Fantasy world creation

[Prometheus]

I'm creating an alternative Earth for a fantasy story and wondered if people may help me speculate on certain features of it, and how 'Earthlike' they would render the world. I don't need realistic so much as believable - but i would like it to be believable to a scientifically literate crowd.

 

For instance, would it be possible to have the world orbit in the goldilocks zone of a binary system and still be 'Earthlike'. I guess this would alter the seasonality of the world, but i don't know enough astronomy to speculate what difference that might make. Would it depend on the relative orbital speeds of the two stars and the the world?

 

Further, would the stars both have to be yellow like our own sun. If even one emits a different spectra there would be significant consequences for the biodiversity surely?

 

And how would eclipses of one sun relative to the world effect its weather and climate?

 

Loads more questions like that, so any pointers, tips, references appreciated.

 

[Ophiolite]

Firstly, despite earlier reservations about the stability of planets in a binary system I believe it has now been established (through simulations?) that stable orbits could exist. I half suspect that at least one such system has been found by Kepler, but I may be imagining that!

 

The issue for your tale is really how large do you want the second sun to be and how close. If it is as massive as the first, then I think, to avoid an unstable orbit, it may need to be placed so far away that it would appear as no more than a very bright star. On the other hand if it small enough to be able to be placed closer, then it will - again - appear more like a bright star than a dim sun.

 

Of course you could include words to the effect that "Contrary to expectations of the early theorists it was possible for a stable orbit to exist in the system, despite the proximity of the twins." And I have read plenty of SF stories in which the issue is not even discussed and they rarely suffered from a suspension of suspension of disbelief.

 

 

Edit: Wow, am I out of date!

 

From 2007, for example, this abstract:

 

The existence of planets in stellar binary (and higher order) systems has now been confirmed by many observations. The stability of planetary orbits in these systems has been extensively studied, but no precise stability criteria have so far been introduced. Therefore, there is an urgent need for developing stringent mathematical criteria that allow us to precisely determine whether a planetary orbit in a binary system is stable or unstable. In this Letter, such criteria are defined using the concept of Jacobi’s integral and Jacobi’s constant. These criteria are used to contest previous results on planetary orbital stability in binary systems.

You will find several articles on google Scholar searching for "stable planetary orbits" AND "binary systems".

Edited December 31, 2015 by Ophiolite

[Prometheus]

Good to know it's at least somewhat feasible.

 

I first imagined the suns would, at times of the year, both appear roughly the same size and luminosity in the sky, while at other times one would hide behind the other such that only one sun would appear in the sky. I think your latter suggestion would work fine in this case - i can easily believe in such a stable system even if it is unrealistic.

 

Given we accept this system as given, how would that influence the seasons? When there are two suns in the sky would it not get hotter, such that it would influence seasons more than the reasons we experience seasons on Earth? Could it then be that summer and winter occur at the same time all over this world?

[dimreepr]

Given we accept this system as given, how would that influence the seasons? When there are two suns in the sky would it not get hotter, such that it would influence seasons more than the reasons we experience seasons on Earth? Could it then be that summer and winter occur at the same time all over this world?

 

 

Our seasons are due to the moons stabilising effect, keeping the Earths tilt between 23 and 26 degrees, in the system you propose, without a moon, seasons would have little meaning; with a moon then I would imagine it’s tides that would be most affected.

[Prometheus]

I wasn't aware it was the moon that kept the earth at that angle. Actually i imagine there would be three moons: a smaller than ours but closer, one about the same size but farther (such that they both are roughly the same size in the night sky) and a small irregular shaped one with a more elliptical orbit. No idea how stable this would be.

[dimreepr]

I wasn't aware it was the moon that kept the earth at that angle. Actually i imagine there would be three moons: a smaller than ours but closer, one about the same size but farther (such that they both are roughly the same size in the night sky) and a small irregular shaped one with a more elliptical orbit. No idea how stable this would be.

 

 

I doubt many people would have any idea, me included, so why not accept this system as equally viable and add a few quirks that enhance your storyline?

Edited December 31, 2015 by dimreepr

[fiveworlds]

Further, would the stars both have to be yellow like our own sun. If even one emits a different spectra there would be significant consequences for the biodiversity surely?

 

What makes you think our sun is yellow??

[Fred Champion]

I wonder if you have ever considered what life would be like on a planet like Uranus. With the axis tilted almost 90 degrees to the orbital plane, the seasons would be so much different from Earth's. It could be that both winter and summer would be so extreme that the population would have to migrate back and forth over the equator. At the distance of Uranus a complete seasonal cycle would be just about an average lifetime. It would be interesting to see a history of navigation on such a planet (without a pole star).

 

Good luck

[imatfaal]

I wonder if you have ever considered what life would be like on a planet like Uranus. With the axis tilted almost 90 degrees to the orbital plane, the seasons would be so much different from Earth's. It could be that both winter and summer would be so extreme that the population would have to migrate back and forth over the equator. At the distance of Uranus a complete seasonal cycle would be just about an average lifetime. It would be interesting to see a history of navigation on such a planet (without a pole star).

 

Good luck

 

 

I think the more extreme problem would be that day length (not in terms of the planet's rotation - but in terms of sun in sky) would last nearly as long as the seasons.

 

If we set a situation in which the "North pole" is in complete shadow and with the "South Pole" having the sun at the highest point in the sky. Even though the planet rotates about its access every 17 and bit earth hours - the North Pole will not see the sun again till the whole planet has moved around in its orbit for almost 21 years! And it will be a whole 42 years before the sun is over head at the north pole. When axis on to the sun there is only a small moving region of the planet that is diurnal

[swansont]

What makes you think our sun is yellow??

 

Because that's what the peak emission is, at around 550 nm. That's around the division between green and yellow.

 

 

Our seasons are due to the moons stabilising effect, keeping the Earths tilt between 23 and 26 degrees, in the system you propose, without a moon, seasons would have little meaning; with a moon then I would imagine it’s tides that would be most affected.

 

The moon stabilizes the earth, but to say the moon causes the seasons is a stretch. The seasons are caused by the tilt.

 

One could envision seasons (of a sort) being caused by an elliptical orbit that's more pronounced than ours.

Good to know it's at least somewhat feasible.

 

I first imagined the suns would, at times of the year, both appear roughly the same size and luminosity in the sky, while at other times one would hide behind the other such that only one sun would appear in the sky. I think your latter suggestion would work fine in this case - i can easily believe in such a stable system even if it is unrealistic.

 

Given we accept this system as given, how would that influence the seasons? When there are two suns in the sky would it not get hotter, such that it would influence seasons more than the reasons we experience seasons on Earth? Could it then be that summer and winter occur at the same time all over this world?

 

Two suns might just mean the orbit needs to be further away, or the suns have a lower power output (i.e. cooler, more reddish in color)

[Delta1212]

 

 

I think the more extreme problem would be that day length (not in terms of the planet's rotation - but in terms of sun in sky) would last nearly as long as the seasons.

 

If we set a situation in which the "North pole" is in complete shadow and with the "South Pole" having the sun at the highest point in the sky. Even though the planet rotates about its access every 17 and bit earth hours - the North Pole will not see the sun again till the whole planet has moved around in its orbit for almost 21 years! And it will be a whole 42 years before the sun is over head at the north pole. When axis on to the sun there is only a small moving region of the planet that is diurnal

You'd essentially have seasonal day-length, wouldn't you? (More so than on Earth, I mean). During the time of year when the poles were aligned with the sun, the entire world would act like the poles on Earth, either being bathed entirely in light or darkness at all times of "day" with each happening to both hemispheres once a year. When the poles are perpendicular with the sun, you'd have a normal day/night cycle as the planet rotated, which would happen twice a year. And of course most of the year would be spent transition between those two states.

 

So it might be a full revolution before the sun was back to being directly overhead, but you'd still get daylight from a normal day/night cycle during a solid chunk of the year between those extremes of light and dark.

 

Edit: Although it wouldn't really be variable day length so much as variable brightness, wouldn't it? If you start in 'permanent' day, you'd start getting slightly dinner periods as the poles moved out of alignment with the start and they'd keep getting simmer until you had true day and night with each rotation. Then the days would start getting dimmer as you moved into the long period of darkness. And then the period of rotation that was previously night would start getting brighter again as the planet moved into position for the second day/night cycle of the year but on the opposite side.

Edited January 8, 2016 by Delta1212

[dimreepr]

The moon stabilizes the earth, but to say the moon causes the seasons is a stretch. The seasons are caused by the tilt.

 

 

That wasn’t the intent just that the moon’s stabilising effect is the reason for the seasons we have.

 

 

One could envision seasons (of a sort) being caused by an elliptical orbit that's more pronounced than ours.

 

 

 

But without a moon wouldn’t the Earth's wobble make the seasons chaotic in this scenario?

[swansont]

 

 

That wasn’t the intent just that the moon’s stabilising effect is the reason for the seasons we have.

 

 

 

 

But without a moon wouldn’t the Earth's wobble make the seasons chaotic in this scenario?

 

 

Possibly more chaotic, but we'd still have seasons. It would depend on the parameters. Also, remember that this wobbling is over a fairly long time scale, The nature of the seasons might change over millions of years, but not from one year to the next. That has certain evolutionary impacts, to be sure, but not immediate ones.

 

Take an orbit that changed the distance to the sun by ~10% between aphelion and perihelion, but no axial tilt. That's about a factor of 20% change in power. Some pretty significant temperature swings would probably result over the course of a year. The distinction would be that there is no difference between the hemispheres — it would be e.g. winter everywhere when the planet was far from the sun. How much of an effect would wobble be for such a planet?

[dimreepr]

My imagined repost to your reply, would be the predictability of a season but I now concede that, however, the possible affect on the life of said planet may mean complex life would struggle to emerge.

On reflection I see your point, maybe conditions would be very different to the Earth but a global winter/summer, depending on severity, wouldn’t exclude complex life; but given the OP there are many scenarios that could be included in a, believable, science-fiction story.

Edited January 8, 2016 by dimreepr

[Moontanman]

I am toying with the idea of a habitable planet in a triple star system but the story is more important than the scientific accuracy.

[Prometheus]

The distinction would be that there is no difference between the hemispheres...

 

I'm thinking of having no axial tilt , with seasons due to the elliptical orbit.

 

I also imagine that the seasonal variations could be exacerbated by how many suns there are visible in the sky at the time of aphelion or perihelion. If there is only one sun visible during the aphelion then it would be a particularly harsh winter over the entire world.

 

I imagine this would particularly motivate civilisations at high latitudes to study celestial mechanics to predict the coming of these harsh winters.

 

I wonder if you have ever considered what life would be like on a planet like Uranus...

 

I briefly considered this but i thought it would create life and civilisations too different from that on Earth for the setting i want.

[swansont]

I'm thinking of having no axial tilt , with seasons due to the elliptical orbit.

 

I also imagine that the seasonal variations could be exacerbated by how many suns there are visible in the sky at the time of aphelion or perihelion. If there is only one sun visible during the aphelion then it would be a particularly harsh winter over the entire world.

 

I imagine this would particularly motivate civilisations at high latitudes to study celestial mechanics to predict the coming of these harsh winters.

 

 

Perhaps, but the suns are going to block each other many times over the course of the year.

[Prometheus]

True, but i think i will just limit this to about 4-5 times per year.

 

Now just have to consider the impact the three moons would have on the planet...

[imatfaal]

True, but i think i will just limit this to about 4-5 times per year.

 

Now just have to consider the impact the three moons would have on the planet...

 

A 7-body problem? Good luck with that. BTW - have a read of the Three Body Problem - Liu Cixin

[Fred Champion]

I've been trying to remember anything in the Star Wars movies that gave an indication of climate cycles. Mostly I remember sand and barren landscapes. Did they just avoid the issue? I'll bet if we ever do get around to colonizing other planets we will want to know all about the climate before we go.

[Roelort]

I am writing a fantasy / sci-fi story and I do not take this into account. I have several other ideas I take into account, I have enough other aspects to check and to regulate. This will just be accepted by my readers as fact. Also, how important can it get? I assume you do not need to explain all these things into your story , so having it covered is nice, but probably unnecessary to be spending tons of time on.

[Prometheus]

The world i'm creating is for a tabletop RPG: i need to be able to narrate the story on the fly. I'm thinking by creating quite a detailed world that when someone does something unexpected i will be better prepared to improvise. Who knows when someone might want to plan some elaborate naval attack involving complicated tides. But, like you say, i don't want to think about it too much - this thread represents pretty much everything i have on the celestial mechanics of the world at the moment.