Some questions about planets for a sci-fi novel?

That would no be a stable orbit, that would be a LaGrangian point, L2 to be specific. It's unstable in one axis. Without periodic corrections, the moon would either fly away from the planet or crash into the planet. Such a moon could not exist for more than a few million years.

You could heat the Moon with geothermal energy from the tidal forces from the gas giants. It would be much like Europa, a surface of ice, with a liquid ocean underneath and perhaps life near hydrothermal vents.

The gas giant could develop stripes from high speed winds, there's little reason to believe it could become polarised in terms of color but you might be able to claim tidal locking if it's in a very close orbit with it's Sun as causing such an effect ( several exoplanets have been found to be larger than Jupiter in orbits closer than Mercury so the old belief that gas giants could not be close in is no longer the case ). It would be what we call an eyeball planet.

In order for the perpetual eclipse thing to work, the gas giant would have to be really close to its sun (scientists call these "hot Jupiters"). This is because a gas giant's moons orbit it in a matter of days, so the gas giant would have to do the same around the star.

If the sun is blocked entirely by the planet, I'm not sure if it matters how close it is and if the star can "shine around" it, it's probably so close that it would have evaporated before any kind of advanced life forms could evolve.

Heat is also a tricky thing, in space. Our Mercury is closer to the Sun than any other planet, but it still gets super-cold at night because it has no atmosphere that could transfer any heat from the lit side (the moon you're describing has no lit side).

Though, it's possible to get some heat from tidal forces caused by the planet and its other moons.

The different sides wouldn't work, since gas giants are always very windy, especially when close to the Sun. Because of this, the gases would mix very quickly if it ever looked like that.

In conclusion, it would be a better idea to place the gas giant in the so called Goldilocks Zone, which is the life-friendly distance from a star. It couldn't be very "artistic" either, since the gas is left entirely to the mercy of the laws of physics. It couldn't eclipse the sun forever, but you could play around a little with tidal locking, which is the thing that causes the Moon to only show us one side.

Gas giant moons tend to act this way as well, so this means that only one side of that world would be able to see the planet. Perhaps this would allow for some fun cultural and historical details for the people of that moon?

Good luck with your novel!

1. The Moon might not need to be in orbit around the Planet, to be in permanent solar eclipse. You should read up on the L2 Lagrange Point. Of course, the diametrer of the star and the gas giant have to be just right for it to be a permanent total eclipse, because the distance where L2 is stable is a very specific distance once you've established the mass of all three objects. This orbit is unstable over time if there are other nearby planets and satellites, so it mighy need to be the only moon of the planet, and the next closest planet in this system would have to be very low in mass, or very far away, preferably both. This would be more realistic than a moon orbiting the planet in perpetual shadow, because it would have to be very distant from its planet to orbit so slowly, which makes the angular size of the planet much smaller and the angular size of the star much larger. You would be more likely to have a permenent annular eclipse, rather than a total one.

2. But maybe that annular eclipse is what you want. This will allow for some 'sun'light to reach the moon, perhaps enough for photosynthesis to take place. Our experiences with life on Earth is, no matter how it got here, once it did, it has done an excellent job of adapting to anyplace where conditions are terrible, but not impossible.

3. The bi-color nature of a gas giant would not be very plausible. It would suggest two very different atmospheric gasses present, and that one would always stay on the night side and the other always on the day side very unlikely... the atmospheres of all our known gas giants rotate at different rates at different latitudes. You would very quickly get the gasses to mix, or at the very least, have banding similar to what you see on Jupiter.

Gases tend to mix so a gas giant wouldn't necessarily be one color on one side and another color on another side. Also, the gas giant wouldn't exist close to the sun at all so you can forget about a warm moon that is eclipsed by the sun. In fact, the gas giants are always very, very far from the sun. If there were a gas giant too close to the star, then the massive gravitational field of the star would just strip away the gas part of the gas giant. That does exist. It's called a Chthonian planet.

Life requires chemical and thermal energy to survive. Moons in our solar system are too small to form an atmosphere so there is no greenhouse effect to keep them warm. If your moon was larger, say the size of earth, it could have an atmosphere and hold heat. Also, geothermal activity could produce heat from the planet instead of gaining it from the star. This occurs on Earth but to a smaller extent because the Earth has largely cooled though looking at Io, one of Jupiter's moons, we see it maintains a very high surface temperature due to the geothermal activity.

Gas Giants tend to have their gasses mix but being a sci-fi novel I am sure you could introduce something to alter this. Many novels simply add unnatural phenomenon that characters notice but largely ignore. This shows readers that you didn't just ignore reality and leaves a place open for an introduction to advanced beings in the future. Or the source of the phenomena could continue to be ignored.

First, its a sci fi novel. I know you want legitimacy, but it doesn't have to be.

Both answers are no.

However. The moon life thing. Why would your species need sunlight. They could evolve not to require it. They could live with cold and little energy. It's science.fiction, its whatver you want.

secondly, your gas giant can't be half and half. That would mean the pressence of two kinds of gasses on the planet in a 50 50, split ratio, which isn't possible.

Still. Extend the universe. Physics has proven there are 10 to the 500 other universes in existance. Say its one of them where these laws don't apply.

Oh, I want to be a writer too. I've written a fantasy book I'm trying to get published, so I just want to say good job and buona fortuna

Addressing your initial answer and not your "Additional Details".

The moons Io and Europa orbit Jupiter and pass through the shadow of the gas giant planet every orbit. The moon's rotation has nothing to do with it. So they experience a solar eclipse on every orbit. You could change that by having the orbit of your moon more inclined.

To avoid being an icy ball, the moon would either have to be large enough to have a rocky core with elements that produce heat by radioactive decay, or, as in the case of both Io and Europa, tidal flexing to create heat by being stretched by Jupiter and other moons.

Gas giants would have to be very large to have enough gravitational pull to retain an atmosphere after their central star initiated nuclear fusion within its core. That event creates a powerful solar wind that would blow away atmospheres from inner planets. So the gas giant would have to be very massive. But its large moons could be like Earth and even be in the "green" zone as they orbit the gas giant and the gas giant orbits its star.

As to the color of the gas giant, normally the rotation of the giant is so rapid that many circulation cells are created and lots of mixing takes place. However, like Jupiter, there could be a huge storm. Your gas giant could have one really, really, really big storm on one side. In addition, perhaps your gas giant could have been knocked over on its spin axis (like Uranus). Its polar regions could be very different depending on gas compositions, spin rates and distance from its sun.

Your moon could have heavy volcanic activity and thus not an iceball

It could also not be an iceball if it doesn't have an atmosphere, thus there is nothing to freeze (even if there isn't water i think its reasonable the gases would become solid at this temperate; Mars ice caps are solid carbon dioxide)

The gas on the gas giant could contain a chemical which changes when it contacts light. This does mean that to anyone on the moon it appears all one color.

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