# Do stars in a binary star system fall along the ecliptic?

I am wondering about Tatooine, and was reading about binary systems here, which provided a lot of good basic food for thought. My specific question is not addressed at that link, and so I pose it here.

Would the two suns (a la Tatooine's, not the other possible binary configurations at the link) likely both lie in an ecliptic? I believe they should, but am not certain. if they do (please confirm):

• I believe at some timepoints, one sun is visible in the sky (a sun-on-sun eclipse).
• At other times, they could be discerned as two bodies, and the distance between (and rate of change) would depend on how quickly they are orbiting one another.
• Intriguingly, there might be two sunsets (again, similar to Luke's view) - and I think if so it would have to do with how far apart they are. I think the rotation of the planet allows two sunsets for two suns lying in an ecliptic with a planet. Is my space perception way off, here?

There could be double-shadow effects too, which could be used for navigation or other maths, I suppose.

• Unless there is major planetary perturbation Commented Aug 30, 2017 at 3:55
• I am curious if any answers will appear that directly address the bullets in my original question. Are these all correct extensions of the geometries? Commented Aug 30, 2017 at 15:21
• @DPT I edited my answer earlier to address them; does it help, or do you want a bit more? I think the answers are simple enough, so I kept them short, but I can elaborate more if need be. Commented Aug 31, 2017 at 1:35

## Most likely, yes.

I wrote an answer here discussion how binary stars are born. The relevant part here is that there are several main theories for how binary stars form:

1. Capture: In an interaction between three stars, two become bound together while the third one is ejected away. This is unlikely to be the major formation mechanism for most binary systems.
2. Separate nuclei: Two stars formed very close to one another, close enough to start orbiting each other. This is also very unlikely to happen.
3. Fission: A protostar collapsing from a gas cloud to a normal star splits in two after some instability forms. Modeling so far rules this mechanism out.
4. Fragmentation: Fragmentation is to some extent like fission, except it involves the gas cloud itself splitting before a protostar forms. This is generally viewed as the accepted way most binary star systems form.

Depending on which hypothesis is true, circumbinary protoplanetary disks should form in the same plane as the orbits of the stars, meaning that the ecliptic would match up (see these notes for more details). There might be some variation - maybe less than 10 degrees - but not much.

In the case of planets orbiting only one of the stars, things are different. There are plenty of cases of single-star disk misalignment in young binary systems. For instance, Jensen & Akeson (2014) looked at the young system HK Tauri and measured a misalignment of 60 to 68 degrees between the protoplanetary disks around each star, meaning that at least one is rotated quite far outside the orbital plane of the stars.

I believe at some timepoints, one sun is visible in the sky (a sun-on-sun eclipse).

This will be possible, yes, but bear in mind that one star may not fully cover the other, so you'd almost certainly be able to see parts of both.

At other times, they could be discerned as two bodies, and the distance between (and rate of change) would depend on how quickly they are orbiting one another.

This is correct.

Intriguingly, there might be two sunsets (again, similar to Luke's view) - and I think if so it would have to do with how far apart they are. I think the rotation of the planet allows two sunsets for two suns lying in an ecliptic with a planet. Is my space perception way off, here?

Yes, this is also possible. For a planet orbiting both stars, assuming Earth-like rotation, there will always be instants where each star sets separately, as viewed from a certain point on the planet.

Yes.

Star systems form when a cloud of interstellar gas and dust collapses under its own weight. See Jeans instability. When this happens, the cloud is almost certainly going to be rotating just a bit. This means that the cloud has some angular momentum. As the cloud collapses, it starts spinning faster and faster so as to conserve its angular momentum, causing the cloud to flatten into a disk. And a disk it will remain, even as the gas and dust coalesces into planets and a star, and the stellar wind from the newly-formed star blows the rest of the gas and dust away.

Some binary systems* form in exactly the same way, except by producing two star-sized clumps of matter instead of just one. If these form close together at the center of the protostellar disk, the result is a Tatooine-like system that could well have planets orbiting the stars' combined center of mass. If they form very far apart (a few dozen AU, perhaps), then the result could be the other type of binary system, with planets orbiting each star individually.

In either case, the stars and the planets will all lie in the same plane (that is, the ecliptic plane), at least at first, since that's where all the matter that formed them was. However, that's not to say both stars and all the planets will always lie in the ecliptic. A close call with another star or another star system can disrupt the orbits of the planets or even the stars themselves. Do note, however, that such encounters will almost certainly affect the planets in a Tatooine-like system before they affect the stars. So a Tatooine-like system with multiple planets where the only things orbiting on another plane are the stars is highly unlikely.

* Binary systems can also form when a single-star system captures another star from somewhere else, but you won't get a Tatooine-like system this way without destroying both stars' planetary systems completely.