# Solar system level worldbuilding



## caters (Jun 8, 2016)

I like the way Artefexian does his worldbuilding(astronomy level first)

But I can't find minimums and maximums for a lot of different planet types. For example, how massive can a gas planet be to be called a mini Neptune? What is the maximum radius for a mini Neptune(1 Jupiter radius, 1 Neptune radius)? Just how massive can a dwarf planet be before it is called a planet?

These are just a few of several questions I would be asking and I bet a lot of astronomers would either not know the answers or be too busy searching for planets or the supposed "Nemesis" that is far from Neptune and in a binary pair with the sun.

The people on mythic scribes have suggested modeling my solar system for my story based on existing solar systems.

But I don't agree with them.

Why should I model them on existing solar systems when a fictional solar system is more likely to be closer to ours in composition than other solar systems?

Most solar systems have 1 tidally locked planet orbiting around a red dwarf star(which can be as low as .08 solar masses). And most that aren't around red dwarfs only have gas giants such as hot Jupiters and mini Neptunes. And most rocky planets aren't in the habitable zone, much less earth like. Ejected planets are relatively common and most of them are ejected because of a supernova.

So my solar system would be most like ours. My Kepler solar system has only rocky planets, no gas giants. 2 of those are earth like both having 4 moons and 2 are binary planets.

Because of this, I think personally that doing it from scratch the way artefexian does it, no matter how hard it is, is better than modeling it off of our solar system, much less other solar systems.

I planned on having a hot Jupiter with anywhere from a 5 day orbital period to a 20 day orbital period(hot Jupiters can orbit their stars as short as 3 days or as long as 131 days) to determine the week length on the planet.

The earth like planet I planned to have a moon with an orbital period at least 3 times as long as the hot Jupiters orbital period to determine the month on the earth like planet.

However many times the moon orbits the planet as the planet orbits the star or stars is the length of the year in lunar months. And however many times the hot Jupiter orbits the star or stars during 1 moon orbit is the length of the month in weeks.

Mini Neptunes could determine time periods longer than a year. Dwarf planets probably wouldn't determine any time but still might be important to the humans.


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## bdcharles (Jun 8, 2016)

Hmm. I wonder if you're not overthinking this. Are you going for total scientific accuracy? If so, I would be inclined to base some parts of your solar system on our own, simply because, leaving aside the preponderance of solar systems out there, ours is the only known inhabited one so use it as a model, as you say. As for deciding when to call a planet a gas giant and whatnot (would you say mini-Neptune, as Neptune implies a link with our world), why restrict yourself? If a planet is big and gassy, immerse us in that big load of gas and depict its size in ways other than mere measurement - other than a rough size that you keep in your head or notes for reference, few people are going to go and actually measure it, so why not have your story actors go and explore it. Ditto the periods - do you need to exposit all that about how long a "year" is, or can you sneak the info into the story there, in the slow passing of the seasons. Is the duration of your lunar months important? Can you conceivably leave it out, invoking dual moons or whatever if you need a bit of light?

I suppose one question that might arise is what effect these different physical properties might have on people or whatever races and species you have. That's hard to determine with much scientific accuracy other than a readup of the matter on, say, the ISS or the moon landings. You could explore it, examine it as a theme. Posit that with two moons there are twice as many tides and twice the chocolate sales  good luck!


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## caters (Jun 8, 2016)

The moon would have a huge impact on everyone if it disappeared. And lunar tides are essential for life like that on earth(large, multicellular life).

If a planet disappeared, it would have impact but not as strongly as if the moon disappeared.

And if the moon, hot Jupiter, or 1 of the mini neptunes disappeared, a certain time period would no longer be known since the civilization depends on astronomers to keep track of the planets.

This is why orbital periods are absolutely necessary.

Now no I don't need to know if the mini neptunes are going to perturb the orbit of the earth like planet in any way or capture it's moon(most likely it wouldn't since mini neptunes are usually far out from the frost line). But still, it is good to know the minimums and maximums for different planet types since if I don't, I could have a 10 earth mass, 2 earth radius planet and call it a mini Neptune when the density of it would be in the rocky planet density range.


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## Ultraroel (Jun 8, 2016)

Ok. 

But is it necessary to go all through this worldbuilding for your story ?
I mean, scientifically correct yes or no. THe majority will not doubt your explanation, or non-explanation and hopefully be intrigued into the story.
How far is the need to clarify it all, becuase you have the knowledge yourself. 

Most of you readers probably won't. I personally, would probably skip the infodump on that one.


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## caters (Jun 8, 2016)

Yes it is since soon before the humans are teleported to this planet with dinosaurs(and I mean weeks in solar time(our sun, not the star or stars in this other solar system)) the aliens would most likely be telling them about this other solar system. And even if they aren't, people reading it would still want to picture what the solar system looks like, even if it is different from my perspective.


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## bdcharles (Jun 8, 2016)

caters said:


> The moon would have a huge impact on everyone if it disappeared. And lunar tides are essential for life like that on earth(large, multicellular life).
> 
> If a planet disappeared, it would have impact but not as strongly as if the moon disappeared.
> 
> ...



Okay, but how much of this needs to be written about in your story? How many earthlings know the density or size of the earth yet manage to lead fulfilling and reasonably productive lives? If your story hinges on these astronomical details you may need to go deeper, I grant you (and all I can suggest there is research, research, research), but if not ... maybe there's a way round it narratively.


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## Cran (Jun 8, 2016)

caters said:


> But I can't find minimums and maximums for a lot of different planet types. For example, how massive can a gas planet be to be called a mini Neptune? What is the maximum radius for a mini Neptune(1 Jupiter radius, 1 Neptune radius)?



A mini-Neptune can be up to 10 Earth masses; after that it would be a standard ice giant planet. Uranus and Neptune are 15 and 17 Earth masses respectively. 

A mini-Neptune is distinguished from a super-Earth by its composition, not by its mass. 

The radius of an ice giant depends on a number of factors including its rotational period, tidal influences, and overall density.* 

Ice giants are distinguished from gas giants by their composition more than relative size or mass. Ice giants are thought to have larger fractions of heavier volatiles - methane, ammonia, water, carbon dioxide, etc - within or below their hydogen-helium atmospheres. 

Gas giants are ice giants that grew quickly enough to gravitationally attract and hold the pre-stellar (or proplyd) hydrogen and helium in their orbital paths before stellar fusion and the emergence of the stellar wind. Gas giant planetary growth is thought to be very fast; possibly as short as ten thousand years. 

Again, to illustrate that planets are defined more by composition than mass or size, planet Kepler-138d is thought to be a gas dwarf. Its mass is about equal to Earth's, but with a radius at the lowest for its type at ~1.7 Earth radius. 
_
*For upper limit radii of cold gas giants and ice giants, see Jupiter. The compressibility of gases and volatiles mean that above ~1.6 Jupiter (~500 Earth) masses, gas planets tend to shrink into denser critical point forms. In hot gas giants, this density gradient is partially countered by outward thermal pressure. Hot Jupiters - gas giants which orbit close to their primary stars - can have very distended shapes (like an egg on its side) due to the internal thermal pressure and the stellar wind distortion of the planet's magnetic field added to the normal tidal distortion. _




> Just how massive can a dwarf planet be before it is called a planet?



A dwarf planet is not determined by an upper mass limit, but by its dominance of its orbital path. A dwarf planet is a planetary mass object which does not comprise the largest fraction of its orbital, and which does not exert the dominant gravitational influence of the other objects which share its orbital. 

Pluto was reclassified as a dwarf planet not because of its mass but because it did not substantially influence the movements of the thousands of other objects - some of which are also of planetary mass - in the Kuiper Belt. 

The same is true for Ceres in the Asteroid Belt, and indeed for moons (planetary satellites) of planetary mass. To be a planet, you gots ta rule!

*ETA*: Ceres sits at the lower limit for a Earth-type planetary mass object, which is defined as the mass needed for hydrostatic equilibrium and to force sphericity - Ceres: radius 473km (~0.07 Earth); mass 0.00016 Earth.

For an ice dwarf: Mimas (satellite of Saturn): radius 198km (~0.03 Earth); mass 0.000006 Earth.


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## Non Serviam (Jun 8, 2016)

caters said:


> Why should I model them on existing solar systems when a fictional solar system is more likely to be closer to ours in composition than other solar systems?  Most solar systems have 1 tidally locked planet orbiting around a red dwarf star(which can be as low as .08 solar masses)...



The solar systems we know about are the solar systems we can detect, and our methods preferentially detect orbital perturbations.  So the large number of hot jupiters, eccentric orbits and so on that we find don't reflect the real universe.  They reflect observation bias.

If you're writing a hard science fiction then I would suggest that you begin by deciding which star this is.  If you know you're writing about, say, Sigma Draconis, then you'll have parameters such as luminosity and temperature and age which will inform the large-scale structure of the solar system you're developing.

If you're writing a soft science fiction then set it on the planet Wilberforce in Gertrude's Cluster and don't sweat the details.


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