# Can a planet that is tidally locked to its sun have a satellite that is not tidally locked to its sun?

A planet orbiting an Red m-dwarf is tidally locked to its sun. Can it have a satellite which is not tidally locked to the sun? Also can the satellite be tidally locked to the planet ?

The satellite to the tidally locked planet CANNOT be tidally locked to the star. The satellite, by definition, is orbiting the planet. While some quirk of resonance or chance could possibly result in the "moon" always having one side facing the star, such a set-up would not be through the mechanism of tidal-lock.

Tidally locking the moon to the planet is possible. Depending on distance form the star, the moon will likely lock to the planet prior to the planet locking to the star.

Mutually locking the moon to the planet (orbiting geosynchronously and always showing the same face to the planet) while the planet is tidally locked to the sun is likely impossible. The planets rotation takes a full year to make a single turn. This means a tidally locked moon would need to take a full year to orbit this planet. To do so would require the moon to be a significant distance away from the planet, far enough away that it would likely be stripped away from the planet into its own, independent, orbit of the star.

• This doesn't seem right. The moon would not need to take a year to orbit, it would need to take a year to rotate. I agree though, tidal locking is to the parent body which in this case is the planet. Commented Feb 27, 2017 at 16:21
• That is correct. I was, for some reason, assuming that the moon was locked over the same spot on the planet, a mutual lock, between them. Commented Feb 27, 2017 at 16:27
• The mutual locking scenario is potentially possible if the "satellite" sits at one of the sun-planet Lagrange points, most likely L4 or L5, although I'm not sure if the "satellite" could still be called a satellite in this case. It should still get tidally locked to the sun (and planet), though- probably before the planet does, even. Commented May 2, 2018 at 4:43

Can the satellite not be tidally locked? Yes. Just look at a real world example of it on a smaller scale and you can see it's easily possible. Our moon is tidally locked to the planet but it's still possible to have satellites orbiting the moon. We have done so multiple times, even having people orbiting, and we still currently have a few active satellites still orbiting the moon.

Can the satellite instead be tidally locked? Also yes. All it takes to make something tidally locked is to have the time it takes to make a rotation around its axis the same time it takes to revolve around the main body. So you could in fact make a satellite locked to a moon, that is locked to a planet, that is locked to the star. Naturally it might not be easy but with some advanced technology and manipulation it's entirely possible.

The only thing that wouldn't be possible is simultaneously being locked to the planet and the sun. As the moon is tidally locked to the planet, like it is with our planet, we see the same side of it but the direction of the light from the sun changes, this is what causes the phases of the moon. But if you had a moon that was locked with the star and rotating freely from the planet I imagine it would be somewhat different. You'd see the entire body of the moon during its full revolution around the planet still leading to phases but more of the moon's surface would be visible. But I imagine you'd still be left with a "dark side" of the moon.

It wouldn't strictly be a satellite of the planet, but if the moon were at the L1 or L2 Lagrange Point, both bodies would probably be tidally locked to the star, and would keep the same faces toward each other. This would give the appearance of the satellite being tidally locked to the planet since the same face of the satellite would always face the planet. If there are any other large bodies in the solar system, this would not be a very stable arrangement, but it would be cool to see while it lasted.

If a planet is tidally locked to a star and has a moon orbiting, the tidal forces between the planet and the moon would either move the moon out of the planet's grip causing it to go into orbit directly around the star or would cause the orbit to decay, break apart and create some very nice rings then fall onto the planet.

• Welcome to worldbuilding.SE! When you have a moment, please take our tour and visit our help center to learn more about us. This is an interesting piece of information, but it doesn't answer the OP's question, which is if it's possible for the satellite to not be tidally locked to the sun.
– JBH
Commented May 1, 2018 at 22:50