# Any astronomers/cosmologists/planetary scientists in the house?



## Trick (Jan 12, 2017)

I have created a world in my WIP that has some differences from earth and, as much as it is fantasy, I'd like to get some things right. 

I could list all the questions but in case no one on here is an "expert" (a term I use loosely), I'll just start with one.

If a planet has shorter years than Earth, and is decidedly smaller, what are some things I need to consider?


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## DragonOfTheAerie (Jan 12, 2017)

Uhhh...my credentials extend to a bunch of books I read when I was 5, one or two documentaries, and occasionally looking up at the sky, but I'll try to help.  

If a planet has shorter years than Earth, that means it'll orbit its sun at a shorter distance. I don't think the shorter years are a problem. The shorter distance probably is. There's a very small margin of distance from the sun at which a planet could sustain life. A weaker sun might mitigate this, but we may have other problems. 

A smaller planet will probably have less gravity. Less gravity means less ability to hold an atmosphere, for one thing. I don't know what other effects it might have. You could make your planet denser, but the core would need a different composition, and you want to keep your outer core kind of the same as Earth's to have a magnetic field...so everything doesn't die of radiation. 

You get problems almost anytime you deviate from Earth, because Earth is about ideal for sustaining life.


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## TheKillerBs (Jan 12, 2017)

Frankly, there are just so many variables to consider that you can get away with just changing the length of the years and the size of the planet. However, since you are changing the length of the year, I would also suggest playing with seasons (look up Artifexian's axial tilt video on YouTube), day and month length, just to avoid straining the willing suspension of disbelief of some readers.

ETA: one thing that _will_ be constant is that the horizon will be closer if the planet is smaller.


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## ThinkerX (Jan 12, 2017)

> If a planet has shorter years than Earth,



depends on the type of star it's orbiting.  A star of the same type as ours (G2V), a shorter year means a hotter world, though axial tilt could mitigate this.  If the planet is orbiting a cooler star - say G8V or K0V - then the shorter year becomes necessary if you wish an earthlike range of climate.  In this case, about the only thing that would stand out is the 'sun' would be more orange than yellow.

Things start to get seriously freaky if the planet in question orbits an M type red dwarf.  With these stars, the 'habitable zone' is very close in; talking about a 'year' of no more than a couple earth months.  More, in many cases, the planet in question would be tidally locked or have a very slow rotation (1 day = multiple earth weeks, maybe).  Hence, one side scorching hot, the other a deep freeze.  Even if not tidally locked, and having a normal day/night cycle, the world would still be on the cold side in most cases - think glaciers/ice caps to within 20 degrees of the equator.



> and is decidedly smaller,



Main thing here is density.  As 'solid' planets go, the earth is on the dense side.  That, combined with its size, gives the earth sufficient gravity to retain an atmosphere and the magnetic fields necessary to protect that atmosphere - and living things on earth from an impressive assortment of lethal radiation.  That said, IF the density were comparable, Earth could be half it's current size and still retain a breathable atmosphere and the necessary magnetic fields.    

Mars lacks the earths density and magnetic fields.  Hundreds of millions of years ago, Mars was a FAR more hospitable place than it is now.

Part of the magnetic field thing comes not just from density, but plate tectonics - vast slabs of the planets crust moving against each other.  Mars doesn't have this either, just giant dead volcanoes.  For plate tectonics to function properly, you need water - lots of water, as in oceans.

That said...had Mars been a bigger physically and just a shade denser (say, about the same size as Venus), it might have had a much denser original atmosphere that would have weathered the radiation attrition much better.  In that case, there is a fair shot that Mars today, in that alternate reality, would be...a lot less unpleasant, though 'habitable' is likely a stretch.

Then there is the option I used with my worlds - terraforming.  Apart from being about the right size and in the right orbits, most of my worlds were 'giant dead rocks' until aliens showed up tens of thousands of years ago and improved things considerably.  The primary world, Char, is about half earths size, with just one true ocean.  Another is almost a water world.


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## Trick (Jan 13, 2017)

TheKillerBs said:


> Frankly, there are just so many variables to consider that you can get away with just changing the length of the years and the size of the planet. However, since you are changing the length of the year, I would also suggest playing with seasons (look up Artifexian's axial tilt video on YouTube), day and month length, just to avoid straining the willing suspension of disbelief of some readers.
> 
> ETA: one thing that _will_ be constant is that the horizon will be closer if the planet is smaller.



That video not only reminded me of my lessons about axial tilt in college but it opened my eyes to a major solution for another question I hadn't asked in here yet. I subscribed to his channel, Thanks for pointing me there!


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## Trick (Jan 13, 2017)

ThinkerX said:


> depends on the type of star it's orbiting.  A star of the same type as ours (G2V), a shorter year means a hotter world, though axial tilt could mitigate this.  If the planet is orbiting a cooler star - say G8V or K0V - then the shorter year becomes necessary if you wish an earthlike range of climate.  In this case, about the only thing that would stand out is the 'sun' would be more orange than yellow.



This is the exact kind of info I'm looking for, thank you! I can easily mention the orange(ish) sun and incorporate it very mildly without any info dump stuff. This is great!

EDIT: I just looked into an orange sun and learned that it is postulated that an orange sun would result in a green sky. Love this idea!


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## Trick (Jan 13, 2017)

DragonOfTheAerie said:


> Uhhh...my credentials extend to a bunch of books I read when I was 5, one or two documentaries, and occasionally looking up at the sky, but I'll try to help.
> 
> If a planet has shorter years than Earth, that means it'll orbit its sun at a shorter distance. I don't think the shorter years are a problem. The shorter distance probably is. There's a very small margin of distance from the sun at which a planet could sustain life. A weaker sun might mitigate this, but we may have other problems.
> 
> ...



I think your credentials can just be "knowing more about this than Trick" - that's plenty good enough for me. 

Yes, denser for the smaller planet is good, I don't want too much deviation from Earth's gravity because I want the basics of life to be the same. Liquid outer core -check. The year length is not terribly shorter than Earth but the days are shorter also so ThinkerX's suggestion of the different type of star may be the perfect solution. And I definitely need tectonic plates.


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## SergeiMeranov (Jan 13, 2017)

If you really want a bunch of in-depth info.  I'd recommend the book World-Building.  It's available on Amazon.  It goes through, in excruciating detail, how and why planets form and how to take into account year length, climate, etc.  It is also a good resource for connecting the various parts which is really important if you're doing hard sci-fi or, if it exists, hard fantasy(?).  It's available here.

I'd say, without consulting the book, that if it has a shorter year than earth and is smaller you have a couple considerations.  Does gravity work the same way?  If it's a shorter year length and years are calcuated the same way as on Earth then it means it's closer to its star.  If it's closer to its star then is it hotter?  If not, does that mean that its star is dimmer or older?  There's probably other good questions that are related to year length but the book would be a better resource for figuring that out.

That said, if your book is fantasy and you don't care TOO much about accuracy, I'd say you can take the GRRM route and just never explain it or elaborate.  Fantasy readers, myself included, seem pretty forgiving of not totally accurate world building from a geological and celestial perspective.


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## DragonOfTheAerie (Jan 13, 2017)

SergeiMeranov said:


> If you really want a bunch of in-depth info.  I'd recommend the book World-Building.  It's available on Amazon.  It goes through, in excruciating detail, how and why planets form and how to take into account year length, climate, etc.  It is also a good resource for connecting the various parts which is really important if you're doing hard sci-fi or, if it exists, hard fantasy(?).  It's available here.
> 
> I'd say, without consulting the book, that if it has a shorter year than earth and is smaller you have a couple considerations.  Does gravity work the same way?  If it's a shorter year length and years are calcuated the same way as on Earth then it means it's closer to its star.  If it's closer to its star then is it hotter?  If not, does that mean that its star is dimmer or older?  There's probably other good questions that are related to year length but the book would be a better resource for figuring that out.
> 
> That said, if your book is fantasy and you don't care TOO much about accuracy, I'd say you can take the GRRM route and just never explain it or elaborate.  Fantasy readers, myself included, seem pretty forgiving of not totally accurate world building from a geological and celestial perspective.



Hard fantasy is a thing but it refers more to strictly defined magic rules than realism, I think.


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## Trick (Jan 13, 2017)

SergeiMeranov said:


> If you really want a bunch of in-depth info.  I'd recommend the book World-Building.  It's available on Amazon.  It goes through, in excruciating detail, how and why planets form and how to take into account year length, climate, etc.  It is also a good resource for connecting the various parts which is really important if you're doing hard sci-fi or, if it exists, hard fantasy(?).  It's available here.
> 
> I'd say, without consulting the book, that if it has a shorter year than earth and is smaller you have a couple considerations.  Does gravity work the same way?  If it's a shorter year length and years are calcuated the same way as on Earth then it means it's closer to its star.  If it's closer to its star then is it hotter?  If not, does that mean that its star is dimmer or older?  There's probably other good questions that are related to year length but the book would be a better resource for figuring that out.
> 
> That said, if your book is fantasy and you don't care TOO much about accuracy, I'd say you can take the GRRM route and just never explain it or elaborate.  Fantasy readers, myself included, seem pretty forgiving of not totally accurate world building from a geological and celestial perspective.



Thanks for the tip, I will check that book out. 

I don't want to write "hard fantasy" per se but I want to know the world I'm working in. Most of these details will never make it into the book but I'll know them and that, I believe, will lead to a better world and possibly open new opportunities in the story.


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## Trick (Jan 13, 2017)

So, I'd consider that question answered, as far as necessary for now, though I still will welcome any info people think to be relevant.

My next question is about the moon. With a small planet, with density adjusted to be at the same gravity as Earth, how big can a moon legitimately be?

And since I'm seriously considering a 60 degree axial tilt, are there other things I need to consider?


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## ThinkerX (Jan 13, 2017)

If the overall density (mass) of the planet is the same as earths, give or take a fraction, then the moon could be about the same size as earths moon.  Note that many tens of millions of years ago (dinosaur times), the moon orbited far closer in than it does now, and appeared correspondingly larger in the sky.   The current dominant theory is that a large moon is desirable for a habitable planet - gets into tides and tectonic plates.

60 degree axial tilt - get set for some severe seasons.  Climate zones are also likely to be compressed.


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## Trick (Jan 13, 2017)

ThinkerX said:


> If the overall density (mass) of the planet is the same as earths, give or take a fraction, then the moon could be about the same size as earths moon.  Note that many tens of millions of years ago (dinosaur times), the moon orbited far closer in than it does now, and appeared correspondingly larger in the sky.   The current dominant theory is that a large moon is desirable for a habitable planet - gets into tides and tectonic plates.
> 
> 60 degree axial tilt - get set for some severe seasons.  Climate zones are also likely to be compressed.



A large moon is what I'm going for so I'll keep that plan for now. I wonder if it can be on a narrower elliptical track than ours is so that the supermoon is more severe than our measly 14% apparent increase?

In the Artifexian's axial tilt video on YouTube, which TheKillerBs told me about, he demonstrates that a 60 degree axial tilt would actually switch the climate zones, making what we think of as the equator into and arctic zone and separating the two hemispheres into two tropical zones. It is so perfect for my book that it's like a mini miracle.

However, the extreme seasons are something to consider and are fortunately already a part of my world to some extent. I'll need to decide if they can be more extreme than I planned without damaging the plot.


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## ThinkerX (Jan 13, 2017)

> In the Artifexian's axial tilt video on YouTube, which TheKillerBs told me about, he demonstrates that a 60 degree axial tilt would actually switch the climate zones, making what we think of as the equator into and arctic zone and separating the two hemispheres into two tropical zones. It is so perfect for my book that it's like a mini miracle.



Halfway to a tidally locked planet. I say 'halfway' because the 'tilt' switches as the planet orbits.  Essentially, what you get (for the polar regions) is a 'day long summer' (lasting several earth months) featuring temperatures at its peak that would make the Sahara Desert look cool followed by a months long, utterly dark winter with temperatures colder than wintertime Antarctica. 

(I am considering a world like this for a long planned tale of my own, but need to think through the mechanics.)


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## Trick (Jan 14, 2017)

ThinkerX said:


> Halfway to a tidally locked planet. I say 'halfway' because the 'tilt' switches as the planet orbits.  Essentially, what you get (for the polar regions) is a 'day long summer' (lasting several earth months) featuring temperatures at its peak that would make the Sahara Desert look cool followed by a months long, utterly dark winter with temperatures colder than wintertime Antarctica.
> 
> (I am considering a world like this for a long planned tale of my own, but need to think through the mechanics.)


I think what you're describing is more like a 90 degree tilt. 

I have another planned book that involves something similar and the only way humans survive is by constantly traveling within the livable zone and bringing their farming with them on huge land barges that also can float. That story needs its mechanics worked out also.


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## K.S. Crooks (Jan 15, 2017)

Shorter year means the planet is closer to their star, thus the star must be cooler to allow the planet to have liquid water to sustain life as we know it. If the planet is smaller this could be taken two ways: The planet has less volume and/or the planet has less mass. For example Saturn has a larger volume than the Earth, but is far less dense and would float in our oceans if it could fit. As opposed to Jupiter which has more mass than all the other planets, moon and asteroids combined.

Less mass has a bigger effect as this dictates the strength of gravity. This then has an effect of the species living on the planet and the likelihood of impacts from stray objects, which is a bigger concern as the planet is closer to its star. It depends on the type of story you are writing whether any of this matters. If its a fantasy story and the people never obtain flight, travel into space or have any astrological beliefs then none of this matter. If they are a spacefaring species then the proximity to the star and strength of gravity could be used as factors, such as launch windows to travel to another planet.


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## TheKillerBs (Jan 15, 2017)

Trick said:


> My next question is about the moon. With a small planet, with density adjusted to be at the same gravity as Earth, how big can a moon legitimately be?



As Thinker said, as long as the mass stays the same, you can have a moon of the same mass as ours, no problem. Things to keep in mind: the distance between both centres of mass will be the same as between the Earth and Moon, but the distance between their surfaces probably wouldn't (unless you want to make your moon extremely large with very low density). Also, unless you make the moon somewhat larger, it would appear smaller on your planet's sky.


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## Trick (Jan 18, 2017)

K.S. Crooks said:


> Shorter year means the planet is closer to their star, thus the star must be cooler to allow the planet to have liquid water to sustain life as we know it. If the planet is smaller this could be taken two ways: The planet has less volume and/or the planet has less mass. For example Saturn has a larger volume than the Earth, but is far less dense and would float in our oceans if it could fit. As opposed to Jupiter which has more mass than all the other planets, moon and asteroids combined.
> 
> Less mass has a bigger effect as this dictates the strength of gravity. This then has an effect of the species living on the planet and the likelihood of impacts from stray objects, which is a bigger concern as the planet is closer to its star. It depends on the type of story you are writing whether any of this matters. If its a fantasy story and the people never obtain flight, travel into space or have any astrological beliefs then none of this matter. If they are a spacefaring species then the proximity to the star and strength of gravity could be used as factors, such as launch windows to travel to another planet.




My goal is to have a denser planet, e.g. less volume, same mass. But that does create the question of effects on gravity. If a planet were smaller than Earth but had the same mass, would the gravity be the same or would there be a difference because of the difference in volume? I honestly have no clue. I would hope that, with the assumption that life forms there are similar to here, that any variance in gravity would be compensated for by its own effect on those lifeforms, thus things would seem the same even if they were different per a scale. It's like the old joke: If you woke up tomorrow and everything was 10% smaller, would you notice?

It is a Science Fantasy or MagiTech story. There is a lot of tech but it's all magic based and relatively recent. There is no space travel and I don't think there ever will be in any of the stories I write in this world.


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## Trick (Jan 18, 2017)

TheKillerBs said:


> As Thinker said, as long as the mass stays the same, you can have a moon of the same mass as ours, no problem. Things to keep in mind: the distance between both centres of mass will be the same as between the Earth and Moon, but the distance between their surfaces probably wouldn't (unless you want to make your moon extremely large with very low density). Also, unless you make the moon somewhat larger, it would appear smaller on your planet's sky.



I want the moon to appear larger. As for mass, I don't mind playing with density to keep it doable but enlarging the moon to get the effect I want. Also, I'm looking at the possibility that the moon could simply contain more of some material that would raise the albedo (make it reflect more light). The brightness is really the important factor, although the size, specifically at the supermoon is the most important thing. 

Basically, in this world the sun supplies the energy for almost everything - there is a magic that is very efficient at harnessing solar power, i.e. one hour of sunlight can make a particular task possible for days. In direct sunlight, there is no delay in function for said task. At one point, it will harness the UV light reflected by the moon just enough to accomplish that same task for a few moments. I think this will only be believable if the moon is larger and brighter than our supermoon.


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## TheKillerBs (Jan 18, 2017)

Trick said:


> I want the moon to appear larger. As for mass, I don't mind playing with density to keep it doable but enlarging the moon to get the effect I want. Also, I'm looking at the possibility that the moon could simply contain more of some material that would raise the albedo (make it reflect more light). The brightness is really the important factor, although the size, specifically at the supermoon is the most important thing.
> 
> Basically, in this world the sun supplies the energy for almost everything - there is a magic that is very efficient at harnessing solar power, i.e. one hour of sunlight can make a particular task possible for days. In direct sunlight, there is no delay in function for said task. At one point, it will harness the UV light reflected by the moon just enough to accomplish that same task for a few moments. I think this will only be believable if the moon is larger and brighter than our supermoon.



If you want the moon to appear larger, you could place it closer and give it proportionally less mass to keep the gravitational pull equal.


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## AJ Stevens (Jan 24, 2017)

I'd like to offer the flip side of the argument for a shorter year. It doesn't _necessarily_ mean that your planet is closer to the star. A more massive star means more gravity means the planet has to orbit faster means a shorter year. I'm not too hot on the knock-on effects of such an occurrence however.


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## Duncan M (Jan 24, 2017)

Hello! Aspiring Astrophysicist here. I'm certainly no expert, but when it comes to moons I'll say the sky is the limit (bad).

Since you're looking for a (relatively) large moon orbiting a smaller planet, you may want to consider a tidally-locked planet-moon system, where the moon and planet are close enough that both perpetually face one side towards the other. Thus, the planet rotates once per lunar orbit, which could potentially be shorter than a day. From the surface, the Moon would never appear to move across the sky, instead just wobbling back-and-forth predictably.

This would result in a world where only one side of the planet receives moonlight. Additionally, tides on this planet would be interesting: the moon would cause a kind of perpetual "high-tide" on the regions facing directly towards it (and opposite), intensified when the Sun is high in the sky and lessened during the dawn and dusk when the Sun pulls as counterweight.

Pluto and Charon are a great example of this. The tidal forces between the two are so strong that bits of Pluto's upper atmosphere have been found bridging the gap to Charon, and Charon is massive enough that their center of mass is outside Pluto, and the two just spin about each-other.

Since you also mentioned an axial tilt, there will (as stated by ThinkerX) be severe seasons. With a 60Â° axial tilt, your arctic and antarctic circles will be about three times as wide as Earth's. They'll begin at around 30Â° N and 30Â° S respectively. Much of these areas will experience perpetual sunlight for half the year and none for the other half, which means that for half of your planet the Moon may be the only source of natural light for that Wintry season. Between those regions and around the equator you'll have a kind of temperate zone, but the sun's position in the sky will be low for most of the year.

Should you decide against a tidally-locked system, then the moon's motions will remain interesting given the axial tilt. The Earth's Sun and Moon follow a similar path across the sky, but if your planet has a 60Â° tilt, the Moon's location will be much harder to predict, with it often crossing or coinciding the Sun.

I hope some of this proved interesting or helpful. Cheers.


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## Trick (Jan 26, 2017)

Duncan M said:


> Hello! Aspiring Astrophysicist here. I'm certainly no expert, but when it comes to moons I'll say the sky is the limit (bad).



Sounds like you know more than me about this, which is the only qualification I'm looking for 

The sky being the limit is actually pretty good news. If that's the case, I'm basically able to place the moon in the sky where and when I need it for the story, within reason. 



Duncan M said:


> Since you also mentioned an axial tilt, there will (as stated by ThinkerX) be severe seasons. With a 60Â° axial tilt, your arctic and antarctic circles will be about three times as wide as Earth's. They'll begin at around 30Â° N and 30Â° S respectively. Much of these areas will experience perpetual sunlight for half the year and none for the other half, which means that for half of your planet the Moon may be the only source of natural light for that Wintry season. Between those regions and around the equator you'll have a kind of temperate zone, but the sun's position in the sky will be low for most of the year.



The reason I chose a 60 degree axial tilt was from this video. 

Seasonbuilding 101: Axial Tilt | Worldbuilding - YouTube

According to Artifexian, at 60 degrees, the arctic circles and equator switch places and the center of the planet becomes an unlivable barren waste of ice. If the tilt would also lead to severe weather in that area, it would be even better for the book. I need no one to travel from one side of the planet to the other and a ring of ice/wasteland with unpredictable and deadly weather is quite the deterrent. 

In your estimation, if Artifexian is right about the arctic circles and equator switching, what kind of climates am I looking at for land masses where the arctic and Antarctica are on Earth? I'm hoping for a place very similar to Venezuela.  



Duncan M said:


> Should you decide against a tidally-locked system, then the moon's motions will remain interesting given the axial tilt. The Earth's Sun and Moon follow a similar path across the sky, but if your planet has a 60Â° tilt, the Moon's location will be much harder to predict, with it often crossing or coinciding the Sun.



Tidally locked won't work unfortunately. I need month-based seasons like earth. I need the phases of the moon (months) to be 26 days long, with 13 months per year. 

Is that an impossibility with a 60 degree axial tilt?


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