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Sticks
2008-May-04, 01:33 PM
I just thought that with the collective wisdom here, then those of us who fancy themselves as writers, or those who think someone has made a booboo could come here to see if the world designed has any flaws in it's conception. I.e. it could not exist or would not be stable.

As for mine

The world would be an inhabited world with higher order life forms, orbiting a binary star system, which consists of two main sequence stars. (Sort of like Tatooine from the original 1979 Star Wars.) The planet would also have four small moons.

Would such a system be stable, and able to support life?
What would tides be like? Assume the moons have different orbit times.

If all the moons were to line up on the same side, would that be a problem

:think:

KaiYeves
2008-May-04, 06:39 PM
That's 'original 1977 Star Wars'.

Sticks
2008-May-04, 07:11 PM
That's 'original 1977 Star Wars'.

:doh:

I stand corrected

But could Tatooine have existed from the view point of orbital mechanics? I did read somewhere that they believe they had found binary star systems with stable planets.

Ilya
2008-May-04, 10:16 PM
:doh:

I stand corrected

But could Tatooine have existed from the view point of orbital mechanics? I did read somewhere that they believe they had found binary star systems with stable planets.

Yes. Tatooine's two suns were only a few solar diameter apart, so Tatooine could comfortably orbit them both.

mike alexander
2008-May-05, 11:32 PM
As for tides, they would depend in the size of the moons.

eburacum45
2008-May-07, 09:10 AM
Looking at the question of binary stars first; there appears to be two ways that you can have a planet in a stable orbit in a binary system.
(from this page )
http://www.solstation.com/habitable.htm

Indeed, stable orbits may extend as far as one third of the closest separation between any two stars in a binary system, but according to NASA's Kepler Mission team, numerical integration models have shown that there is a range of orbital radii between about 1/3 and 3.5 times the stellar separation for which stable orbits around two stars are not possible

First the two stars might be close together in the centre of the system- a good rule of thumb is that the stars should be separated by no more than one third of the radius of the planet's orbit; for Earth that would be 1/3 AU, or a bit less than fifty million kilometers. In this case the planet would orbit both stars.

Or the second star could be separated from the first by more than 3.5 times the planet's orbital radius; for Earth that would be 3.5 AU, nearly as far as Jupiter. In this case the planet would only orbit one of the stars, not both - figure-of-eight orbits are almost impossible.

I'd add a safety margin to both of these figures - make the stars in the first case ten million kilometers closer together or so, and in the second case make the second star at least 4.5 AU distant- but that is up to you.

----------
As for the moons, there is an interesting thread here about multiple moons
http://www.bautforum.com/questions-answers/71168-planet-two-moons-double-eclipse-possible.html

It looks like you could have at least two moons orbiting an Earth-like planet, although it would be a little difficult to explain how they were formed. One moon at 50 or 140 thousand kilometers, and one at 380 thousand kilometers seems to be stable. You could then add a single close-in asteroid-type moon, a bit like Phobos; note that this moon will eventually hit the planet. And for a fourth moon you could have a smallish retrograde moon at about 500,000 km or so, going the other way round the world.

eburacum45
2008-May-07, 10:03 AM
The tides from the two major moons in such a system would be interesting- the two smaller moons would probably have little effect, but the other two moons would each cause tides, which would periodically reinforce each other, and would also each be periodically reinforced by the central sun or suns.

Each time one of the two moons is aligned with the other, or with the two suns in the centre (which would act, most of the time, as if they were only one sun for tidal purposes) there would be a high tide. In the case where there is only one sun in the centre of the planet's orbit and another several AU distant, I suspect that the second sun would have little tidal effect.

Note that 'aligned with' means that the objects concerned form a line with the planet; they can both be on one side of the planet, or on opposite sides. An extra big tide would occur when the two moons were both aligned with the central sun or suns.

geonuc
2008-May-07, 11:30 AM
Wouldn't the magnitude of the tides be significantly determined by the geography of your world? Earth has some monstrous oceans which are quite affected by the Moon. If your world had less water or smaller individual water bodies, the tides wouldn't be so big.

eburacum45
2008-May-07, 11:43 AM
Absolutely. Even on Earth the Mediterreanean, a large land-locked sea, has no tides to speak of.

Sticks
2008-May-07, 01:55 PM
This is quite good

As I went along with my "franchise" in the early days I was more concerned about the political make up - This world I created is actually situated in another membrane universe, but for strange unknown reasons the inhabitants were formed to be like the creatures and entities from the imagination, folk law, nightmares and collective unconsciousness of humans on Earth.

Somehow a number of the higher order intelligent beings became aware of the existance of Earth and humanity, and using what they considered "magic", (But as ever in this kind of thing never explained), they had constructed devices to travel between their universe and ours. In the earlier story lines I had it that some of the ancient gods humans worshiped were actually entities from the other world in human form. As time wore on and humanity grew up, the entities mostly returned to their world, although some stayed and the forces of law and order in the other world considered it a crime to further interfere in our world. A law, for storyline purposes was clearly honoured in the breach.

In the later storylines I did, I had a collusion of agencies from that world and secret Earth agencies to keep the existance of this alien world in another universe secret and to monitor those entities living among us (Men in Black - Shades of)

This franchise has been kind of going since 2001, but I ressurected it last year. Only in some of the more recent story lines was I turning to the geography, my next story line I am half thinking of definitely requires more thought given to how this world would actually work.

In terms of the binary star system, I thought of something like Tatooine from the original Star Wars, so the suns are close together and the world orbits both.

In terms of the moons, in one of the more recent story lines I had a description where three of the moons were in the night sky, but it would be a lot later in the night before the forth one appeared. eburacum45 idea is intriguing, with that set up, which would be the late moon one wonders, or perhaps over time each one would have turns of being late. One question, how would one get a retrograde moon, do any of our planets in the solar system have a moon that orbits backwards to others around the same planet?

As for formation, our moon is thought to be the result of a collision between two worlds where a larch chunk broke of and became the moon. Perhaps you could have this scenario, and then there was a further collision, either with the planet, or the moon.

So how did Larry Niven work out his Ring World?, I understand that he got quite a load of flack at a sci fi convention once by a rampaging mob of respectable physicists - or was that the inventor of the infinite improbability drive :whistle:

eburacum45
2008-May-07, 02:09 PM
In terms of the moons, in one of the more recent story lines I had a description where three of the moons were in the night sky, but it would be a lot later in the night before the forth one appeared. eburacum45 idea is intriguing, with that set up, which would be the late moon one wonders, or perhaps over time each one would have turns of being late. They would all take turns. The second moon, at 50K would be interesting- it would orbit the world in about a day and a half, so would move quite quickly against the fixed stars, but relatively slowly across the sky. Its motion would look quite different to anything we normally see from Earth.
The fastest moon would be the innermost- you could probably see it move quite easily.

One question, how would one get a retrograde moon, do any of our planets in the solar system have a moon that orbits backwards to others around the same planet? Retrograde moons have to be captured, I believe; more than half of Jupiter's satellites are retrograde. Having two or three other moons makes the capture process slightly more feasible, if I recall correctly.

AndreasJ
2008-May-07, 08:11 PM
Note that a small captured moon at 500,000 km away would be visible only as a starlike dot.

ngc3314
2008-May-09, 07:29 PM
Retrograde moons have to be captured, I believe; more than half of Jupiter's satellites are retrograde. Having two or three other moons makes the capture process slightly more feasible, if I recall correctly.

Captured it may be, but Triton is a counterexample to retrograde moons usually being small. The dynamics of intermingling sizeable direct and retrograde moons cannot fail to be interesting...

ravens_cry
2008-May-10, 12:54 AM
I found this on Elfwood, and while I have yet to use it personally, reading it through really gave me a lot of ideas. Hope it helps. http://www.elfwood.com/farp/thewriting/liljenbergworlds/liljenbergworlds.html

grant hutchison
2008-May-10, 01:26 AM
So how did Larry Niven work out his Ring World?, I understand that he got quite a load of flack at a sci fi convention once by a rampaging mob of respectable physicists - or was that the inventor of the infinite improbability drive :whistle:His physics was good; a neat hybrid of the Dyson sphere and the rotating habitat. The "flak" came from MIT students at the 1970 World Science Fiction Convention, who pointed out that he hadn't explored the (in)stability of Ringworld's rotation around its sun.

Grant Hutchison

Sticks
2008-May-10, 05:46 AM
And he would have gotten away with it had it not been for those pesky meddling MIT students :naughty:

KaiYeves
2008-May-10, 05:51 PM
And he would have gotten away with it had it not been for those pesky meddling MIT students
Come to think of it, that's probably where Velma did go.

timb
2008-May-10, 08:16 PM
Looking at the question of binary stars first; there appears to be two ways that you can have a planet in a stable orbit in a binary system.
(from this page )
http://www.solstation.com/habitable.htm


First the two stars might be close together in the centre of the system- a good rule of thumb is that the stars should be separated by no more than one third of the radius of the planet's orbit; for Earth that would be 1/3 AU, or a bit less than fifty million kilometers. In this case the planet would orbit both stars.

Or the second star could be separated from the first by more than 3.5 times the planet's orbital radius; for Earth that would be 3.5 AU, nearly as far as Jupiter. In this case the planet would only orbit one of the stars, not both - figure-of-eight orbits are almost impossible.

I'd add a safety margin to both of these figures - make the stars in the first case ten million kilometers closer together or so, and in the second case make the second star at least 4.5 AU distant- but that is up to you.


That's pretty sound advice. A different sort of "binary" -- a Jovian class planet in the inner solar system -- is observationally quite common. The orbit of such an intruder has to meet conditions similar to but less stringent than those on a binary star for there to be stable planetary orbit in the habitable zone. If the gas giant itself is in the habitable zone then there's the possibility of its possessing a habitable moon.

Another thing to keep in mind is the life cycle of stars. Don't cast a late cycle degenerate star such as a white dwarf in any role but a very distant companion. Black holes and neutron stars are pretty much a no-no anywhere in a life-bearing system.


----------
As for the moons, there is an interesting thread here about multiple moons
http://www.bautforum.com/questions-answers/71168-planet-two-moons-double-eclipse-possible.html

It looks like you could have at least two moons orbiting an Earth-like planet, although it would be a little difficult to explain how they were formed. One moon at 50 or 140 thousand kilometers, and one at 380 thousand kilometers seems to be stable. You could then add a single close-in asteroid-type moon, a bit like Phobos; note that this moon will eventually hit the planet. And for a fourth moon you could have a smallish retrograde moon at about 500,000 km or so, going the other way round the world.

In the solar system moons around terrestrial planets are uncommon and seem to be the result of "unusual" mechanisms such as asteroid capture (Phobos and Deimos) or mega-impact (Luna). I'm not sure that a captured moon would necessarily have a weird orbit: Phobos and Deimos have very vanilla (circular, prograde, low inclination) orbits.

If you like your moons' orbits weird the author could consider a pair of co-orbital moons. Retrograde orbits, high inclination orbits, and orbits at or below geosynchronous are all plausible ways to add variety. A debris disk could substitute for a really close in moon.

I've noticed that when people design worlds they usually make it just like Earth only better: more moons, more suns, a bigger planet, a brighter night sky etc. Don't over do it.

m1omg
2008-May-10, 09:33 PM
And he would have gotten away with it had it not been for those pesky meddling MIT students :naughty:

Well, ringworlds can be constructed, just use the right materials...
http://www.orionsarm.com/worlds/Ouaddai.html

jokergirl
2008-May-13, 11:23 AM
His physics was good; a neat hybrid of the Dyson sphere and the rotating habitat. The "flak" came from MIT students at the 1970 World Science Fiction Convention, who pointed out that he hadn't explored the (in)stability of Ringworld's rotation around its sun.

Grant Hutchison

And if I remember correctly he used this feedback rather well in the later books where the instability becomes a major plot point.

Back on topic.

The system that Sticks described had four moons, which would needs be rather small (at least some of them) to work. I'd also be thinking about the possibility of some "moons" actually being captured debris in the Lagrange points instead of in orbits of their own, although I don't know how big objects could be captured in such a way.


Personally I've always toyed with the idea of a world with two big moons - the locals having created an elaborate calendar based on their relative position.
But for the calendar system to work, the orbits would need to be arranged so that one moon could obscure the other from time to time (the timespan could be rather long though, but should be some sort of harmonic). Also, both moons should be large enough to be discernible disks of not too different diameter. Is such a system even physically possible?

;)

Sticks
2008-May-13, 12:51 PM
With multiple moons, do you get a form of tidal locking, where a bigger one, drags the smaller one?

Another idea, just off of the top of my head, is where two of the moons orbiting each other around their own barry centre as well as orbiting the planet. (this could in theory have been one moon that had been impacted by a larger object that could be one of the other moons)

I vaguely recall that Saturn (or possibly Jupiter) has two linked sheparding moons which swap places from time to time.

Tobin Dax
2008-May-13, 05:32 PM
With multiple moons, do you get a form of tidal locking, where a bigger one, drags the smaller one?
You can. The orbital periods or the inner three Galilean moons around Jupiter (Io, Europa, and Ganymede) are in a 4:2:1 resonance, which is a form of tidal locking.


Another idea, just off of the top of my head, is where two of the moons orbiting each other around their own barry centre as well as orbiting the planet. (this could in theory have been one moon that had been impacted by a larger object that could be one of the other moons)

I vaguely recall that Saturn (or possibly Jupiter) has two linked sheparding moons which swap places from time to time.The latter is far more likely, I think, considering the tidal effects and orbital speeds involved as the two moons orbit. If one moon is much smaller in mass than the other, you could probably get a binary moon in orbit around the planet (along the lines of Earth and the Moon around the Sun). If they're of similar mass, you'd end with with a co-orbiting situation like Janus and Epimetheus (which are around Saturn).

Sticks
2008-May-14, 07:21 AM
This is kind of an aside, and I have used this elsewhere on BAUTForum but not sure where.

A few years back I came across this advertisement at a bus stop
http://img128.imageshack.us/img128/9927/o2advert001a0ec.th.jpg (http://img128.imageshack.us/my.php?image=o2advert001a0ec.jpg)

I doubt that such a planet would be stable with a moon that close to the planet, but what do advertising executives know :rolleyes::wall:

Delvo
2008-May-14, 12:37 PM
The picture doesn't tell you anything about how close the moon is to the planet.

What is a "top-up"? Something that turns upside-down things right-side-up again?

jokergirl
2008-May-14, 12:53 PM
When I was designing worlds for scifi comics, I used some simple applet to figure out whether or not certain moon masses and configurations would work. I wonder if it's still around (and if so, where)...

;)

Tobin Dax
2008-May-14, 06:51 PM
Delvo is rigt about not knowing how close it is. My question is where is the light source? The same one can't illuminate both bodies like that.

Sticks
2008-May-14, 08:47 PM
Delvo is rigt about not knowing how close it is. My question is where is the light source? The same one can't illuminate both bodies like that.

Yeah, what do illustrators and graphic designers know eh? ;)