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.

Click image for larger version.  Name: Earth vs Gigantica.png  Views: 92  Size: 18.3 KB  ID: 133395

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:

Click image for larger version.  Name: Just the Physical Features.png  Views: 65  Size: 51.0 KB  ID: 133393

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.)

Click image for larger version.  Name: With Tectonics.png  Views: 81  Size: 77.6 KB  ID: 133394

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