# Habitable zone for binary star systems

## Main Question or Discussion Point

Does anyone know which type of orbit is most likely to result in habitable worlds in a binary star system -- a planet orbiting one of the two stars, or orbiting both of the stars, or are they both very likely/unlikely? Wikipedia quotes a paper that simulated binary stars and found that 50-60% could support terrestrial planets, but didn't address my specific question as to the preferred type of orbit.

I'm making up the system for a story, so I have complete flexibility as to star orbits, planetary orbits, etc.

Any help would be appreciated.

Thanks,
Dirk

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mfb
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I didn't see statistics yet, both options are possible. For orbital stability, the radii should differ by a factor of at least ~5, better ~10. As an example, if you add another star to our solar system, the distance between sun and the other star should be less than .1 AU or more than 10 AU.
The latter option is problematic for the evolution of planets in the system, it is easier if the separation is much larger (like 100-1000 AU).

Thank you.

There's been a lot of research into this subject. Here's how one does it: one finds a planet's motions by doing numerical integration over lots and lots of orbits, and one follows what happens to that planet. If you have some programming ability, you could try it yourself.

Planetary orbits in binary stars R.S. Harrington, Astronomical Journal, vol. 82, Sept. 1977, p. 753-756.

If the stars are separated by a, then a planet's orbit will be stable if it orbits one of the stars closer than about a/3 or else if it orbits both stars farther than about 3a.

Long-Term Stability of Planets in Binary Systems - Abstract - The Astronomical Journal - IOPscience from 1999. It has similar results, but extended to nonzero eccentricities and different relative masses of the stars.

I recall that someone or other once claimed that some SF stories have featured planets moving in figure-8 orbits around binary stars, with each star in one half of the 8 shape. Has anyone found such a story?

But that sort of orbit would not be stable, as calculations show. I remember once trying to find one, without success. After a few orbits, the planet would either escape or orbit one of the stars.

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Choosing plausible star parameters is surprisingly difficult. The mass difference between a star too cool to be an attractive option and a star so hot that it would be too short lived is quite small. From the SF point of view the Lagrange L4 & L5 points have their attractions, but then we want the stars to differ by about a factor of at least 25 in their mass. Not much hope, unless the smaller star is practically cool enough to walk on, assuming you could handle the gravity.

You easily could have your two stars so close that your total orbit around them is much like the orbit around a single star, and it would lead to some dramatic but livable climatic and visible effects, but you would have to work them out very carefully if you wish to construct a plot around them.

Or you could have two stars further apart (say 2 astronomic units) one a lot redder than the other, the planet in question orbiting the red one fairly closely, so that it doesn't get QUITE enough heat and light, but so that the brighter star is cruelly hot when the planet is at its closest, but not quite hot enough at its furthest.
That would give you lots of scope for elaborate ecologies and religious views!

This is helpful. Thank you all.

it's sci-fi, does it has to be correct?
it's about imagination more than reality?

it's sci-fi, does it has to be correct?
it's about imagination more than reality?
That question is more about story construction and story intention than about physics.

If you are writing abstract, "soft" SF or even fantasy, then it doesn't matter whether you put your civilisation inside the sun, as long as it contributes fundamentally to the story. If it is not essential to the story then do not sully the writing with irrelevant nonsense; leave it right out! It does not contribute to the story and destroys the suspension of disbelief. For horrible examples read almost any of the teenage crud on the Internet, and a fair proportion of popular published SF of the mid-to-late 20th century. For good examples read Vonnegut, much of Wells, Wyndham, Tolkien, Stevenson, or classical mythology for example. Or Browning's Pied Piper of Hamelin.

OTOH, if you are writing hard SF, like some of the works of Wyndham, Wells, Niven, Clement, Clarke etc. then you must omit every item that can possibly be omitted whether scientifically arguable or not, but those details that are of interest to the story must be either accurate and defensible, or clearly speculative (is there really liquid water on... Can we really find a drug or treatment that... etc) because otherwise it not only fails to contribute to the story, but destroys the suspension of disbelief as well.

What you certainly cannot do as a competent writer, is to say: "Oh, it is only a story, so it doesn't matter what sort of #\$%^&* I write; it is up to the reader to enjoy it." That would hardly work for Jane Austen at one extreme or Evelyn Everett-Green at another, let alone SF.

In general, SF is fiction in which the science matters; if it doesn't matter it shouldn't be in the story; matter extraneous to ANY story should be omitted, leaving non-technical fiction. If it does matter it should be as real and convincing as possible.

mfb
Mentor
The question is more than one year old.

The question is more than one year old.
Errr.... And? I am not sure of the intention of that remark in context. The question that I answered certainly was recent.

mfb
Mentor
It means dbaezner is unlikely to see Xyooj's questions, and speculating about what dbaezner meant does not help either. If there is something new to discuss, it should go into a new thread, that's why I close this one now.