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Sun and Sun and Moon and Moon and Moon
November 30, 2012 10:56 AM   Subscribe

A world with binary suns and multiple satellites that can still be livable by a diverse (even lush) assortment of life forms - true or not?

Feel free to answer the question as asked or explore any of the expanded pondering for more targeted responses.

I'm clearing big blocks out of the way for a story I've been writing for a long time (previously). Getting the details sorted for the type of world I'm most interested in building has been somewhat stickier than I'd hoped.

1. In the original images I had of this world, it had two suns. Not right on each other like Tattoine, but definitely visible at the same time at certain points of the day. Then I decided it wasn't a believable world. Earlier this year, though, a study was published saying twin-sunned habitable worlds are more possible than thought. The floodgates of potential opened again - it would change the world's colours, plants would behave differently, animals (including human-like forms) would need adjustments - which is so appealing that the words just want to spill and spill. I still desire a believable world, so guidelines would be really important to keep it disciplined. The study is clear about livable proximity. One or both suns can be further back and improve habitability with fewer strictures. Ideally: one would be almost as close as Sol is to Earth, the other somewhat higher and further back, and separated by the width of outspread arms at the peak of their shared time in the sky along the horizon.

A. How flexible can I be with their position in the visible sky without them eating each other or having major unpredictable effects in their solar system & impact on the world the story is on?
B. How much control do I need to surrender about having a real night of at least some few hours? How about seasons? Temperature?
C. Are there restrictions on what colours these suns can be?
D. I read an article a while back and lost it on the ways another sun would change the colours of plants and similar effects on living things - any thoughts on this or a link to that article would be vastly appreciated.

2. I'd already wanted multiple moons and hadn't ever dumped that idea, because I know it's workable (with considerations). I would like one moon to have an organic chemistry in its atmosphere. The rest can be airless. I was thinking 3-7 satellites. Some can be shepherd moons or captured asteroids. I do want them to have tidal influence, and am open to other effects.

A. The world's water mass is 50% or less, with 30% of it in a semi-divided ocean. What are likely behaviours of that amount of mass when influenced by a higher number of satellites?
B. Does the organic chemistry moon have to be huge, like Titan? Can it be as close as Luna (or closer)?
C. How many moons are too many, world-effect-wise?
D. I read an article about how Earth may have been a little different when it had two moons before they crashed to become Luna, but lost that, too - again, a link to that or related thoughts much appreciated.

3. How will these things combine to effect weather and planetary activity? I have a mega-continent, smaller continent, and sprinkling of islands in various sizes.

4. What size does my world need to be in order to survive this arrangement? Must I have more water?

Really, any links, studies, articles, whatever on the topic of realistic multiple-sun/satellite scenarios/considerations for a livable world will be helpful...including access to experts in this field (I am already planning on writing to the linked study's author with a boiled down question set).

I really wish there were lay-person accessible modeling software or even a game that can show effect of adding and moving satellites and suns to a world and system. If you know of anything like that, I'm interested.

Thank you so much! All answerers go on the thank-you page.
posted by batmonkey to Science & Nature (11 answers total) 6 users marked this as a favorite
 
This Bad Astronomy article may be of interest.
posted by Slap*Happy at 10:58 AM on November 30, 2012 [1 favorite]


Wouldn't the suns' apparent proximity in the sky change as the planet completed its orbits as well as during the dance the suns would do in space? The orientation you've described would constantly be changing.

Also: Celestia
posted by Chutzler at 11:21 AM on November 30, 2012


Slap*Happy: that's based on the linked study, so, yep, of interest - while I've found that one and several other articles cogitating on that study's findings, there's no guarantee I've seen a particular one, so those are welcome.

Chutzler: I sure hope so! Those kinds of thoughts are precisely what I'm interested in.
posted by batmonkey at 11:26 AM on November 30, 2012


This post from Starts With a Bang from a couple days ago talks about what would happen if the moon were closer to Earth than it was. Might give you a place to start thinking about how multiple satellites would affect the system. Or not, I don't really know.

I'm not sure if he's ever talked about the potential habitability of binary star systems but have a poke around, there's all sorts of good info over there.
posted by Mister_Sleight_of_Hand at 11:38 AM on November 30, 2012


The short answer is that a binary star system with a large separation on the sky is very tough to make last a long time (unlike those kepler systems whee the stars are very close) unless you make it so that the stars are so far apart that they are barely orbiting each other, in which case the movement of the companion would be different than what you are hoping for, I think. And the probably very near impossible if you want to have a lot of moons around an planet orbiting them both unless they have a very very large or very small separation. I can put together a more detailed response, but also - there is a game! Universe Sandbox will basically let you make little solar systems and see what happens. I mean, I wouldn't publish a paper with it and it certainly has limitations, but it can give you an idea of how these gravitational forces work.

A system with the two stars close together is much much more doable.

But honestly, the universe is big enough that even incredibly unlikely things can happen through the sheer number or solar systems that exist, and so for a science-fictiony setting even as an astronomer/physiscist myself I wouldn't have a problem with letting suspension of disbelief smooth the rough edges of plausibility here if its needed to tell the story that you want to tell.
posted by McSwaggers at 11:40 AM on November 30, 2012 [2 favorites]


This is going to sound like a flip answer, but it is not. The answers to all of your questions (that don't involve finding articles - I cannot help you there) are:

You're the writer. You decide.

Tatooine, for example. The science in Star Wars, in any given instance, tends to run the gamut from laughable to nonexistent. I seriously doubt that any thought was given to the actual science of a planet orbiting a binary star system during the film's production - all of that came after. The decision was a stylistic one. By having characters hanging out in the desert with two suns in the sky behind them, you have an easy and powerful visual indicator that we are not on Earth or even in any place we have a name for. We have the scene of a young man looking into the sunset and contemplating his future - but here, there are two suns. They exist for a dramatic purpose and the science of how they could exist is never discussed, because it's not important.

So taking that into account:

I still desire a believable world, so guidelines would be really important to keep it disciplined.

A believable world in science fiction is one in which everything hangs together in a way that's not jarring and doesn't contradict itself. If you have a consistent set of rules in your head (see above - you're the writer and you can decide what those are) and you follow those and don't contradict yourself, you'll be all right. Bear in mind that we have no working model of life on a planet orbiting more than one star; we have guesses and possibilities. All you really need to do is map out where this planet is in your head, which it sounds like you've already done. Unless your story has characters sitting around and talking about astrophysics, it won't come up.

It sounds like you have a lot of good reasons for wanting two suns: life would be different and you have a scene in mind where they're an arm span apart.

So go with that. Describe animals and human-like creatures and resist the temptation to point out that they're different because of the two suns. When the arm-spanning scene happens, don't spend any time explaining how it's possible - write with confidence and the reader will not even question that it's possible. Your reader will not have been spending the last fifty pages paying close attention to whatever else you might have said about the positions of the stars and calculating the apogee and perigee to notice that the scene couldn't possibly happen the way you describe it, and frankly if they do, they have problems that your book can't help them with and you shouldn't worry about it. Science, when it comes up, should serve the story, not the other way around.

There will be (and already have been) a lot of helpful answers in terms of physics, so please consider this another approach to the problem and pick whatever solution works for you. Remember that realism and believability are different things, and the latter is always preferable to the former.

As an aside, a small bit of trivia: Though their latin names are Sol and Luna, as far as I know the scientific names for our sun and moon are just "The sun" and "The moon."
posted by FAMOUS MONSTER at 11:56 AM on November 30, 2012 [5 favorites]


On the one hand, I agree with FAMOUS MONSTER: you're the author, do what you need to do to make the story work. We've found a remarkable diversity of systems in the past 20 years; I wouldn't rule too many possibilities out. That said...

How flexible can I be with their position in the visible sky without them eating each other or having major unpredictable effects in their solar system & impact on the world the story is on?

Stable planets around binaries have mostly been considered in two regimes: 1) two stars orbiting at a great distance, with the planet around one of them, and 2) two stars orbiting fairly close together, with the planet orbiting both at once. To consider case 1, you might follow the orbit of Neptune (say) and the Sun in a simulator like Celestia, and see how close/far away they get on the sky. They'll be at the "width of outspread arms" at some point, but they won't maintain that position throughout a year. Case two would put them reasonably close consistently.

I suppose in theory you could consider a planet at the L4 or L5 point of a binary; this would tend to keep the stars in the same relative location at ~60 degrees apart. Is this reasonable for a planet? Who knows! Asteroids do live at these Trojan points for most Sun-planet combinations in our solar system, including Earth. No idea if there's a reasonable formation scenario that leads to a planet stably there---it's extremely unlikely to form in situ.

Are there restrictions on what colours these suns can be?

Two restrictions: 1) they should match real stellar color properties, and 2) they should be age-appropriate: O stars have short (~million year) lifetimes, while M stars have very long ones. A very blue star may not live long enough to form a planet and start life.

D. I read an article a while back and lost it on the ways another sun would change the colours of plants and similar effects on living things - any thoughts on this or a link to that article would be vastly appreciated.

You may be thinking of this article, which is based on these papers. More generally on the topic, the Virtual Planetary Laboratory is not a bad place to start.

The world's water mass is 50% or less, with 30% of it in a semi-divided ocean. What are likely behaviours of that amount of mass when influenced by a higher number of satellites?

You might be better off thinking in terms of % surface cover than % mass here; water is less than a tenth of a percent of the Earth's mass. A half-water planet would likely be entirely covered by water to a very great depth (hundreds of kilometers), where you start getting interesting phases of water from the high pressure at the bottom before hitting the rocky/iron core.

4. What size does my world need to be in order to survive this arrangement? Must I have more water?

Without solving the dynamics, I'd note the key to survival in a binary system is long-term stability, which will probably rely more on planetary location than size. Any rocky planet is simply not going to be massive enough to do more than perturb the system slightly, so I wouldn't worry so much about that.

My concern would be atmosphere loss: if it's too small, the particles in the upper atmosphere will start occasionally hitting escape velocity and leaving the planet forever, bleeding the light elements and molecules preferentially into space. I probably wouldn't go as small as Mars.


Other links that you may find useful: the Solar System Planetary Fact Sheets, the Extrasolar Planets Encyclopaedia, and the Habitable Zone Gallery.
posted by Upton O'Good at 1:07 PM on November 30, 2012 [1 favorite]


Resisting commenting on the awesome stuff y'all are bringing forth to clarify one thing: I absolutely meant surface area rather than mass, and have no idea how I let myself continue that brain fart so egregiously.
posted by batmonkey at 1:25 PM on November 30, 2012


Some thoughts on the physics, but I wouldn't let this constrain you like everyone is saying. This is hand-wavey type physics thought experiments not necessarily how things would actually work, but my hope is more that it might provide something interesting consequences to mentally play with rather than a 'this is a solar system' type of thing. As a side note somebody wrote a serious of scifi/fantasy novels about a stellar system with a very very very large separation, but with a large eccentricity such that usually the stars were very far apart but for a few hundred years every several millenia they were very close, and the story telling spanned spanned generations that might be an interesting read if I could remember the name. Which I can't, sorry.

1: Lagrange points might be an interesting thing for you to read up on on wikipeda - they are for a 3-body system which is similar to this situation, but the consequences of having moons and presumably other planets as well as a third dimension means that things are honestly less stable than the lagrange points wiki would suggest. Especially when the two bodies are closer in mass like stars rather than a star plus a planet.

But, basically, for the most stable configuration, you really want the two stars orbiting each other pretty closely with one much larger than the other like in that kepler article, and the planets orbiting from outside the pair like in our solar system, but with those two stars kind of where our one star is. Stable here means 'able to last a long time without gravitationally slingshotting the stars and or planets away'. If the stars are close together, their gravitational forces act similarly on all the planets meaning that they are essentially cooperating in having the planets orbit around their mutual barycenter. However if they are far apart such that the planet orbits *between* the stars there is a tug of war that goes on that will typically kick the planet out of the system. Unfortunately, the close-orbit configuration will force them to always be near each other in the sky, say, within ~3-6 times the width of the star kind of like you see on tatooine or in the kepler article, which sounds like a no go. If they are close together than sometimes they will be overlapping and sometimes they will be say ~6 suns away, and their distance will go back and forth. Now, if you make the stars far enough apart that the tug from the second star is minuscule, the problem for your purposes two fold - 1: The second star is now fairly small in the sky. Think Jupiter/Mercury not sun sized, and two - it would move in the sky very very slowly ….like on the order of centuries, so it would behave more like a constellation that you could see during the day and night, but where the time of day that it was visible changed, say, every 300 years. So for your storys star behavior, you want an intermediate separation which is hard to do realistically.

My best thought at making this intermediate separation in the sky is this, which is physically motivated but not nescessarily realistic for a whole whole bunch of reasons that dont matter really - You probably want one very massive but not too hot star and one very small also relatively cool star. What you probably do is try and have the red dwarf (the small star) orbit the the big star at some relatively large distance. Think Saturn-Pluto kind of range here. Then, you orbit your earth-type very close to your red dwarf, so that it is essentially a mini-solar system going around another star. This is a tricky balancing act - you need the big star to be massive enough that the systems barycenter is relatively close to the giant, but you also want the small star to be far enough out that it can maintain its own mini solar system type deal (think Jupiter + moons, but scaled up). Since both stars are 'cool', the earth will need to be a fair bit closer to the small star, which will look much much bigger on the sky than the larger star that is farther away (and much bigger than the sun looks to us most likely). This will also decrease the 'year' of the planet around the small star, and you will have a sort of 'great year' as the small star goes around the big star over several decades to a century. Now, if this orbit is elliptical, or the orbital planes are inclined you could get some interesting cyclical season patterns - basically, you would have your regular yearly seasons around the small star, but then when you are close to the great star everything would be warmer by quite a bit for several decades, and then cooler as you move to the far point every orbit. This could be anywhere from non-existant to ice ages to the atmosphere freezing out and sticking to the worlds surface kinda like pluto on the cold side depending on how hot your nearby star is and how elliptical your orbit is. Your Night would also vary with the great year+year periods -if the stars were in the same area on the sky you would have a regular night, while if they were on opposite sides of the planet you would have say a day, plus a constant twilight or second dimmer day, and two short nights separating them, or a constant day with a super bright/hot time twice a day when the stars temporarily overlap depending on the details. This would vary over the course of a small year basically, as well as over a great year to some extent.

As far as colors go, 'blue' is basically hot and 'red' is basically cold, with the other colors varying as you would expect. The more massive a star is, the closer to Blue it would appear and the larger it would be. There are both very large and very small red and blue stars, although the small blue stars and large red stars are not your typical star. But, if you wanted them you could, say, have life evolve during the red-giant phase of a star even though this portion of the stellar cycle doesn't last that long (few million years, so statistically unlikely timeframe for life to show up over the universes billions of years), or after a sun-type star evolved into a white-dwarf (small, blue at first, red much much later) which honestly would only have happened pretty recently, and so is again unlikely. Large Stars are typically more massive than small stars and live for far less time and tend to explode violently, so there are also problems with really blue massive stars for example. But you could certainly play around with this.

Now, this is having everything in the same elliptical plane.... if its not, then things can get really really wacky.

In terms of plant life and suns, chlorofill which plants use to make their energy is basically evolved to absorb the most abundant 'colors' of light that gets through our atmosphere. Which is green, and is why leaves are green - right? If you have two suns, you could imagine plant life which say is mottled to take in both kinds of light, or an in-between color to take in both stars light sorta ok, or you could have species a that is say red and uses primarily the nearby stars light while you have another species that is green that uses a hotter, more distant stars light, or all of the above combined really. You might have plants that are tuned to work with the larger, more massive star hibernate for decades or a century if the orbit is elliptical around the big star - having them be active in a great-summer and dormant in a great winter.

You also *might* need a saturn orbiting the big star ...somewhere... not sure whether close in or like 2-10x as far out as the small star is better, but sometimes they can help stabilize systems due to orbital harmonic interractions.

2. The tides would be erratic, basically, with that many moons. That would be the big thing. How erratic and how they varied would really depend upon the specifics and even then. Basically, instead of having one high/low tide you might have 20, or none, or sometimes 20, sometimes 1 or zero, or completely unpredictable with occasional tidal-tsunamis - make up whatever you want here.

The problem with a moon with an atmosphere is that you have to be large enough that you gravity can hold on to it. So it needs to be *massive* at the very least. Typically this means 'big', but if you have an extremely large iron core you can shrink the size somewhat (it would still be big, but not quite earth big). A large moon is ok though too, because earths big moon has helped to stabilize it, and is thought to have contributed to the stability of our axis, seasons, and day night cycle. The problem is mostly then that your earth has to be large enough that the moon still wants to orbit it, so you are looking at like a 'super earth' as they say, say up to ~10 times larger than the earth, which will also increase its gravity, the quantity/thickness of its atmosphere, and should hopefully allow it to hold onto the big moon. Alternatively, you can come up with some method that its atmosphere gets 'replenished' faster than it evaporates. large reservoirs of gas that geyser out or something handwavy. Or, maybe it orbits relatively close and 'steals' atmosphere from your earth, or some sort of ring system with water-ice chunks that would probably not be very stable. You could have this ring system be replenished by water-ice comets that come in from an oort cloud and come too close to the star/weird earth system and break up and turn into rings.

Any moon at all is unlikely, really, but the best case I can think of if for the other moons would need to be small, and honestly would probably not be stable and would get ejected from the system. So if I wanted to be realistic-ish I would keep the total number of moons small and have the non-atmospheric moons be small, and some are probably more like asteroids that have been caught than honest to goodness moons.

4. Water surface area percentage amount … doesn't really matter. Mass does, and water is on the light-ish side of things that add mass to worlds. Typically if the world was more massive and is not a gas giant you would, say, increase the size of the iron core (would also increase the planets magnetic field most likely and help it maintain at atmosphere probably) which would then scale up the planet a bit it terms of radius, atmosphere size and density, etc - but for your purposes, your water percentage can really be whatever you want it to be. From this stand point and can be explained away in various manners if you feel it is really nescessary.

So your earth would probably need to be more massive than our earth unless you want t o rock a replenishing atmosphere of some sort. But it probably shouldnt go over ~12-20 of our earths because then unless something drastic happened you should have started forming a gas giant.
posted by McSwaggers at 1:25 PM on November 30, 2012


I should also have mentioned - the tides would be dominated by your atmospheric moon. Also, for weather, with this much gravity going on I would expect lots of volcanoes/geysers - both on your earth and probably on your moon if it is 'big'.

Also, I can't find a cite for it at the moment, but venus earth and mars started out pretty similarly, but are currently very different - I don't have any on hand, but you might be able to google some interesting discussion of why which would touch quite a bit on 2 - 4.
posted by McSwaggers at 1:33 PM on November 30, 2012


I misread your question at first, but I figured I'd post this as the information may prove useful. I thought the organic chemistry moon was supposed to support life and would be around a different planet, which I figured would have to be a gas giant.

On the subject of moons around gas giant (which is what I'm assuming if they can hold life) One thing you might want to consider and perhaps make use of (or ignore if you like, it's your call) is Van Allen radiation belts, which, if I understand things correctly, are tied to having a strong magnetosphere. A strong magnetosphere is useful for protecting a planet from solar wind (see Mars for what happens without one), but if it is too strong, it can create strong radiation output of its own which can be hazardous on the moons. (See the wikipedia article on colonization of Europa for more details.) You can probably avoid some of this by giving the moons their own magnetospheres, although this would result in some spectacular northern/southern lights over most of the moon.

Back on the subject of your post, now that I have re-read it, and trying not to write anything McSwaggers just wrote, one thing to think about is that our moon, over the course of the life of the earth has massively slowed down our day/night cycle. If we had not had a moon, our days would be roughly 8 hours long. To have a moon capable of having an atmosphere (probably not livable), you could have your planet be a "super earth" and have it have a large enough mass that it is not in a system that is closer to a double (in this case dwarf) planet setup like Pluto/Charon, where the orbital point is still in the planet, not between the planet and its moon. The super earth, depending on your choices, could either have more or less tectonic actiity, however either way, the volcanic eruptions would be more spectacular. (Which could potentially lead to strange seasons, however this is probably dependent on the greenhouse gasses in the atmosphere, given atmospheric content is what makes Venus the hellish place it is.)

However, without axial tilt, the volcanoes could be the source of seasons. It's probably unrealistic, but if you had a regular volcanic cycle, it could give a regular seasonal cycle over the entire planet without shortening/lengthening days.
posted by Hactar at 1:39 PM on November 30, 2012


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