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 On Creating a Stable System... (Read 10168 times)
 LordHyperbole Uploader I Love YaBB 2! Posts: 9 On Creating a Stable System... 03/18/08 at 14:10:04   Hi, I've been trying to set up a stable stellar system for my fictional world to inhabit, and the simulator is excellent and a lot of fun.  I appreciate the existence of this forum and I have a few questions:     I had the thought that it would be cool to have a planet in the middle of the habitable zone (the 'main planet'), with one at the outer edge and one at the inner edge (in reality this would be halfway through the MS lifetime of the star, but presumably we would assume that this is when the simulation starts). It seemed a pretty implausible scenario from the little I know about orbital mechanics, but for almost 100,000 years the three planets orbited discretely.  Every time, though, they end up crossing orbits eventually.  For the reason I explain below, I end up having to put all of the planets exactly in the ecliptic each time.  Does this increase the chance that they will perturb one another?  Also, I have not been setting the eccentricity at zero, since I thought it would be unrealistic to have perfectly circular orbits.   Ignoring the rest of the system for now, the three planets intended (optimistically) to inhabit the HZ were:   Planet 1:  SMA = 178,883,748,934 m  //  e = 0.001215  //  Mearth = 0.891472155   //  d = 12,487,000 m Planet 2:  SMA = 199,015,029,488 m  //  e = 0.000523  //  Mearth = 1.200213009   //  d = 13,834,000 m Planet 3:  SMA = 219,086,227,216 m  //  e = 0.001007  //  Mearth = 1.0915276409  //  d = 13,667,000 m   These were all in the ecliptic, although my random values for their obliquity ('inspired' by our Solar System) should have been 5.75, 7.77 and 6.19 degrees respectively.  Obviously I will keep changing these values if need be until I find something that works.     The star is 1.1654141576188 solar masses and 1,531,253,667 metres in diameter.     I wondered if it was possible that other planets at the right distances inside and out might help to stabilise this configuration?  My gut feeling is that they're just too close for comfort, though...   During the long, stable period, each 'edge' planet comes to almost exactly 20 million km of the middle one at its closest.         I have a problem in that when I set the planets to orbit around their barycentre with the sun, they inevitably fly off into space.  I can resolve this by clearing the position and velocity boxes and setting the length of the SMA as the X coordinate in metres and the mean orbital velocity in the Y box in m/s.  The problem with that is that in doing that, I must be killing the obliquity of the orbit and resetting it to the ecliptic.  My maths knowledge is poor, and I'm struggling with certain aspects of the orbital mechanics.  There's no way I can work out the relative positions for X, Y and X to give the correct length of SMA at a particular inclination, let alone the correct velocity component required for each direction.     Tony, since I notice that you are continually updating and improving the software, I was going to ask if it would be possible to add an input field for velocity in the Create Object box at some point in the future?  It would also be great if it was possible to edit eccentricity & inclination in the Edit Object box, and another cool addition might be to add an 'orbital period' field to the Orbital Elements box. I don't know if these things are possible, though.       My other questions were, what is the fastest timestep that you would recommend using with the latest beta version, and should I set the Update Graphics Interval as high as possible?  What is the function of the DoEvents interval?     Oh, and I'd be grateful if someone could point me in the direction of an explanation of rotating frames, I can't figure it out.  I'm not even sure what the time unit used is. I see that they seem to be useful in determining resonances, though.  I tried to induce a couple of resonances - 2:1 between the outermost planet and the next one in, and 4:1 between two of the smaller planets close to the star. I did this by deciding their orbital periods first and deriving their distances and velocities from that, and then starting them in the same position.  Does this work?  If so, they were both unstable...I just don't know. Back to top IP Logged
 frankuitaalst Ultimate Member Great site Posts: 1510 Gender: Re: On Creating a Stable System... Reply #1 - 03/18/08 at 14:36:16   At first glance , as I see your initial conditions and separation between the planets  I think your system should be stable on the long term . The distance between each planet  is about 20Mio km and the eccentricity is small . Adding more planets should be possible . A feature which can influence the stability slightly is the argument of perifocus and the longitude of the ascending node as both have an influence on the MOID (Minimum Orbit Intersection distance ) . Back to top IP Logged
 LordHyperbole Uploader I Love YaBB 2! Posts: 9 Re: On Creating a Stable System... Reply #2 - 03/18/08 at 15:24:36   Ah, well, thanks for the feedback.  So I should definitely be inclining my orbits.  Although I still have the problem of having to enter the velocities, unfortunately, since I don't know how to calculate them in three dimensions.     I have a 3.8 jupiter-mass beast at about 1 billion km which stays stable for as long as I've run the sim, although it seems to like to migrate in just a little, to 800 Mil.  There's a 2.5 J giant which starts at about 1.8 billion km and comes in a little to about 1.5 (IIRC).  The outer two giants have problems, though.  There's a 0.4 J 'Saturn' at 3.5 billion km (approx.) and a 0.075 J 'Neptune' at 5.5 billion.  One or the other, usually the Neptune, ends up with eccentricity increasing until it ends up being thrown out the system. This still happens with the 2:1 resonance I tried to instigate.     The three (four since one is a double planet) planets interior to the HZ have a similar problem currently, in that they appear unstable on a timescale of 100 Ma. Back to top IP Logged