World-building for fantasy story

Main Question or Discussion Point

I'm doing some world-building at the moment for a fantasy setting and would like some help, please. When it comes to physics etc, I'm like the kid who looks at the cool pictures of kung fu masters doing awesome kung fu stuff; less experienced than the student wielding a brand new white belt. Some guidance from you kung fu masters would be beneficial.

Here's the general concept:
1. The setting is located in a binary system with a sun slightly larger and brighter than our own, and one about half as such.
2. More precisely, the setting is a moon that orbits around a gas giant, which in turn orbits the two stars within the system's Goldilocks zone.
3. The gas giant is approximately the size of Jupiter, with 4-5 total moons.
4. The second moon is where everything within the setting takes place; although slightly smaller than Earth, it is more replete with heavy material, and thus of nearly equal gravity.

So here are my first set of questions:
1. What is necessary to include in this setting to ensure that earth-like civilizations could develop? For example, shielding from radiation, tidal forces from the gas giant, etc?
2. What kind of tidal forces would the moon experience from both the gas giant and the other surrounding moons? I'm assuming that a moon with sufficient distance from the gas giant wouldn't be trembling with volcanic activity.
3. What kind of weather effects would be occurring as a consequence of having a giant ball of gas hovering nearby? Would it be, all things considered equal, warmer/dryer/wetter than Earth?
4. Would reflection from the gas giant negate darkness in whatever regions of the moon were facing the gas giant, but not the suns?
5. How would light from the suns work? Would they appear as a large blob of light or separate points? Would they "rise" and "set" together? What about shadows cast by objects?
6. Would it make more sense to have the suns closer together and all of the satellites circling both or have one stationary and the other moving?

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Matterwave
Gold Member
Most of your questions require detailed knowledge of the system. A lot more details than we are currently given. Just that the gas giant is "in the Goldilocks zone" is not enough to tell us everything.

If you really want to know how the environment on this moon would be like, you will need the detailed orbits of the planet around the stars and the moon around the planet, as well as the orbits of the other moons. The main difficulty that I see here is finding stable orbits of a binary system where your planet will remain in the Goldilocks zone throughout its orbit. As you can imagine, figuring out the stable orbits for a binary system is far more difficult than for a single star. That's where I would start my analysis, if I had to have a binary system as the parent stars.

DaveC426913
Gold Member
Seems to me, there's going to be quite a problem having a planet orbiting a binary system. It would have to be very distant from the two in order to be stable. But then how could it be in the Goldilocks Zone?

A more stable sitch is to have the planet closely orbit one of the stars, with the second star in a long period orbit outside the planetary system.

Thanks for the quick reply. Using Astrosynthesis v2.0, I get the following information:

Multistar system
Star 1: Mass 0.55, Radius 0.62, Luminosity 0.13 sols (all relative to our sun)
Star 2: Mass 1.57, Radius 1.44, Luminosity 4.8 sols (all relative to our sun)
Total of 12 primary satellites, with the gas giant in question the 5th planet

Gas Giant
Distance from Star 2 (assuming it orbits only one star) 125,695,818km
Eccentricity of orbit 0.18
Inclination (in degrees) 1.75
Ascending node (deg) 50.31
Periapsis Angle (deg) 34.78
Time past Periapsis (days) 7

Moon
Distance from gas giant 1,252,540km
Gravity 1.02g
Rotation 28hrs
Eccentricity of orbit 0.0185
Inclination (deg) 5.36
Ascending node (deg) 76.34
Periapsis angle (deg) 107.09
Time past Periapsis (days) 0

Matterwave
Gold Member
Star 2 is 4.8 times brighter than our sun, and the planet is about halfway between the distance from our Sun to Venus and the Earth...I'm guessing it's going to be quite hot over there.

Moons orbiting a jovian will always be tidally locked. (Unless the system is very young, or recently disrupted.) As a result, the "day" on these worlds will always be equal to the time they take to orbit their planet. The closest moon to Jupiter is Io. It's orbit is 1.769 days, and it gets hit with lots of nasty tidal forces causing volcanism.

So you are likely to get stuck with very long days on your moons.

Star 2 is 4.8 times brighter than our sun, and the planet is about halfway between the distance from our Sun to Venus and the Earth...I'm guessing it's going to be quite hot over there.
Yeah, I was thinking the same thing when looking at distance, although not quite at the same level of technical understanding. Pushing the distance further away would help cool things down.

Moons orbiting a jovian will always be tidally locked. (Unless the system is very young, or recently disrupted.) As a result, the "day" on these worlds will always be equal to the time they take to orbit their planet. The closest moon to Jupiter is Io. It's orbit is 1.769 days, and it gets hit with lots of nasty tidal forces causing volcanism.

So you are likely to get stuck with very long days on your moons.
For clarity, tidally locked means there is no rotation? The same side of the moon faces the gas giant the entire time? So essentially one side will always be day and the other night?

Seems to me, there's going to be quite a problem having a planet orbiting a binary system. It would have to be very distant from the two in order to be stable. But then how could it be in the Goldilocks Zone?

A more stable sitch is to have the planet closely orbit one of the stars, with the second star in a long period orbit outside the planetary system.

Matterwave
Gold Member
For clarity, tidally locked means there is no rotation? The same side of the moon faces the gas giant the entire time? So essentially one side will always be day and the other night?
The "one side will be day and the other night" would only apply if the central body was the star, and not the gas giant. Tidally locked to the gas giant means one face will always face the gas giant. In terms of day and night, how ever long the period of orbit around the gas giant is is the period of day and night.

The "one side will be day and the other night" would only apply if the central body was the star, and not the gas giant. Tidally locked to the gas giant means one face will always face the gas giant. In terms of day and night, how ever long the period of orbit around the gas giant is is the period of day and night.
At the moon's distance, the software is telling me the orbit around the gas giant is approximately 10 local days. So this means it would take that many days to transition from day to night and back? So this could be sped up by moving the moon closer to the gas giant, which then introduces problems with increased volcanic activity etc. Is there another way to increase the speed of the day?

For clarity, tidally locked means there is no rotation? The same side of the moon faces the gas giant the entire time? So essentially one side will always be day and the other night?
Tidally locked means the rotation exactly matches the revolution. Earth's moon is tidally locked to the Earth, so the sun will rise and set every 28 days, but the Earth will always be in (almost) the same place in the lunar sky.

Edit: So this means it would take that many days to transition from day to night and back? So this could be sped up by moving the moon closer to the gas giant, which then introduces problems with increased volcanic activity etc.

That's right.

Matterwave
Gold Member
Yeah, I was thinking the same thing when looking at distance, although not quite at the same level of technical understanding. Pushing the distance further away would help cool things down.
5 times the flux means 5 times the heat input. Of course, without knowing the green-house effect, and albedo (reflectivity) of your planet, we can't really predict an average temperature.

But let's say there's no green house effect, and Albedo is 0. In-coming flux: $F=\frac{L}{4\pi d}\approx 5*\text{solar~constant}\approx 7000W/m^2$. Total incoming heat (assuming Earth sized moon): $U_{in}=F\times\pi r^2\approx9\times 10^{17} W$. The outgoing radiation is $U_{out}=4\pi r^2\sigma T^4$ We can solve for the equilibrium temperature: $$T_{eq}=\left(\frac{1}{4\sigma}F\right)^{1/4}\approx 150^\circ C$$ Which is pretty hot...above boiling anyways. One might object that this calculation did not take into consideration the albedo and green house effects (among a vast number of other effects), but as a comparison, doing this exact same calculation for Earth gets you 5 degrees Celsius (the green house effect keeps us about 10 degrees warmer on average). Of course you need an atmosphere to distribute the heating more evenly between the day and night sides. An atmosphere means green-house effect, which means more heating. With tidal locking taken into account, probably this planet will be scorching hot on the day side, and like an oven at night.

Probably this planet is not in the habitable zone of the parent star.

Here we go. From Wikipedia:

In contrast, the outer natural satellites of the gas giants (irregular satellites) are too far away to have become locked. For example, Jupiter's natural satellite Himalia, Saturn's natural satellite Phoebe, and Neptune's natural satellite Nereid have rotation periods in the range of ten hours, while their orbital periods are hundreds of days.

Here we go. From Wikipedia:

In contrast, the outer natural satellites of the gas giants (irregular satellites) are too far away to have become locked. For example, Jupiter's natural satellite Himalia, Saturn's natural satellite Phoebe, and Neptune's natural satellite Nereid have rotation periods in the range of ten hours, while their orbital periods are hundreds of days.
So the moon needs to be further out from the gas giant in order to remain free from tidal lock?

By the way, all of this is very helpful. Thank you everyone.

5 times the flux means 5 times the heat input. Of course, without knowing the green-house effect, and albedo (reflectivity) of your planet, we can't really predict an average temperature.

But let's say there's no green house effect, and Albedo is 0. In-coming flux: $F=\frac{L}{4\pi d}\approx 5*\text{solar~constant}\approx 7000W/m^2$. Total incoming heat (assuming Earth sized moon): $U_{in}=F\times\pi r^2\approx9\times 10^{17} W$. The outgoing radiation is $U_{out}=4\pi r^2\sigma T^4$ We can solve for the equilibrium temperature: $$T_{eq}=\left(\frac{1}{4\sigma}F\right)^{1/4}\approx 150^\circ C$$ Which is pretty hot...above boiling anyways. One might object that this calculation did not take into consideration the albedo and green house effects (among a vast number of other effects), but as a comparison, doing this exact same calculation for Earth gets you 5 degrees Celsius (the green house effect keeps us about 10 degrees warmer on average). Of course you need an atmosphere to distribute the heating more evenly between the day and night sides. An atmosphere means green-house effect, which means more heating. With tidal locking taken into account, probably this planet will be scorching hot on the day side, and like an oven at night.

Probably this planet is not in the habitable zone of the parent star.
So if the gas giant is moved further out, it'll cool down the temperatures. Add some atmosphere and albedo to keep it from getting too chilly. Or is it albedo that decreases heat? I have a hard time remembering that one.

Matterwave
Gold Member
So if the gas giant is moved further out, it'll cool down the temperatures. Add some atmosphere and albedo to keep it from getting too chilly. Or is it albedo that decreases heat? I have a hard time remembering that one.
Albedo makes the planet reflective. In general, it cools the planet down. You'd have to move the planet out a ways. However, remember that you are in a binary system. Where's the other star? I'm guessing the other star is farther out? There might not be a stable orbit at all distances and configurations.

Matterwave
Gold Member
I should probably also point out that with such a bright star, you are going to get a large amount of ultra violet rays...which might be damaging to cellular organisms. So you would probably need some kind of protection by e.g. having a thick atmosphere or having your civilization live under water or in rocks or something.

I'm not married to the luminosity of the star, so it may be easier to just tone it down one or two magnitudes. As for the distances between the two stars, I'm not entirely sure. Based upon the article I referenced earlier, it would seem that having the two stars close and orbiting one another would be the most advantageous for allowing life to develop and flourish on a moon.

And to clarify why I'm aiming toward a binary system with a habitable moon is to break away from more "traditional" fantasy settings that, for good or bad, are more or less Earth with a different paint job. The question becomes what kind of cultures would develop where there are two suns in the sky and giant planet watching over you constantly. What would the effects be on agriculture and the economies. (Not to mention the enormous impact of magic on social structures, technology, meanings, etc.)

Matterwave
Gold Member
The biggest problem, if you want to do "hard physics", like I stated is that you have to find the stable orbits for this Jovian planet. If you have a binary far away, they will both rise and set at the same time, so you'd just always see 2 Suns in the sky (or none). The distance you gave in post #4 means your planet is quite close to the larger star. If the other star is even closer still, it seems it would have to be making some very tiny orbits. I wouldn't know much more details than that though.

The biggest problem, if you want to do "hard physics", like I stated is that you have to find the stable orbits for this Jovian planet. If you have a binary far away, they will both rise and set at the same time, so you'd just always see 2 Suns in the sky (or none). The distance you gave in post #4 means your planet is quite close to the larger star. If the other star is even closer still, it seems it would have to be making some very tiny orbits. I wouldn't know much more details than that though.
I can push the gas giant out, thus decreasing the temperature. Are there not instances of binary systems with planets that have stable orbits? I don't have the faintest idea how to calculate Jovian orbits, let alone any type of orbit. (Jovian means gas giant, is that correct?)

What if the moon was tidally locked but had no rotation?

Also, would proximity of sorts to the gas giant have any affect on the moon's temperature? What about reflected radiation from the gas giant? I'm assuming that the moon's atmosphere would absorb enough to keep it all at safe levels.

Edit:
Doing some research on orbits, this is what I pulled from wikireality:
Orbital stability
For a stable orbit the ratio between the moon's orbital period Ps around its primary and that of the primary around its star Pp must be < 1/9, e.g. if a planet takes 90 days to orbit its star, the maximum stable orbit for a moon of that planet is less than 10 days. Simulations suggest that a moon with an orbital period less than about 45 to 60 days will remain safely bound to a massive giant planet or brown dwarf that orbits 1AU from a Sun-like star.

Is that along the lines of what you are mentioning?

Last edited:
Matterwave
Gold Member
I can push the gas giant out, thus decreasing the temperature. Are there not instances of binary systems with planets that have stable orbits? I don't have the faintest idea how to calculate Jovian orbits, let alone any type of orbit. (Jovian means gas giant, is that correct?)

What if the moon was tidally locked but had no rotation?

Also, would proximity of sorts to the gas giant have any affect on the moon's temperature? What about reflected radiation from the gas giant? I'm assuming that the moon's atmosphere would absorb enough to keep it all at safe levels.

Edit:
Doing some research on orbits, this is what I pulled from wikireality:
Orbital stability
For a stable orbit the ratio between the moon's orbital period Ps around its primary and that of the primary around its star Pp must be < 1/9, e.g. if a planet takes 90 days to orbit its star, the maximum stable orbit for a moon of that planet is less than 10 days. Simulations suggest that a moon with an orbital period less than about 45 to 60 days will remain safely bound to a massive giant planet or brown dwarf that orbits 1AU from a Sun-like star.

Is that along the lines of what you are mentioning?
That's one problem to deal with, but the other problem is that orbits around binary stars is hard to figure out because you have a restricted 3-body problem instead of the standard 2 body problem. Your planet is orbiting 2 stars which must, in fact, orbit each other. If you are far enough away from the center of mass of the 2 stars, you can treat the 2 stars as 1 plus perturbations, but in general this is a quite difficult problem to solve. You might consider trying to google "stable orbits around binary systems".

Here's the general concept:
1. The setting is located in a binary system with a sun slightly larger and brighter than our own, and one about half as such.
2. More precisely, the setting is a moon that orbits around a gas giant, which in turn orbits the two stars within the system's Goldilocks zone.
3. The gas giant is approximately the size of Jupiter, with 4-5 total moons.
4. The second moon is where everything within the setting takes place; although slightly smaller than Earth, it is more replete with heavy material, and thus of nearly equal gravity.

So here are my first set of questions:
1. What is necessary to include in this setting to ensure that earth-like civilizations could develop? For example, shielding from radiation, tidal forces from the gas giant, etc?
2. What kind of tidal forces would the moon experience from both the gas giant and the other surrounding moons? I'm assuming that a moon with sufficient distance from the gas giant wouldn't be trembling with volcanic activity.
Tidal forces from gas giant depend on eccentricity.
3. What kind of weather effects would be occurring as a consequence of having a giant ball of gas hovering nearby? Would it be, all things considered equal, warmer/dryer/wetter than Earth?
Little effect. The solar warmth on Earth varies by 6 % due to Earth orbital eccentricity. This is not noticeable, and seasons are caused by tilt of Earth axis instead.
4. Would reflection from the gas giant negate darkness in whatever regions of the moon were facing the gas giant, but not the suns?
Yes.
5. How would light from the suns work? Would they appear as a large blob of light or separate points? Would they "rise" and "set" together? What about shadows cast by objects?
Your later specifications - 4,8 times solar brightness for the bigger and 0,13 for the smaller stars - suggests little effect on shadows when suns are high in sky. Though perhaps some.
But compare with Kepler-16, that actually exists.
Binary orbital period - 41 days
Binary distance - 0,22 AU
Planet´s orbital period - 229 days (so 5,6 times binary period)
Planet´s orbital distance - 0,705 AU.
So the binary distance is 0,32 times planet´s distance.
Compare Mercury.
Period 88 days (so 4,2 orbits per year)
Orbital distance average 0,387 AU, but with eccentricity varies between 0,307 and 0,467 AU.
So, looking from Kepler-16b, the orbit of 16B looks much like the perihelion of Mercury.

Would they rise and set together? Yes, like Mercury and Venus rise and set together with Sun.

But B is tremendously brighter than Mercury!

On Earth, Venus us something like 600 million times dimmer than Sun. Venus shines in evening and morning twilight brighter than any other star, but it is never bright enough to outshine the combined light of all other stars, or cast shadows.
Moon does outshine combined light of stars and cast shadows. Full Moon is 400 000 times dimmer than Sun. When Moon is near Sun, however, it is a narrow crescent and very much dimmer than when at full. Still, a large crescent Sun can easily be seen in broad daylight.
But a star like your B would shine plainly visible high in sky, and if it is above horizon when A is underneath (or even close) it would outshine the twilight.

Yes, they would sometimes merge into a blob. But such mutual occultation would last something in the region of a few hours. Although, with the dazzling disc of sun in the sky, it might not be particularly easy to notice, just like small phase partial solar eclipses are not easy to notice.
6. Would it make more sense to have the suns closer together and all of the satellites circling both or have one stationary and the other moving?
More sense than what? What sense of "stationary"?

2 more ideas:
-put the planet (with its habitable moon) in Lagrangian point http://en.wikipedia.org/wiki/Lagrangian_point
-make everything orbit the big star and put the small star as far as we have Neptun (like 30 AU)

If it is a moon with long day night cycle:
-make the atmosphere dense (water boils in higher temperature) and less big continents and more islands with sea around that would make those temperature jumps lower (or just make interior of continents a hell)
-adjust flora and fauna accordingly - think about hairy leaves and coniferous-like plants that survive such jumps more or less fine, while animals hide in to holes
-if the atmosphere is much denser then any flying creatures have much easier life

If you have a star much bigger than the Sun you must take in to account that it has shorter life and a billion years earlier was much dimmer. (was the planet a while ago hot enough?)

2 more ideas:
-put the planet (with its habitable moon) in Lagrangian point http://en.wikipedia.org/wiki/Lagrangian_point
http://en.wikipedia.org/wiki/Lagrangian_point
http://en.wikipedia.org/wiki/Lagrangian_point
Binary stars don´t have them.
http://en.wikipedia.org/wiki/Lagrangian_point
-make everything orbit the big star and put the small star as far as we have Neptun (like 30 AU)
Compare Alpha Centauri. Small stat 35,6 AU from big star at apoapse. And big star does qualify under "slightly brighter than Sun".