Some words of wisdom? Fixing an old project to fit with a higher standard I've set.

[Scrit]

The orbital period of a planet depends on both the radius of the orbit, and the mass of the star, and the range of habitable radii depends on the luminosity of the star, which depend on its mass and age.

Awesome. Somehow I overlooked this yesterday morning. Thanks for pointing it out.

Also, you only needed the third law. Assume the orbits have approximately the same eccentricity, and the semimajor axes will be proportional to the mean distances. If your planet is 1.3 times as far from its primary as Earth is from Sol, and it also has an approximately circular orbit, then the semimajor axis is 1.3 times that of Earth's. In fact the semi-major axis of a near-circular orbit, is pretty close to the mean distance from the primary.

This is good to know. I had used all three laws for the most part, to flesh out some feasible distances and work out, at the time, and before passing out, I believe an orbital eccentricity I was comfortable with, as well as a revised axial tilt to better determine believable climate and seasons. I hadn't realized, at least for the bit of work shown, that I only needed the third law. You are right in your example, whether you intended to be or not, that I would be using a near circular orbit, similar to Earth's in my model.

The planet is going to receive less energy that far out, so you need to make the star brighter to keep the planet from freezing, you need to raise the luminosity by the square of the change in radius to do so. So the star needs to have a luminosity 2.2 times that of Sol. If it's the same age as Sol (4.7 Ga), I think you would need an F9 class star (I'm working backwards using the star system design rules in GURPS Space at this point) which would have a mass of 1.15 solar masses.

The square of the orbital period is inversely proportional to the cube of the mass of the primary, so doing all the math: (1.297^3/1.15^3)^(1/2)=1.198, Multiply by 365.25, and we get 437.5 earth days.

Seems my math was way off. I blame the lack of coffee and hour of my post.
Thinking about it, you're right. One of two things come to mind. Either a more luminous star, or an atmosphere that was thicker, with more greenhouse gasses and/or stronger magnetic field ... Mars for an example, I believe had a good portion of it's atmosphere stripped away by solar wind. A stronger magnetic field could have prevented that, however the world would still be fairly cold without a semi-Venus effect going on. Another consideration could be gravitational forces playing at the very shape of the world, expanding and contracting like one of Jupiter's moons, and through that causing geothermal energy to be released on the surface, as well as causing more active tectonic activity ... I think. Even then, while it is something I will keep in mind for the time being, it is something I would like to avoid as that would provide for some drastic changes and results.

Greenhouse gasses and magnetic field. The world being larger than earth, should have a sufficiently strong magnetic field -- perhaps strong enough to allow for more frequent atmospheric phenomena, like the Aurora/St. Elmo's fire. Greenhouse gasses would give me some room to play with visual effect in writing, like subtle play of color in the sky, sunrise/set and such, but I digress, most of that information wouldn't be of much use to a player or reader unless it were a more modern setting. It would, for the most part, be information stored in my notebooks, for the sake of practice and the possibility of working out future, sci-fi settings ... which is good, I don't mind the work behind it, and will save me hours, or days in research later on, while costing me the same in this project.

I am more familiar with the DnD 3.5 system than I am with GRUPS however, I do live with my DM, and he has had experience with the system as well as a mutual friend. I'm sure I could work the GRUPS Space system (for the sake of further expanding my understanding) back into the scientific standard.

Thanks for the information and the correction, Hai-Etilk. Have some rep.