Celestia Forums

In a binary system, with planets orbiting the main star, do the orbits form along the star's rotation or the orbital plane of the entire system?

Thanks,
Tim

Yes.



Where they form and where they are billions of years later are not necessarily the same. There's going to be some non-uniform motion in the original collapsing gas and dust cloud, which will cause variations in the initial orbital and rotational planes of all of the bodies in a system, although they'll probably be close to the same plane. Gradually, over time, rotational and orbital energies will be dissipated due to tidal and other effects, resulting in planetary orbital planes nearer to the equatorial plane of the nearest star. Close interactions among bodies can throw them into non-coplanar orbits, though.

So, in the case of Sirius A, if enough time has passed for an Earth type planet to form, it would more than likely settle in an orbit like in image 1.
Correct?
I made a fictional system around Sirius A and the orbits are 90 degrees off the orbit that Sirius A takes around its barycenter. But they are in-line with Sirius A's rotation.
Which makes sense to me, but I'm no scientist, so I thought I would ask.

Thanks,
Tim

Here is a small part of my scc file to show what I mean.

Regarding Sirius, the white dwarf star was originally more massive, and the system would have had a smaller semimajor axis. That is not good for forming planets in the original system, around Sirius A, as Sirius B would have severely reduced the protoplanetary disc.

As for strongly misaligned orbits, Kozai oscillations become a major factor - the inclination and eccentricity of the orbit couple and are exchanged back and forth. (Unless something else, e.g. tides, planet-planet interactions, general relativity, etc. has enough influence to disrupt the oscillations - which is why Uranus can support a regular satellite system)

See this page on the Gravity Simulator website: note that for inclinations near 90 degrees, the maximum eccentricity during the Kozai cycle approaches 1, and the planet could end up crashing into the star. Not good for stable planetary systems, and it is postulated that this is responsible for forming misaligned hot Jupiters, and the extremely elliptical orbit of HD 80606b.

IIRC Celestia doesn't specify the rotation of Sirius A, so it is taken by default to be in the ecliptic plane (as for all stars which do not have rotation models specified), the star's rotation is almost certainly different in reality.

So the info for the Sirius system in Celestia is lacking, or wrong?
It is a fictional system, so if it is not 100% that's ok, but I would like to get close.
I took 2 pics with Celestia. 1-is straight on with the orbits of Sirius A and B around their barycenter . 2-straight on with the equator of Sirius A, with the star rotating west to east.
Maybe I'm just thick headed, but to me (without any advanced education) picture 2 makes more sense.
What changes would need to be done to make it at least close to reality?
Thanks for the information so far guys.

Tim

fungun wrote:So the info for the Sirius system in Celestia is lacking, or wrong?

Well in some sense "lacking" is the same as "wrong", so...

In any case the rotation is NOT specified in any of Celestia's data files, that means it uses the default. Celestia's default stellar rotations have the rotation axis normal to the ecliptic plane, and a rotation based on the stellar class. This is obviously not going to produce accurate results for the majority of stars, except for the few that fortuitously happen to actually have a rotation corresponding to the default.

Actually measuring stellar rotation in 3 dimensions is difficult: most often the quantity that is supplied is v*sin(i), where v is the rotational velocity and i is the (typically unknown) inclination. There may be other indications of what the actual rotation period is (e.g. starspots), but Sirius is not a type of star that typically shows starspots, so I don't know if there's any indication of this. In any case, even figuring out the inclination is not going to fully specify the orientation of the axis, for that you need a second angle (the position angle of the axis). If the star is sufficiently large and close-by and is a fast rotator, it might be possible to resolve the shape of the oblate star, which may provide this information. However Sirius is (as far as I am aware) a slow rotator, so I don't expect that it is oblate enough to make this kind of measurement.

Might be worth trying to dredge up some information on SIMBAD. However given the separation of the binary it seems fairly likely that the rotation is aligned with the binary orbit.

I'm inclined (heh) to say that planetary orbits within a binary/multiple-star system would be no different from that of moons around a gas giant of a given stellar system- After all, it's best to treat the secondary star as just a bigger planet, whose gravitational interaction with the primary (the more massive the objects, the greater the interaction) would keep the companion star close to the plane of the primary, and the planets of the companion close to the companion's plane. The interaction would also treat the two stars as each other's gas giants, in terms of clearing away protoplanetary debris. I could be wrong, but I don't know of too many multiple-star systems (3 or greater) whose orbital planes differ significantly, unless one or two of the stars are at significant-enough distances that the gravitational interaction would be weak enough to allow for a greater inclination away from the primary's plane.

d.m.f.

Ok, I think I understand all the info I have been digesting (even did some Googleing on the subject).
So what is the best way to get the planets to line up on the same plane as the star's orbits?
I tried using inclination, it was close, but still way above the plane. I tried everything from 70 to 120.
With Sirius A rotating at a right angle to the star's orbital plane, how would you line up the planetary orbits?

Thanks,
Tim

You'll also need to have longitude of perihelion and ascending nodes be determined, as these do affect the orbit's 3D orientation in space. You could just copy the values in the .stc for Sirus A & B.

fungun wrote:So what is the best way to get the planets to line up on the same plane as the star's orbits?
I tried using inclination, it was close, but still way above the plane. I tried everything from 70 to 120.

You need to match the inclination and the ascending node of the orbit.

fungun wrote:With Sirius A rotating at a right angle to the star's orbital plane, how would you line up the planetary orbits?

Sirius A probably does NOT rotate perpendicular to the binary orbit: that is an artifact of the rotation not being specified in Celestia. You can reorient the rotation to be in the same plane as the binary orbit by adding a UniformRotation block to the star definition, and matching the inclination and the ascending node of the binary orbit (contained in nearstars.stc). E.g.

Thank you all very much for this information.
Since this came up, I have found 3 other addons I am going to have to fix now.
Guess I'm going to have to back away from the system when checking to see if the addon is done or not.

Thanks,
Tim