Two stars mutually orbiting and planet in between?

I am reading a SF series by Catherine Asaro (a real polymath genius type in real life) and one of her solar systems is a double star, I suppose like Alpha Centauri (I know it's a triple but the tiny one doesn't do much to perturb possible planets around either large star).

I was thinking, what about two stars mutually orbiting one another, say at 2 AU, and they are stars maybe a bit dimmer and smaller than our sun, could a planet exist at the center of the orbit? Halfway between the two stars? Suppose the whole system was moving through space, the milky way, whatever, and the planet was going the same group velocity, could such a relationship between the two stars and planet be stable?

Would it be like some kind of gravitational well like a Lagrangian?

Originally posted by sonhouse
Would it be like some kind of gravitational well like a Lagrangian?

It would be a Lagrangian point. I don't know much about them, but from brief reading there are 5 such points of which 3 are unstable (I don't know which 3), and two result in a sort of wobble.
I don't know if anything other than spacecraft have ever utilized Lagrangians.
Does anyone know of asteroids or other bodies found at Lagrangian points in any system?

Originally posted by twhitehead
It would be a Lagrangian point. I don't know much about them, but from brief reading there are 5 such points of which 3 are unstable (I don't know which 3), and two result in a sort of wobble.
I don't know if anything other than spacecraft have ever utilized Lagrangians.
Does anyone know of asteroids or other bodies found at Lagrangian points in any system?

I think they are alternatively called Trojans and there are hundreds of them +/- 60 degrees from Jupiter's orbit.

I thought it might be a nice sci-fi plot to have a planet in an arrangement like that. Not sure if you could even call it an orbit since it looks to me like the planet stands still and the sun rotates around the planet from the planet's pov.

It would be interesting if intelligent life formed on such a planet, maybe an Aristotle like philosophy would start and in this case be true! He said the Earth is the center and the sun rotates around it, which is a natural conclusion to make since we see the sun go around from sunrise to sunset.

It looks like tides would maybe cancel out also.

If the day was the same length as it is here, then you would have two sunrises and sunsets and if there was a tilt like here and the stars had a mutual orbit period of one year, it seems likely to be 8 short seasons. Does that sound reasonable? If not, how many seasons would there be?

I don't think a planet between two stars orbiting eachother is especially stable. Semi-stable, yes, but this is theoretical. STable? No, I don't think so.

And if I'm wrong in this, even if it is stable, weakly stable, then a planet shouldn't be able to form there from the proto-cloud.

No, I think it's unlikely. Interesting, but unlikely.

Originally posted by sonhouse
I think they are alternatively called Trojans and there are hundreds of them +/- 60 degrees from Jupiter's orbit.

Thats interesting. I'll read up on it.

I thought it might be a nice sci-fi plot to have a planet in an arrangement like that.
It seems from further reading that only the +/- 60 degrees points are stable.

Now the question is, if two bodies are of nearly equal size then where are the Lagrangians? And how many are there?
Possibly as many as 10, with 4 stable ones!

It looks like tides would maybe cancel out also.
The tides on earth are mostly from the moon. In the scenario you describe the tides would be twice as strong as they would have been with one sun.

If the day was the same length as it is here, then you would have two sunrises and sunsets
And near constant daylight.

and if there was a tilt like here and the stars had a mutual orbit period of one year, it seems likely to be 8 short seasons. Does that sound reasonable? If not, how many seasons would there be?
At the poles, you would only see one sun at a time - all the time, with each sun being visible for half the year.

As for seasons, that would depend on whether there was a difference in the energy output of the two suns. At temperate latitudes you might have a warm summer and a cool summer.

Our count of 4 seasons is rather more cultural than factual. In reality there is only 'more sun' and 'less sun' for higher latitudes and the same but twice a year at the equator.

Originally posted by twhitehead
Thats interesting. I'll read up on it.

[b]I thought it might be a nice sci-fi plot to have a planet in an arrangement like that.
It seems from further reading that only the +/- 60 degrees points are stable.

Now the question is, if two bodies are of nearly equal size then where are the Lagrangians? And how many are there?
Possibly as many as 1 more sun' and 'less sun' for higher latitudes and the same but twice a year at the equator.[/b]

Yes, spring and fall are transition times, not long in duration. Why wouldn't the tides cancel out? You would have a pull on the planet from one direction like with the moon or sun but you would have one exactly 180 degrees apart in the opposite direction. Seems to me they should cancel out and be a null point gravitationally.

So you would have two different sources of light, maybe they would have different spectra, so you would have a red day say, and a blue day next. Maybe we should pepper the atmosphere with great clouds so the day would have some break from the suns. Maybe the winds go not equatorially but polar wind patterns so the clouds would be in vertical stripes, allowing for a respite from the constant sun.

Originally posted by sonhouse
So you would have two different sources of light, maybe they would have different spectra, so you would have a red day say, and a blue day next. Maybe we should pepper the atmosphere with great clouds so the day would have some break from the suns. Maybe the winds go not equatorially but polar wind patterns so the clouds would be in vertical stripes, allowing for a respite from the constant sun.

If there are two types of spectra, then it must come from two stars with different masses. Blues stars are hot, massive, and young stars (because they don't get very old), or hot white dwarfs. Red stars are large red giants in their final old stage, or small, not so hot, red low-mass dwarfs. This makes the orbits unsymmetric, and therefore even less stable orbit for the planet in between.

Sorry, this configuration, with two stars orbiting eachothers with a planet in some lagrange point in betwsen, is highly unlikely.

Forget the tides. The planet would enter a synchronous rotation in its orbit quite quickly. One fix bulge on either side. No seasons, no rising or setting of either sun, only one sun visible at the time, never two, which one is dependant on the location on the planet. Sorry. This will be a very boring planet.

Originally posted by FabianFnas
The planet would enter a synchronous rotation in its orbit quite quickly.

I disagree. That is not a forgone conclusion. The tidal force would be twice as strong as the sun exerts on earth but that isn't a whole lot. The moons effect is stronger and has not yet managed to get the earth into a synchronous rotation.

Originally posted by sonhouse
Why wouldn't the tides cancel out? You would have a pull on the planet from one direction like with the moon or sun but you would have one exactly 180 degrees apart in the opposite direction. Seems to me they should cancel out and be a null point gravitationally.

http://en.wikipedia.org/wiki/Tide

The moon causes a tide on the opposite side of the earth too.
A 'spring' tide occurs when the moon is on the same side as the sun, or the opposite side. They only 'cancel out' when they are at 90 degrees.

So, if the planet is at the 60 degree Lagrange (which is stable), the tides might nearly cancel out.

Originally posted by twhitehead
http://en.wikipedia.org/wiki/Tide

The moon causes a tide on the opposite side of the earth too.
A 'spring' tide occurs when the moon is on the same side as the sun, or the opposite side. They only 'cancel out' when they are at 90 degrees.

So, if the planet is at the 60 degree Lagrange (which is stable), the tides might nearly cancel out.

Ah, that is a good point, the planet does not have to be at the center, it can be off +/- 60 degrees also. That would put it in a stable trojan orbit too. Have to think about that.

Originally posted by sonhouse
Ah, that is a good point, the planet does not have to be at the center, it can be off +/- 60 degrees also. That would put it in a stable trojan orbit too. Have to think about that.

As I asked earlier:
The normal Lagrangian points are for a small mass orbiting a larger mass (and the trojan is of negligible mass. I would like to know what happens when the two main masses are nearly equal.
The +/- 60 degrees is on the orbit of the smaller mass. If there are two nearly equal masses they each have an orbit (nearly coinciding) around their center of gravity. Where are the Lagrangian points?

Originally posted by twhitehead
As I asked earlier:
The normal Lagrangian points are for a small mass orbiting a larger mass (and the trojan is of negligible mass. I would like to know what happens when the two main masses are nearly equal.
The +/- 60 degrees is on the orbit of the smaller mass. If there are two nearly equal masses they each have an orbit (nearly coinciding) around their center of gravity. Where are the Lagrangian points?

I don't think the lagrangians depend on the relative masses of the objects, the gravity would still tend to cancel at +/- 60 degrees and a smaller body could hang out there I think in a small orbit around that point. Of course it would be corkscrewing it's way round the system but it should be stable.

Originally posted by sonhouse
I don't think the lagrangians depend on the relative masses of the objects, the gravity would still tend to cancel at +/- 60 degrees and a smaller body could hang out there I think in a small orbit around that point. Of course it would be corkscrewing it's way round the system but it should be stable.

I didn't realize just how common objects at L4 and L5 are. Wikipedia points out that there are quite a number, including small moons around Saturn:
http://en.wikipedia.org/wiki/Lagrangian_point#Examples_2

Based on the above I would guess that such objects would not be uncommon in binary star systems.

Originally posted by twhitehead
I didn't realize just how common objects at L4 and L5 are. Wikipedia points out that there are quite a number, including small moons around Saturn:
http://en.wikipedia.org/wiki/Lagrangian_point#Examples_2

Based on the above I would guess that such objects would not be uncommon in binary star systems.

Maybe in a lagrangian orbit, you might sneak in some night time too. Looks like it geometrically speaking. I guess you would have a very short night, if it was a 24 hour day like here, you would get 4 hours of night or so. Think what that would do for agrigulture!

I imagine there would be a lot of interest in working night shift🙂

Originally posted by sonhouse
I guess you would have a very short night, if it was a 24 hour day like here, you would get 4 hours of night or so. Think what that would do for agrigulture!

It would depend on the latitude. As with the earth, at higher latitudes, in the summer you would get longer days and shorter nights, and in the winter longer nights and shorter days. Just overall, you would have much longer days.
Now, to avoid global warming, you would need a system where the planet was further from the suns than the earth is from our sun. So you would have longer cooler days, and shorter warmer nights.
The day would start of with sunrise, followed later by second-sun rise, then later first sunset and finally second sunset.
In midsummer, it would never get completely dark. But that is the case on earth at high latitudes. It would be like moving the polar circle further south (over summer).