LadiSilverfox said:
Let's say we start over? My restrictions are that M2 is comfortably habitable, has a 32-hour day, and is tidally locked to P. P (inhabitable) still has M1 and M2. (Bonus: M1 was habitable but now is not.)
Potential habitability of the satellite, to a good approximation, should be determined by the distance from the star. I.e., if the parent planet is in the habitable zone, so are its satellites.
Other reasons than solar irradiation can still affect habitability, though. So you can e.g. say M1 used to be habitable, but being small has lost too much of its atmosphere by now. While M2 being larger has retained its atmosphere and is habitable just fine.
This also suggests the parent planet should not be an ice giant, as the ices should melt/sublimate at the distance from the star where the moon can be habitable.
Energy from tidal stretching or otherwise received from the parent planet could alter this picture, but IMO it's better to ignore these for clarity.
Similarly, I'd handwave away any radiation concerns. You can always say it's remarkably low, or the moon happens to orbit away from the main belts, or itself has a strong magnetosphere.
Or, you could say that while M2 orbits away from the radiation belts, M1 has at some point drifted (due to tidal interactions) into one of the belts, which accelerated atmosphere loss and is the reason M1 is uninhabitable. Maybe it still has enough atmosphere to survive on, but it's now too strongly irradiated to support life? There are options here.
Here, use this
spreadsheet.
(Google Docs; use the hyperlink and make a copy to edit - otherwise it's viewing only)
There are two sheets in there.
The first is for the star and its habitable zone. The data should be left alone here, unless you really need more/different masses. But it already spans the range of masses corresponding to G and F-type stars. If you really want to, the masses can be extended on either end by a bit, and the table should still make sense.
Pick a star you like, and an orbit within its HZ, and copy the mass and the desired orbit into the second sheet.
The second one is for the planet-moon system. It's a bit more messy (sorry), just remember that green fields want you to enter something, while blue fields tell you something. It calculates things like the sizes of the two planets, how large the parent planet would look on the sky, if the orbit is not too large for the moon to fly away, or what the period has to be given the radius (or vice versa). A few other things.
The data already in place is for a Sun-like star, Earth-like orbit of the parent planet, a Uranus-like parent planet, and an Earth-like moon tidally locked on a 32h orbit (which sets the length of a day in this case).
There's precious little room for another moon on an even lower orbit, so either raise the orbit of M2, or place M1 on a higher orbit than M2. (M1 is not included in the spreadsheet)
There's nothing there about orbital inclination or axial tilt (yet). If you decide to have axial tilt AND tidal lock, then the orbital inclination will have to be equal to the axial tilt (and vice versa). Without tidal lock the tilt and the inclination can be uncorrelated.
With the current setup, and without inclination, significant daily eclipses would be commonplace. These are not taken into account when deciding habitability, or anything else for that matter.
Try playing with it a bit and see if any of it makes sense. Some cells have comments telling you what's what, but let me know if you need clarification or want something added that you can't do yourself. Or if the link doesn't work for some reason.
Also, I can't guarantee everything's put in correctly. Comments welcome.
