspect of the program.) Also, if you want to model the physical aspects of planetary orbits, the energy flow through their atmospheres and interiors, and tidal interactions of satellites, then choose Universe Sandbox. If, on the other hand, you want to merely simulate procedurally-generated images of planets, stars, and nebulae, including realistic surface maps, without needing to delve into the hardcore physics of your simulations, go with Space Engine. (Space Engine does have an atmosphere model, but I haven't examined it yet.)
I really hope that helps you decide. If you have the finances, I would buy both. Again, thanks for the comment.
Peter
Update:
1) Assuming slightly modified parameters, I've modelled the temperatures using Clima-Sim and obtained the following results for January/July:
January
July
Now here is a higher resolution file showing topography and currents:
Question: Am I correct in my current placements?
Peter
Definitely will be curious to see where your Clima-sim journey takes you.
For the currents, a few things to maybe look into:
- The circumpolar (~80 degrees N/S) currents are probably not going to be warm but may be better depicted as neutral.
- In the center you have a warm current going into a gulf but nothing leaving, so maybe remove that current or add a corresponding 'exiting' current.
- On the south coast of the easternmost continent you should probably add a poleward flowing warm current to complete the gyre in that part of the ocean.
Thanks for the breakdown. Universe Sandbox sounds like it would be more in line with what I am looking for, as I want something with fairly accurate physics modelling capabilities. It's disappointing to hear that the modelling for satellites of brown dwarfs isn't accurate. Is this due to no taking into account tidal locking properly? If so, I imagine this would apply to red and orange dwarf stars as well which would be a challenge for what I would like to do with it.Originally Posted by Peter Toth
For some reason, Universe Sandbox always exaggerated the temperatures on the side of my planet permanently facing the brown dwarf. The computed temperatures were therefore quite higher than I had independently calculated using formulas. I suspect the reason for this exaggeration was a lack of a wind model, in Universe Sandbox, to distribute temperatures across the longitudes.
Updates:
A view showing currents and topography, updated thanks to MrBragg's helpful advice.
Northern Winter and Summer temperature maps, taking into effect currents and altitude (10 C per 1000 meters lapse rate):
Northern Winter and Summer pressure maps:
Now onward to the next step: calculating precipitations and climates, then finally fine topographic detail.
By the way, please let me know if you see any glaring errors.
Peter
Update: Relative Precipitations
Using Azelor's tutorial, I've arrived at the following precipitation patterns for Gigantica. (The aqua blue zones are highlands.)
Northern Winter Precip Pattern:
Northern Summer Precip Pattern:
Now for the difficult part of determining climate zones. Because I didn't use the same colour scheme as recommended in the tutorial, I likely won't find the provided script very useful. Does anyone have any recommendations on how to proceed? I'm thinking of using ExoPlaSim, but the extra-lengthy processing times are right now discouraging me.
Can anyone out there give me some feedback regarding my temperatures, currents, and precipitations?
Thank you so much!
Peter
I’m no expert, but the pressure map looks a little off. The ITCZ band looks a little jagged, and unlike the thermal equator it doesn’t need to zig zag around mountains. I doubt it’ll change much on the final pressure map, but it might be worth setting a smoothed out ITCZ and see how it affects the winds.
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