# How long would a day be on two tidally locked earth sized habitable planets?

So for my story I want to have a binary planetary system, where both planets are about the size of earth and are habitable by human people. I'm also thinking that they will be tidally locked to each other, which means their day will be the same amount of time it takes for them to orbit around their common center of gravity, about half way between the planets.

So my question is, how fast could they be going without going too fast where they would lose their stable circular orbit. I'm just trying to figure out what would be a reasonable length of a day in this situation.

• If they are tidally locked, central part of the hemisphere facing the other planet will always vary between dusk/night/dawn/eclipse cycle. Sep 30, 2016 at 20:51
• The closer they are together, the faster they orbit about one another, and the more stable their mutual orbit about their sun is. So The question isn't how fast can they orbit, but rather how slow. Sep 30, 2016 at 21:15
• So it depends on how close together they are. If very close together they could go faster, but would have more of an eclipse as well, if farther apart, slower but less shadow from the eclipse. Too close together though and they'd not be able to remain spheres and fall apart, too far apart and their days would be too long to maintain habitable temperatures. Sep 30, 2016 at 21:34
• I think you don’t understand orbits. The speed is determined by how close they are: the maximum speed is when they touch. Plug mass and radius into Newton’s laws of gravity to get the actual speed. Oct 1, 2016 at 10:40