locked to have one face fixed facing the star.
A risk. Though Venus is at inner edge of Goldilocks and not tidally locked. But possibly a higher risk on a much smaller star.
highly eccentric ellipsoidal orbit near the star might also get quite a lot of internal heating
Yes this too. Some of Jupiter's Moons are none too happy, cracking!
@sozme
What sort of thing would you like and I'll crunch some numbers?
I'm building a catalogue of the most important 1100 Homeworlds in my Galactic Civilisation/Culture/Council (I have them named! But obviously you would use a different name and location*), I can let you have a few. Given the sheer number of stars with planets in Milky Way, the more likely combinations. A habitable world that is really a larger than Mars sized moon around a Gas Giant is also maybe possible (e.g. the ewok world). But no-one is absolutely sure. Jupiter is maybe about 1/3 to 1/2 the maximum size of a Gas Giant. At some point if large enough the pressure due to gravity kicks in Fusion and you get a Brown Dwarf. They are searching for Brown Dwarfs so as to find a lower limit for size of a star.
Decide on amount of gravity you want. Less than 0.7G is small planet that loses atmosphere more easily. Mountains will be much higher on smaller planet and less atmospheric pressure. About 1.2G is maybe about x3 Earth's surface area. Unless severe vulcanism, the mountains will not be as high as Earth, but at sea level atmosphere denser. At maybe 16,000 feet you have about 1/2 the pressure on Earth. So you could have a situation where deep valleys on a small planet suit us Tellurian Humans (Dead sea is well below sea level, Some villages in Andes you are altitude sick till you acclimatise to low air pressure, so we can acclimatise to a wide range. 8 Atmospheres is about 250ft underwater without a hard suit (about 80m). I don't think that's good for Humans any length of time. But mountain tops might be only 6 down to 3 atmospheres depending on height. The relationship between Gravity and actual surface area and mass of planet is complicated.
The gravity then sets an approximate size of planet. Day length and axial tilt can be whatever you like without affecting much else, but likely 15hours to 48 hours period is a reasonable idea unless you want mad weather and tides!
Decide on Star mass. Too small and you risk a brown dwarf. You want enough output so the World isn't too close. Too large and it burns fierce with too much UV (also might have a fraction of life). It wants to be a main sequence star (probably, though who knows what life is out there), i.e. burning Hydrogen. We think not a good idea to be near one about to change to Helium or burning Helium. I suggest maybe 0.6 to 3.5 Solar masses (0.6 might be tricky due to flares, 3.5 any Humans are going to wear loads of sunblock cream and it may only have 1/10th of life of our sun, but that is still a lot of time)
Based on Star mass we only have small choice on orbit radius. The inner and outer edge of Goldilocks Zone the atmosphere characteristics and gravity (planet size) are more important. The Star mass and Orbit Radius sets the Orbital period (year). Seasons can be by elliptical orbit or tilt or both. We have an elliptical orbit and it reduces winter cold slightly in north and increases summer (at same time) in South. Not enough that you'd notice. Seasons here are due to tilt. I think one planet in the solar system rotates "wrong direction" and one has 90 degree tilt, so day & night are season length, the rotation not creating day and night!
Other features:
You want a liquid Iron core. Without a magnetosphere you are in trouble with Solar Wind and Cosmic Radiation.
A moon is optional, it makes tides (good), but can increase vulcanism (bad). You can throw in some further out small ones just for cosmetic reasons. You do get daily tides without a moon, due to the star. This is why tides here vary (Moon Sun aligned or in opposition)
A Gas Giant (or three!) further out is VERY good as it dramatically reduces likelihood of comet/asteroid/meteor striking your world and having an ice age for 1000s of years due to the dust.
*Location: Too close to centre of galaxy and risk of a supernova wipe out is high. Jury is out on Galactic rim. You don't have to be actually on a spiral arm, there are almost as many stars not on the arms. There is a higher concentration of brighter stars on Arms, which is why we see them as arms. So any place 10,000 light years to 60,000 light years from Galactic core is probably feasible. We are 30,000 light years from Core.
