Hello PF, Alright; I'm working on a big Sci Fi project and, being a lover of science, I want to get the physics right. So I'll spell out what I've got and see what responses I get... First, the star I've chosen for the world is Tau Ceti, for a number of reasons that involve the plot more than physics, just know that's our sun in this case. The planet has roughly 1/9th the mass of Saturn, and orbits within the habitable zone (so far exact distance undetermined, but it is a Sudarsky Class II gas giant.) But the world of interest is a moon of that world, mostly Iron, it has a very powerful magnetic field and it orbits at roughly 2 million km. It's significantly less massive than Earth, but because it's more dense, it's surface gravity is roughly ~8 m/s^2. Q: How can I find the mass of the planet with these requirements? Q: Do we know if Tau Ceti puts out a lot of solar radiation in the form of charged particles? Q: Would the stronger magnetic field induce more or less brilliant Auroras, and how would it effect the latitude they occur at? Q: Would the gas giant's Van Allen belts or it's own radiation influence the Auroras, even though the gas giant has just 1/9th Saturn's mass and is a Sudarsky Class 2? It has roughly 2 bar of atmospheric pressure, with the partial pressure of O2 only slightly higher than Earth's. IIRC, ~2 bar is near the max of human tolerance for breathing air. Is this correct? Also, it's in 1:2:4 resonance with two other moons, it is the outermost and the most massive. Being facelocked with the giant, the life on the surface slowly and continously migrates away from the terminator in order to stay within a certain "time zone", i.e. to continously live at a longitude where it's, say for instance, ~10am, in order to stay in a region with a familiar climate. Although this would mean covering thousands of miles in only 192 days at the equator, the life forms manage to pull this feat off by living near the poles, where the East-West distance across the world is much shorter and the terminator moves much slower. The sapient native life always follows the Auroras to keep from diverging north or south of their habitable zone. Because of it's thicker atmosphere and large oceans, the vast majority of the planet is too hot for them to survive, some areas along the equator reaching 150*F, meanwhile the zone they live in where the Auroras are constantly seen, has a climate more on the order of the Yukon. I'm hoping to make their culture have great significance to the Auroras that mark the path of their continous migration away from night. But the Auroras can't play this role if they're at a higher lattitude than the Axial Tilt, because if that were the case then by always staying under the Auroras, they would fall into a 6-month long night, but the whole purpose of following the Auroras is to stay out of the night... So the Auroras need to be closer to the equator, or nearly equal to the lattitude that corresponds to the Axial Tilt, i.e., the Arctic and Antarctic. The Auroras also need to be at least nearly constant and very brilliant. This may result in a very small Axial tilt?... But there are some buffers that make it so the Auroras can be within the arctic and antarctic and the life still never enter night (or never leave it), if they can tolerate twilight, Dawn, or Dusk, and follow a path parallel to the auroras but with the auroras on the horizon to their right, instead of directly overhead. The distance where "on the horizon" is for the auroras (as in this image.) depends on the radius of the planet and the altitude of the auroras, which depends on the altitude of the upper atmosphere, which I'm completely at a loss as to try to find... But I also want the seasons to reach somewhere on the order of ~80*F in the summer and drop to ~ -40*F in the deepest part of winter, would an Axial tilt of ~14* accomplish this? Hope you don't mind so many questions, a place where I can ask and discuss this is absolutely priceless! Many thanks!