For the background of one of my alien races, I am having them come from a double-planet. (One roughly Earth-sized, one smaller and tide-locked to each other.) My current issue is to to how far apart to set them to get a reasonable balance between stability (because both planets have developed life) and rotational speed. Data Large planet (Urrot): Radius: 6453 Mass: 5.74042E+24 kg (less dense than Earth, approx 0.93g) Orbital radius (approximate mean, derived from solar flux equivilent to Earth from it's star):18 million km (1.2AU) Small planet (Haron) Radius: 4553 Mass:2.21396E+24 kg (approx 0.72g) Star: Radius: 765050km (1.1 Sol) Mass: 2.20729E+30(1.11 Sol) My start for ten was to set the smaller planet Haron's angular diameter at 35'. This gave an orbital radius of 894000km and a rotation period of about 84 days. As I toyed with this today, I realise that put it at 51% of the primary's Hill Sphere - perhaps a bit too high, so I could probably afford to bring it in just slightly whatever. Tidal acceleration functions suggest that the tidal forces on the primary would be approximately 2.4 times Luna's on Earth - though obviously, the planets are tide-locked, I assume this will have some effect of the tides (which I assume will still exist, but only on the level of the effect the alignment of the planet's to the sun on the "standing tide" caused by the pull between the planets). Coming back to this idea after a long break, I had been thinking of a roughly 104-hour rotational period before crunching the astrophysics (and remembering I already had made that first pass on my spread sheet). As an experiment to see what this value would result in, I changed the angular diameter to 253' (making the smaller planet appear eight-and-a-half times larger than the sun; if nothing else, it'd LOOK spectacular...!), which changed the rotational period to 4.3 days. The orbital radius of the small planet around the primary was now 123500km (7% of the primary's hill sphere radius, perhaps a bit close.) The tidal acceleration on the planet was now 920 times Luna to Earth, which I would assume would impart not only some significant disortion to the planet along that alignment, but also have some significant effect on the tides (or rather, on the "standing tide" as it were). I am sort of half-assuming the effect of the sun (which would be pretty insignificant to that sort of tidal value) would be fairly small on the actual solar tidal height variance? Or would it rather be that the solar tides would be enormous? Do either of the values sound reasonable, or would you think I would need something more in-between the two (perhaps to get with the 1/3-1/2 optimal Hill sphere radius). Is the lower radius feasible, or would those sort of tidal forces make planets holding atmosphere and/or habitability impractical? Some suggestions would be very helpful.