Could Gliese 581 g have a moon?

[sderamus]

Gliese 581 g, assuming it's real, is supposedly tidally locked being so close to its parent star. That makes sense. It also means it is probably not a very habitable planet - one side being a burnt desert and the other a frozen wasteland, with only the margins capable of supporting life. Maybe.

But what if it had a moon? Would that "unlock" it and allow it to rotate like the earth? I don't understand the details of the physics of tidally locking well enough to answer it, but intuitively it seems like it ought to.

I was reading that moons around our gas planets don't have moonlets because the gravitational pull of the close giant would disrupt their orbits so much that a moonlet would just get thrown off. Would that be the case with Gliese 581 g? Or is it far enough away from its parent star so that wouldn't happen?

TIA!

sderamus

 

[Algr]

I think that the effect that causes moons to be unable to have moonlets is actually the same as what causes planets to be tidally locked. Our two innermost planets, Mercury and Venus, are the only planets without moons. While they do rotate very slowly, they are not tidally locked.

 

[DaveC426913]

Mercury and Venus ... are not tidally locked.

Uh, well, you're half right...

 

[Algr]

I'm at least 584/224.65ths right.

 

[DaveC426913]

I'm at least 584/224.65ths right.

Ha ha. That's what I get for staying up past my bedtime. Spent too long with my head in the Gliese system, all rusty in the Sol system. :sheepish:

 

[Janus]

Gliese 581 g, assuming it's real, is supposedly tidally locked being so close to its parent star. That makes sense. It also means it is probably not a very habitable planet - one side being a burnt desert and the other a frozen wasteland, with only the margins capable of supporting life. Maybe.

But what if it had a moon? Would that "unlock" it and allow it to rotate like the earth? I don't understand the details of the physics of tidally locking well enough to answer it, but intuitively it seems like it ought to.

I was reading that moons around our gas planets don't have moonlets because the gravitational pull of the close giant would disrupt their orbits so much that a moonlet would just get thrown off. Would that be the case with Gliese 581 g? Or is it far enough away from its parent star so that wouldn't happen?

TIA!

sderamus

There are two factors to consider: How large would the Hill sphere for Gliese 581g be, and how would a moon within effect the planet tidally compared to the star?

Taking the high side estimate of the planet's mass, I get a Hill sphere of 524966 km. Fore an orbit to really be considered stable we need an orbit of about 1/3 that or 174930 km (about 0.45 the distance of our moon).

Tidal force is proportional to mass and inversely proportional to the cube of the distance. Gliese 581 g is 0.146 the distance from the Earth to Sun, and Gliese 581 is 0.31 the mass of the Sun, so its tidal force will be 100 times that of the Sun on the Earth.

Our moon orbiting at 174930 km would exert 11 times the tidal force that it does now, and at it present distance exerts about twice the tidal force the sun does, so a Moon orbiting Gliese 581 g at 174930 km would have to be more than 5 times the mass of the Moon in order to dominate the planet tidally. A Moon the same mass as our Moon would have to orbit closer than a distance of 102,299 km to exert more tidal force than the star.

Also, A moon orbiting with a period less than the rotation period of the Planet will slow the planet's rotation, climbing to a higher and slower orbit. Given Gliese 581 g's smaller hill sphere, I'm going to guess that it would eventually eject the moon.

It will only speed the planet's rotation up if it has a slower period or orbiting retrograde.

A moon formed at the same time as the planet is going to start with a longer orbital period than the planet's rotation and will not be retrograde. Our Moon was likely to have been created by a collision, but even that should leave the planet spinning faster.

This leaves us with the possibility of capture. The capture of an object into orbit is actually easier if it enters a retrograde orbit, so at least this is a factor in favor. But still you are left with a fairly rare event; the capture of a body large enough to significantly effect the rotation of the planet.

So while I wouldn't put in the range of impossible, I would consider it highly improbable.