Can a Giant Moon and Enclosed Desert Planet Coexist Realistically in Sci-Fi?

[ann-omaly]

Greetings Physics Brainiacs,
Please advise about the planetary / solar / lunar relationship I've developed in my sci-fi novel. It should be believable and devoid of issues with gravity. The protagonist wakes up on another world. The reader knows only what she does: the day is 44 hours in length, gravity matches Earth's, the planet has similar rotation (ie, sun rises east). What she sees:
• A desert planet, no plant life
• Towering mountain ranges, rugged impact craters
• No sign of water: no ice, no clouds, no dry lake beds
• Intense, deep blue sky (not dark or black: she's inside a gargantuan, invisible, climate-controlled enclosure)
• Large sun--almost twice Earth's sun
• Giant moon**
** Unsure if there would be gravity issues with this moon: It shows four days straight, filling 15-20% of the sky. It's almost daylight on those nights. Every time the moon appears in the daytime sky, it causes a full solar eclipse. I think this scenario will work if it's a giant gas object with a small, rapidly-spinning core. Would it nevertheless cause the planet's gravity to vary? If so, would a second moon or asteroid belt on the opposite side of the planet in the same lunar orbit create some kind of equalizing gravitational pull? The moonlight plays a critical role in a large segment of the book, so whatever I can do to support it is key. Thank you.

 

[Janus]

Greetings Physics Brainiacs,
Please advise about the planetary / solar / lunar relationship I've developed in my sci-fi novel. It should be believable and devoid of issues with gravity. The protagonist wakes up on another world. The reader knows only what she does: the day is 44 hours in length, gravity matches Earth's, the planet has similar rotation (ie, sun rises east). What she sees:
• A desert planet, no plant life
• Towering mountain ranges, rugged impact craters
• No sign of water: no ice, no clouds, no dry lake beds
• Intense, deep blue sky (not dark or black: she's inside a gargantuan, invisible, climate-controlled enclosure)
• Large sun--almost twice Earth's sun
• Giant moon**
** Unsure if there would be gravity issues with this moon: It shows four days straight, filling 15-20% of the sky. It's almost daylight on those nights. Every time the moon appears in the daytime sky, it causes a full solar eclipse. I think this scenario will work if it's a giant gas object with a small, rapidly-spinning core.

In order for a moon to fill that much of the sky it would have to be either very close or have a very low density. In either case, the planet's tidal force would tear it apart. I don't see how it would be possible to have a moon that looks that large.

Would it nevertheless cause the planet's gravity to vary? If so, would a second moon or asteroid belt on the opposite side of the planet in the same lunar orbit create some kind of equalizing gravitational pull?

In this case the largest concern is the planet's effect on the moon. That being said, the Moon's gravitational effect on the planet would also be tidal in nature. Since tidal effects cause bulges on both the moon side and anti-moon side, a body opposite the moon would only make the effect stronger . (For the same reason we get higher tides during the new moon, when the Moon and Sun are on opposite sides of the Earth.

The moonlight plays a critical role in a large segment of the book, so whatever I can do to support it is key. Thank you.

Have you considered this, Instead of a moon, have your planet as a satellite around a large Jovian planet.

You also might not need the gas giant/moon to be quite as large as you think. For one, our eyes can adapt quite a bit to light levels. A well lit office might be about 500 lux while direct sunlight can be 100,000 lux. So a lot less actual light can look almost as bright to our eyes.

Also, if you are using our moon as a guide, it actually reflects only 12% of the light that hits it (about the same as worn asphalt.), Whereas, Saturn for example, reflects about 34% of the light or almost three times as much. So a cloud shrouded body can more effective in providing nighttime light.

 

[ann-omaly]

In order for a moon to fill that much of the sky it would have to be either very close or have a very low density. In either case, the planet's tidal force would tear it apart. I don't see how it would be possible to have a moon that looks that large. In this case the largest concern is the planet's effect on the moon. That being said, the Moon's gravitational effect on the planet would also be tidal in nature. Since tidal effects cause bulges on both the moon side and anti-moon side, a body opposite the moon would only make the effect stronger . (For the same reason we get higher tides during the new moon, when the Moon and Sun are on opposite sides of the Earth.

Have you considered this, Instead of a moon, have your planet as a satellite around a large Jovian planet.

Also, if you are using our moon as a guide, it actually reflects only 12% of the light that hits it (about the same as worn asphalt.), Whereas, Saturn for example, reflects about 34% of the light or almost three times as much. So a cloud shrouded body can more effective in providing nighttime light.

Janus, a gas giant is the answer. Believable and easiest to convey in writing. Plus the character won't be aware she's on a moon rather than a planet. Thank you so much. Ann

 

[Drakkith]

Hrmm, I tried calculating the apparent size of Jupiter seen from Io and came up with about 19 degrees. (Which is HUGE! The sun is only about half a degree in apparent size I believe.) If correct, that could be a LOT of light reflected onto the moon depending on how much light the gas giant receives from its star.

 

[Janus]

Hrmm, I tried calculating the apparent size of Jupiter seen from Io and came up with about 19 degrees. (Which is HUGE! The sun is only about half a degree in apparent size I believe.) If correct, that could be a LOT of light reflected onto the moon depending on how much light the gas giant receives from its star.

Assuming the planet receives just about the same intensity of light as the Earth does (which, in order for the star to been seen as twice as large as the sun, would make it a class K star about 2/3 the mass of the Sun), and has an albedo like Jupiter's, you should get about 1100 lux when it is in full phase. This is equivalent to an overcast day or typical TV studio lighting.

 

[ann-omaly]

Drakkith and Janus, such specific light calculations--you just improved my novel. The believability of the character's new world is critical, and I feel a lot better about it right now. Thanks to both. The title is The Human.

 

[Drakkith]

Drakkith and Janus, such specific light calculations--you just improved my novel. The believability of the character's new world is critical, and I feel a lot better about it right now. Thanks to both. The title is The Human.

Just remember that while building a believable world is great, telling the story is even more important! Don't get caught up too much in the details of the science and forget about the story.

 

[ann-omaly]

Just remember that while building a believable world is great, telling the story is even more important! Don't get caught up too much in the details of the science and forget about the story.

The story's strong, if I do say so. But I also believe--as the saying goes--that "god is in the details". Thanks again for the detail support.

 

[Cosmo Novice]

The story's strong, if I do say so. But I also believe--as the saying goes--that "god is in the details". Thanks again for the detail support.

Just to make you aware - there are massive psychological inferences from seeing a HUGE object like that in the sky.

I would strongly reccomend two books for a deeper understanding of human reaction to non earth/familiar environments:
Isaac Asimov: Nemesis (This specifically deals with an inhabited Jovian artificial moon.
Arthur C Clarke: Rendezvous with Rama (This has some very good descriptions also)

I just wanted to bring to your attention to crucial psychological factors that will impact your novel. Hope this helps.

 

[ann-omaly]

Cosmo, my own story deals heavily in the psychological. Have read Rendezvous With Rama, and Rama II. Love to read Clarke; Asimov, not so much. Thanks for the input.

 

[vociferous]

It sounds interesting and I think making the "planet" a moon orbiting a gas giant is probably a better decision. Our earth-moon system is one of the smallest ratios of planet to satellite diameters known. Remember that the only known moons on the scale of of Earth's moon orbit planets many times the mass of the Earth.

I'm not sure if your original premise is impossible but it would probably have to be a double-planet system or your character would have to be located on the moon itself. Keep in mind, our moon is a significant fraction of the diameter of the Earth.

[URL]http://www.mhhe.com/physsci/astronomy/fix/student/images/09f03.jpg[/URL]

That does not even come near filling 20% of the sky. For two approximately equal mass rocky planets to do so, they have to be orbiting much closer.

 

[ann-omaly]

Thanks, Vociferous. Hadn't considered a double planet system--an interesting idea. But changing the character's location to a moon orbiting a gas planet seems the simplest solution for the effect I'm after. Hemingway famously compared writing to an iceberg: only 10% of it actually shows; the hidden remainder merely supports it. The object in my character's night sky needs to light the way, and be thought of by the reader as colossal. The actual % of sky that it occupies won't necessarily be explained, nor will the fact that she's on a moon instead of a planet. It just has to work, so that science-based readers like you won't think, Well, the moon's bright but the writer isn't.

 

[Travis_King]

Just a concern of mine with your requirements:

Aren't the Jovian satellites tidally locked? So you would have the massive body in the sky at all times, no?

 

[ann-omaly]

Three clarification questions for you, Travis:
1. do all gas giants necessarily share a tidally locked relationship with their satellites?
2. since the moon that my character inhabits is rotating, wouldn't the gas giant be out of sight for a certain portion of the day?
3. if a gas giant has to show in the sky at all times, would gravity fluctuate or otherwise be problematic?
Again, for the purposes of my novel, gravity must not be an issue, and the main character must have intense light at night. Thanks.

 

[Travis_King]

1. No, I imagine it isn't a necessity. But generally large bodies (like moons) orbiting very large bodies (like a big gas giant) will become tidally locked. Almost, if not, all of the major moons in our system are tidally locked to their parent planets.

2. The moon will rotate, but because it will likely be tidally locked, it's rotation will be syncronized with it's orbit (the definition of tidal lock) http://en.wikipedia.org/wiki/Tidal_locking

3. I doubt gravity would be much of an issue as far as people or structures are concerned.

 

[ann-omaly]

1. No, I imagine it isn't a necessity. But generally large bodies (like moons) orbiting very large bodies (like a big gas giant) will become tidally locked. Almost, if not, all of the major moons in our system are tidally locked to their parent planets.

2. The moon will rotate, but because it will likely be tidally locked, it's rotation will be syncronized with it's orbit (the definition of tidal lock) http://en.wikipedia.org/wiki/Tidal_locking

3. I doubt gravity would be much of an issue as far as people or structures are concerned.

Thanks for the link. I'll definitely scour it prior to the re-write.

 

[Dizzle]

Tidal locking takes some time though right, so if the moon wasn't that old, it could still rotate with respect to the planet, right?

 

[Drakkith]

Tidal locking takes some time though right, so if the moon wasn't that old, it could still rotate with respect to the planet, right?

I believe so, but I don't know the timeframe for tidal locking.

 

[Vanadium 50]

Now that I think of this, I think you probably don't recognize how big 20% of the sky is. The moon occupies 1/26000th of the sky; you're looking at 5000 moons. "Night" would be as bright as a cloudy day.

If you were this close, your planet would be close to falling apart from tidal forces.

 

[ann-omaly]

Now that I think of this, I think you probably don't recognize how big 20% of the sky is. The moon occupies 1/26000th of the sky; you're looking at 5000 moons. "Night" would be as bright as a cloudy day.

If you were this close, your planet would be close to falling apart from tidal forces.

Vanadium, you're right, of course. 1/5 of the sky would be overwhelming, to say the least. It doesn't need to be that huge. The point is to show a prominent, bright body in the night sky but not quantify an exact size. The gas giant will necessarily take up a larger portion of the sky than our moon or sun, but perhaps I can avoid tidal locking and rotational issues (suggested by Dizzle as possible) with something smaller. Again, readers will only see what the character sees, know what she knows--all through her personal filters, assumptions, and limited access to the facts of her new world. She is not a physicist.
All of you physics buffs have been very helpful and interesting. Thanks--Ann

 

[vociferous]

Tidal locking takes some time though right, so if the moon wasn't that old, it could still rotate with respect to the planet, right?

Yes, but you have to realize that this is proportional to distance and mass. An Earth mass moon orbiting a great distance from a gas giant like Jupiter would tend to take much longer to achieve tidal locking than a less massive moon orbiting closer.

Tidal locking would make the novel more realistic, but it is not as if you are violating some cherished law of physics if your moon is not tidally locked. For instance, the impact of a massive body could impart rotational momentum. Winning the lottery is pretty rare, but newspapers do not publish human interest stories about the people who lost. They publish the stories about the winners.

Also keep in mind that if the moon is tidally locked, the only real consequence that I can think of for your story is that the planet is going to be in the same place in the sky (assuming the character is stationary). The "sun" will still rise and set and other moons may occasionally pass close enough to see.

 

[Ryan_m_b]

Don't forget to think about the implications for life on this tidally locked planet. Anything that evolved on the far side is going to get more sunlight than that on the near side, and obviously life on the near side is going to have reflected light from the gas giant. There's some interesting scope there.

 

[DaveC426913]

2. The moon will rotate, but because it will likely be tidally locked, it's rotation will be syncronized with it's orbit (the definition of tidal lock) http://en.wikipedia.org/wiki/Tidal_locking

i.e. it will rotate with respect to the stars; it won't rotate wrt the planet overhead.

Note though that this rotation wrt the stars will be all but impossible to see and therefore useless. When the overhead planet is reflecting any light (pretty much all the time), all but the very brightest stars will be completely washed out and invisible (planet's light will destroy her night vision). The only time revolution against the stars will be visible is when the moon is fully in the planet's shadow (eclipse phase).

 

[ann-omaly]

Yes, but you have to realize that this is proportional to distance and mass. An Earth mass moon orbiting a great distance from a gas giant like Jupiter would tend to take much longer to achieve tidal locking than a less massive moon orbiting closer.

Tidal locking would make the novel more realistic, but it is not as if you are violating some cherished law of physics if your moon is not tidally locked. For instance, the impact of a massive body could impart rotational momentum. Winning the lottery is pretty rare, but newspapers do not publish human interest stories about the people who lost. They publish the stories about the winners.

Also keep in mind that if the moon is tidally locked, the only real consequence that I can think of for your story is that the planet is going to be in the same place in the sky (assuming the character is stationary). The "sun" will still rise and set and other moons may occasionally pass close enough to see.

Vociferous, the moon in my story needs to rotate; the sun needs to rise and set. Tidal locking won't work for my purposes, so I'm calling this the lottery win, to use your analogy. I'm hoping to receive something of a "this scenario is possible" consensus from the various participants in this discussion...?

 

[Travis_King]

Tidal locking of the moon to the planet doesn't mean the sun won't rise and set. The so-called "dark side" of the moon does see the light from the sun as it orbits the earth. If you have body tidally locked to a large planet, the planet will always be in the sky, but the sun will still rise and set. To what extent each will be present will depend on the location on the satellite.

Check out this basic animation and watch the moon as it orbits the earth
http://jove.geol.niu.edu/faculty/stoddard/JAVA/moonphase.html

 

[ann-omaly]

Tidal locking of the moon to the planet doesn't mean the sun won't rise and set. The so-called "dark side" of the moon does see the light from the sun as it orbits the earth. If you have body tidally locked to a large planet, the planet will always be in the sky, but the sun will still rise and set. To what extent each will be present will depend on the location on the satellite.

Check out this basic animation and watch the moon as it orbits the earth
http://jove.geol.niu.edu/faculty/stoddard/JAVA/moonphase.html

Travis, observing the animation you provided: daylight on our moon lasts about two weeks in a single location--? I think my character's moon will need to rotate.

 

[DaveC426913]

Travis, observing the animation you provided: daylight on our moon lasts about two weeks in a single location--? I think my character's moon will need to rotate.

Daylight lasts half of a body's day. The Moon's day is a month long, so its day is two weeks.

Your Moon could have a shorter day by being closer to its planet. Or its planet could be more massive.

But you can't go too far, or you'll get problems with tides, quakes and, ultimately, disintegration.

 

[ann-omaly]

Daylight lasts half of a body's day. The Moon's day is a month long, so its day is two weeks.

Your Moon could have a shorter day by being closer to its planet. Or its planet could be more massive.

But you can't go too far, or you'll get problems with tides, quakes and, ultimately, disintegration.

Dave, now it looks to me that my character needs to be on a planet. I can give her bright light at night via several moons that show simultaneously for a few days. Thanks.

 

[Travis_King]

But you lose your "large object in the sky" effect. If you had two planets of similar size locked to each other, you could have something approaching your conditions, depending on her location on the planet. Since you will have librations so your large planet in the sky will still move around throughout the day (not to mention the filtered effect when the star's light is dominant).

Point is, if you just mention these things, eg. "She looked up in the sky and saw a big freaking planet that is always there", People are going to make up their own minds on why. As long as you don't give numbers, nobody can really prove you wrong as long as what you propose is feasable.

 

[Ryan_m_b]

Dave, now it looks to me that my character needs to be on a planet. I can give her bright light at night via several moons that show simultaneously for a few days. Thanks.

If the moons are low enough (like Phobos) they will orbit every few hours.

But you lose your "large object in the sky" effect. If you had two planets of similar size locked to each other, you could have something approaching your conditions, depending on her location on the planet. Since you will have librations so your large planet in the sky will still move around throughout the day (not to mention the filtered effect when the star's light is dominant).

Point is, if you just mention these things, eg. "She looked up in the sky and saw a big freaking planet that is always there", People are going to make up their own minds on why. As long as you don't give numbers, nobody can really prove you wrong as long as what you propose is feasable.

Indeed. The character could even wonder about the strangeness of it, this is a SF novel after all. It could turn out that the giant moon in the sky is pretty much hollow and put there by ETs nan as a night light.