Tidal locking in a Red Dwarf System - How would large moon affect?

[Althistorybuff]

Question:

I understand that close-in planets in a red-dwarf system are probably tidally locked. 

However, if the planet had a large moon, would the tidal locking be avoided? 

Would the tug of the moon keep the planet from being locked?

Thanks. 

 

[TheVat]

Depends on the mass difference and distance between planet and moon.  If the moon's mass is great enough that the barycenter is outside the moon's sphere then,  as with Pluto and Charon,  both bodies will be tidally locked to each other as they both orbit their common center of gravity which lies between them.    The effect will override any tidal locking from the primary,  the M star.   I'm afraid I don't have the math handy on locating the barycenter.   

The question is if,  unlike Pluto and Charon,  a close-in planet could possibly be close enough to a dwarf star to disrupt the mutual lock between the planet and big moon.   Seems unlikely,  but we could use a second opinion.   

[Janus]

20 hours ago, TheVat said:

Depends on the mass difference and distance between planet and moon.  If the moon's mass is great enough that the barycenter is outside the moon's sphere then,  as with Pluto and Charon,  both bodies will be tidally locked to each other as they both orbit their common center of gravity which lies between them.    The effect will override any tidal locking from the primary,  the M star.   I'm afraid I don't have the math handy on locating the barycenter.   

The question is if,  unlike Pluto and Charon,  a close-in planet could possibly be close enough to a dwarf star to disrupt the mutual lock between the planet and big moon.   Seems unlikely,  but we could use a second opinion.   

While a Moon's tidal influence can dominate, the star will have a tidal influence.  For example. Let's start with a planet-moon system tidally locked to each other, The star will still will still produce tidal effects that will produce drag on the Planet's rotation. If this were allowed to happen, then the Moon would orbit faster than the planet rotates. In this situation, the Moon would spiral in, transferring rotational energy to the planet, speeding up its rotation. 

So what happens is while the Moon does end up keeping the planet tidally locked to itself, it does so at the expense of  it's own orbital energy. Both it's orbital period and the rotational period of the planet shorten.  However, this can't be maintained forever, as eventually the moon would spiral inside the Roche limit and break up. How long this would take depends on the tidal influence of the star.   For example, Proxima Centauri B orbits so close to its star that the stellar tidal forces on it are roughly 1000 times that of the Sun on the Earth.

[Althistorybuff]

OK, thanks for the feedback. It sounds like the answer is that very close-in planets around red-dwarf stars would see tidal forces so strong that it would evntually force a large moon to spiral into and crash. Then, the planet eventually would become tidally locked. I appreciated all the help. 

[Airbrush]

On 9/30/2021 at 7:20 AM, Althistorybuff said:

OK, thanks for the feedback. It sounds like the answer is that very close-in planets around red-dwarf stars would see tidal forces so strong that it would evntually force a large moon to spiral into and crash. Then, the planet eventually would become tidally locked. I appreciated all the help. 

That is similar to what happens to a binary star system that is orbiting, and gets too close to, a supermassive black hole.  The pair gets torn apart, one crashes into the BH and the other get shot out of the galaxy.