Astronomy difficulties in an epic fantasy

[Deleted member 4007]

Hello, I'll get straight to the point. My epic fantasy series is based in a binary star system.

Now there is nothing to do with real sci-fi spaceships or anything: the world has approx. 1800s technology.
I want to focus mainly on scientific possibilities of this planet (Trecataur), so let me give some more details.

– it has a circumbinary orbit around a binary star system. This binary star is the type in which a smaller star orbits a larger star, just like how Mercury orbits the sun. (think of it in that kind of positioning)
– it has 386 earth-days within a year
– it is approximately 1/3 of the size of earth
– it has 8 moons orbiting it. One of these moons has its own tiny asteroid orbiting around it.

My question here is mainly focused around the possibility of life around binary star systems, and more specifically, how seasons might work on such a planet. This also includes how a person observing the sky might see the smaller star, as it orbits the larger one.

Even if you don't know how science works, feel free to give ideas as to how you think the 2 suns would look like in the sky and stuff.

Thanks

 

[Vandor]

Any ideas on how you're going to handle day/night? There's a film, Pitch Black, which deals with a planet that has binary, or even triple suns- two of which are essentially one, and the the other is opposing, and covers the appearance of several astrological incidents. Also, there's Tatooine from Star Wars, where their suns were paired in the sky. If one is orbiting the other, you would have an additional way to track time-over days, though, not hours-as the smaller sun crossed the larger, left of the sun, hidden in combined light, right of the sun. That would lead to very interesting interpretations from a religious or spiritual perspective on your cultures' part.

Seasons will really depend on the type of orbit. As I said, Tatooine seemed pretty stable, though I don't know how accurate that depiction was. If there's a noticeable gap between the orbit, you might have doubled seasons within the space of a year. That could give you intense and weaker versions of each season as time goes on. Hot summer, warm fall, mild winter, cool spring, mild summer, cold fall, dead winter, warm thaw in the new spring.

The moons also have a lot of potential, again depending on proximity. Are they only specks, or are they as visible as our own moon? Our moon and sun greatly affect the tides and ocean currents, which I can get into if you want. Ten celestial bodies would play havoc with nature in those regards. You also have an increased possibility of lunar and solar eclipses. And come to think of it, with the eight seasons from above, you have the potential for a different moon in each season, if the orbit is synced properly.

 

[CupofJoe]

If your planet is 1/3 size of earth, then won't gravity be much reduced as well? If the gravity is less, then you start to loose the thick atmosphere we'd expect to live in.
I think Mars has an atmosphere less that 1% the density of Earth and it is more than half Earth's size. [thanks Philster401]
Having so many moons will probably play merry hell with the regularity and frequency of tides... But lots of fun to write...
As for two suns, if the smaller one is so close and potentially small, then I don't seeing it have much effect, as for most of the time it will be in front or behind the main star and will only be beside it for a day or two every 44/45 days [using Mercury as a model]. They might be considered holy-days, holidays or signs of ill omen as cultures interpret them. I could see it rising before or setting after the main star giving longer twilights...

 

[Philster401]

Mars has a hundred times less dense atmosphere than Earth.

 

[Noldona]

I am also working on a binary star system and going semi-hard science route with mine so I have done some research into this.

The first thing you need to determine is the classification of your stars. These will determine the luminosity, size, distance for the secondary star to orbit the primary, and habitable zone. Once you know what the habitable zone for the combined star system is, you can accurately determine the year length.

Seasons are based upon luminosity and axial tilt. Winter is caused by a portion of the planet being farther away from the sun during parts of the year. Summer is caused by a portion of the planet being closer to the to the sun during parts of the year. The amount of distance change is determined by the axial tilt. Less tilt, less season change. More tilt, more extreme season change.

Day length is based upon rotation speed of the planet. For day length there are actually 2 measurements. Sidereal day and solar day. Sidereal day is how long it takes for the planet to complete 1 complete rotation (360 degrees) while a solar day is how long it takes for the sun to reach the same location in the sky such as directly overhead. Due to the planets orbit of the sun and the rotation of the planet the solar day is usually longer then the sidereal day. This of course assumes our orbiting works similar to Earth.

Gravity of a planet is based upon mass, not specifically size. A smaller, more dense planet could still retain an atmosphere and moons. However, there is a limit to this. The less gravity a planet has the fewer and/or smaller the moons in its orbit.

Moons and suns affect the tides. In the case of Earth, the force from the sun is minor compared to the moon. However, in the case of multiple moons, they would affect the tides more. I forget the exact formula off the top of my head, but you take into account the orbital period of the moon, the distance from the planet, and the mass of the moon. This will show you how much that single moon affects the tides. You then add the results of these together to get the actual tides of your planet.

You can find formulas for how to do all of this on Wikipedia. A good place to start is Kepler's laws of planetary motion - Wikipedia, the free encyclopedia. You can also find programs like Universal Sandbox to help you model the solar system in a mathematically accurate fashion.

Without doing any of the formulas and just looking at your description of the planet. I don't think it is scientifically possible to exist in any sort of stable orbit long enough for life to evolve on the planet.
1. The stars will collapse into each other due to gravitational forces; or the stars would have to be too small to have that close of an orbit and thus not produce enough light.
2. The habitable zone of binary star systems in which the planet orbits both stars is usually way farther out and thus wouldn't have such a short year.
3. As stated previously, a planet of that size would probably not have the mass and thus gravitational pull to hold an atmosphere.
4. A planet of that size probably also wouldn't have the gravitational pull to support that many moons unless your moons are the size of asteroids. As one has an asteroid orbiting it, I am assuming you intended the moons to be larger due to the mass needed for that to happen.

Of course, all of that is based upon the physics of our world, how we understand the universe to work, and what we consider the requirements of life to be. As a writer and world builder, you can make it work however you want/need for story purposes. Maybe your world exists in another universe or dimension where physics works differently. Maybe the inhabitants of your world don't have the same requirements for life as humans. Maybe some great cosmic being built the solar system to work this way and by force of will makes it all work perfectly without science getting in the way.

 

[skrite]

I think if your smaller sun / larger sun model is like mercury and our sun, then days and night could be like what we experience here, since your planet will have once side facing both of them all of the time.

Also, having a planet 1/3 the size of earth might be a problem with radiation. If there is not liquid core in the planet, there will be no magnetic field. These are harder to find as planets get smaller.

 

[Deleted member 4007]

My thoughts on the fact that Mars had little density was this - didn't it once have a better density, and perhaps be habitable for life?

Or has it been scientifically proven that planets 1/2 the size of earth couldn't support life?
What kind of changes and differences would have to exist on the planet if it was this size and could support life?

Thanks for all the help.

 

[buyjupiter]

If your planet is 1/3 size of earth, then won't gravity be much reduced as well? If the gravity is less, then you start to loose the thick atmosphere we'd expect to live in.
I think Mars has an atmosphere less that 1% the density of Earth and it is more than half Earth's size. [thanks Philster401]
Having so many moons will probably play merry hell with the regularity and frequency of tides... But lots of fun to write...
As for two suns, if the smaller one is so close and potentially small, then I don't seeing it have much effect, as for most of the time it will be in front or behind the main star and will only be beside it for a day or two every 44/45 days [using Mercury as a model]. They might be considered holy-days, holidays or signs of ill omen as cultures interpret them. I could see it rising before or setting after the main star giving longer twilights...

Also, if the gravity is 1/3 that of Earth, it'll be harder--much harder--for the planet's gravity well to not only attract moons but keep them as well. Unless they are really really tiny.

And if you have two suns orbiting each other closely, one would most likely be absorbing mass from the other star.

Algol is a tertiary system so you might want to look into how the physics operate there for ideas, as well.

 

[skrite]

Or has it been scientifically proven that planets 1/2 the size of earth couldn't support life?
What kind of changes and differences would have to exist on the planet if it was this size and could support life?

It hasn't been proven at all, in fact, a lot of astrobiologist think that the most likely places to find life off of Earth is on some of the moons that have liquid water oceans beneath a frozen exterior. Water can be liquid because of the heat generated by tidal friction.

 

[X Equestris]

My thoughts on the fact that Mars had little density was this - didn't it once have a better density, and perhaps be habitable for life?

Or has it been scientifically proven that planets 1/2 the size of earth couldn't support life?
What kind of changes and differences would have to exist on the planet if it was this size and could support life?

Thanks for all the help.

Mars used to be tectonically active. When it's mantle and core cooled, the planet's magnetic field weakened. This led to the solar wind stripping Mars' atmosphere, which led to the water on its surface evaporating or freezing. The thin atmospehere is also the reason why the planet is as cold as it is, even though it once had liquid water.

 

[Trick]

My thoughts on the fact that Mars had little density was this - didn't it once have a better density, and perhaps be habitable for life?

Or has it been scientifically proven that planets 1/2 the size of earth couldn't support life?
What kind of changes and differences would have to exist on the planet if it was this size and could support life?

Thanks for all the help.

As enjoyable as it is to understand all of these things and know them as the writer, you're not exactly writing hard SF. If this is a fantasy work, you really only need to get your seasons/calendar down to be consistent. Unless, of course, there is some direct link between the astronomy of your world and the story. If that is the case, explain that aspect of the story and it might be easier to help you.

Are you planning on starting the story with, "On a planet 1/3 the size of Earth with a binary star system, Dirk McFarmboy was minding his own business when the soldiers rode into town like a thunderstorm." ?

The various other issues often don't require explanation because they can be comfortably taken for granted. If a scientist (or nerd, and that's not an insult) says, "by my calculations, this planet would only be 1/3 the size of Earth and thus the gravity would be reduced not to mention that a planet that small couldn't support life, ha!"

You can say, "The people are biologically different that humans. Denser and able to survive in that environment." Or you could just say nothing since that reader won't be likely to ever meet you and you'll probably never hear from someone like that because they are unlikely to read fantasy in the first place. An inability to suspend disbelief does not a fantasy reader make but if you address all of this technical stuff in your book you'll both lose readers and gain critics for any scientific mistake you make.

However, as I said above, if the astronomy ties directly into the plot in some way, other than as part of the setting, describe that in more detail and it will be easier to break down the facts you need for the sake of consistency.

 

[chrispenycate]

I don't think spacing in the order of the sun/Mercury is going to give you stability - at least the lesser star out as far as Venus, and your goldilocks planet a bit further out than Earth (probably quite a bit, as your primary star will probably be a bit more massive, since

Let's try an alternative layout. No need to upgrade the Sun, or change the Earth's orbit for this one. We'll add some mass to Jupiter - or, add some monoliths to increase its density. In theory, we could simply crash a decent sized planet into it, and the shock wave would ignite the fusion, but it'd take some precision aiming, and when you can get the same results with a few gigatonnes of loose hydrogen… Anyway, this is definitely a binary star - a few AUs distance won't change that. And Jupiter's moon system, now rechristened planets (I think. Or perhaps a new nomenclature should be developed for multi-star systems) will get almost all of their heating effect from their local star, with only some light spill from the system primary. It is not likely from radiant energy alone that more than one of them would fall into the liquid water zone, considerably narrower than that of the system primary Sun analogue, but tidal stress and magnetic transduction might well add enough energy to render one or two more habitable (I didn't offer 'comfortable' Earthquake heating -er - leaves me cold?) Ganymede in particular is big enough to be a decent planet now; if Jupiter were twice its present mass, could we not anticipate its analogue being even bigger?

Another way of building a binary system is with two separate protoplanetary disks, at maybe a light year's separation, rotating round a common centre of gravity (all systems rotate round their centres of gravity, rather than their star - it's just that, with our solar system and probably with the 'super Jupiter' one, so much of the system's mass is concentrated in the principal star that the centre of gravity falls inside it.) each with planetary systems that are essentially independent. Not much fun, as such, but definitely a linked pair in telescope observation.

I've set a couple of stories in a binary system (of the 'hot Jupiter' style) so I could look up the details and calculations (mine was SF, so you might want to look into modifying a few physical constants for fantasy) rather than working from memory.