PDA

View Full Version : A moon orbiting a tidally locked planet



eburacum45
2007-Sep-19, 10:01 AM
Just a quick question, with a reasonably straightforward answer, hopefully;

if an Earth-sized planet was tidally locked to (say) a red dwarf star, would it be possible for that planet to have a moderate sized moon? I seem to recall that this is not possible, but I can't remember why.

Another way of asking this question might make more sense. Is it possible for a planet with a large moon to become tidally locked to its star, if the planet is close enough to that star? Or does the presence of the moon inhibit this process?

Mr. D
2007-Sep-19, 04:49 PM
I might be wrong here. But i think the planet will become tidally locked to whichever body has the stronger influence on it. In Earth's case, the moon has a stronger effect on the tides than the sun because it is much closer, so eventually, (In millions of years or something, i'm not sure how long) the earth will be locked facing the moon.

This is because gravitational field strength is inversely proportional to the square of the distance between the two bodies.

So in short, if the moon is sufficiently massive, the planet will be locked to that instead of the star.

It's covered in the "Bad Astronomy" book i believe. I'll have to dig that out and have a look.

I'm sure somebody more knowledgable will be able to give a better answer, that's just my understanding.

eburacum45
2007-Sep-19, 05:09 PM
So a terrestrial planet close to its star might be locked to its moon, and rotate once a local month? That sounds an interesting option.

Other worlds might not yet have reached that situation yet, and rotate more like the Earth and the Moon.

But it seems that a planet with a large moon cannot be tidally locked to the local star, as a planet couldn't very well be locked to the moon and the star at the same time. Unless perhaps the moon occupies a Lagrange point?

Mr. D
2007-Sep-19, 05:24 PM
I'm just guessing based on my own understanding of tidal forces really.

But I think, if the planet is sufficiently close to the star so that the star has a stronger pull on it than the moon does, then the star will have a bigger tidal force and hence the planet will become locked facing the star.

I think it's not so much an issue of whether or not the planet can have a large moon if it is tidally locked, but more that it won't be tidally locked to the star if it has a sufficiently large and sufficiently close moon.

Also, i think the number of moons a planet has will depend on how strong its gravitational effect is, relative to the star it's orbiting. If the planet is too close to the star, it will become easier for the star to strip the moon away from the planet. If you look at this table (http://www.windows.ucar.edu/tour/link=/our_solar_system/moons_table.html), it seems to be the farther out and more massive planets that have the most moons, which makes sense to me.

But now i really am just speculating, best to wait until a real astronomer gets here for a proper answer. :)

EDG
2007-Sep-19, 07:30 PM
Mr D isn't quite correct.

If the planet is tidal locked to the star, it can't have a moon. The reason is that if it had a moon to start with then energy would have been taken out of the planet-moon system during the tide-locking process, which means the moon would spiral into the planet. Once it hits the planet, the planet can then proceed to tidelock to the star.

Earth isn't a good example since we're too far from the sun to be able to tidelock to it anyway (within the lifespan of the solar system)

aurora
2007-Sep-19, 07:38 PM
Mars has some captured moons that are small so Mars would never lock to them. At least one is temporary, and will eventually spiral in, but in the meantime a planet like Mars that was closer to its sun could have a moon temporarily even though the planet was tidally locked?

EDG
2007-Sep-19, 10:20 PM
Any moon would be temporary, since it'd ultimately spiral in :). A planet can start with a moon, which would then spiral in due to the solar tides sucking angular momentum out of the system and then it'd crash into the planet or form a temporary ring system.

eburacum45
2007-Sep-19, 10:27 PM
Thanks for the explanation, EDG. That's just what I wanted to know.

Mr. D
2007-Sep-20, 12:01 AM
Mr D isn't quite correct.

If the planet is tidal locked to the star, it can't have a moon. The reason is that if it had a moon to start with then energy would have been taken out of the planet-moon system during the tide-locking process, which means the moon would spiral into the planet. Once it hits the planet, the planet can then proceed to tidelock to the star.

Earth isn't a good example since we're too far from the sun to be able to tidelock to it anyway (within the lifespan of the solar system)

Aha, i didn't think of that.

So if the planet has a moon, will they both become tidally locked to each other before the moon spirals in? Or will it do that before the planet and moon get chance to lock together, i suppose it depends how close they are to the star doesn't it?

I know that in the Earth's case, the moon is slowly moving farther away, because its robbing the Earth's angular momentum and getting pulled forward in its orbit. So once the two are locked i guess the moon will start to fall back towards the Earth as the sun takes energy from the system. I'd better read up on this.

I probably shouldn't attempt to answer people's questions when i don't fully understand it myself!

I'll get me coat.

EDG
2007-Sep-20, 12:33 AM
So if the planet has a moon, will they both become tidally locked to each other before the moon spirals in? Or will it do that before the planet and moon get chance to lock together, i suppose it depends how close they are to the star doesn't it?

If it's close enough, the moon would probably collide while the planet is still forming. Or very early in its history.


I know that in the Earth's case, the moon is slowly moving farther away, because its robbing the Earth's angular momentum and getting pulled forward in its orbit. So once the two are locked i guess the moon will start to fall back towards the Earth as the sun takes energy from the system. I'd better read up on this.

That's pretty much correct. Though it gets weird because as the moon spirals in, the planet's rotation actually speeds up to maintain the lock with the moon's orbital period (which is getting shorter and shorter). Once the moon hits or breaks up, the planet then gets slowed down by the star's tides again.

Noclevername
2007-Sep-20, 03:49 PM
Make the Earthlike one the Moon, the larger planet will take longer to slow down.

EDG
2007-Sep-20, 04:36 PM
Make the Earthlike one the Moon, the larger planet will take longer to slow down.

All other things being equal, a larger planet tidelocks faster than a smaller one.

John Mendenhall
2007-Sep-20, 04:49 PM
All other things being equal, a larger planet tidelocks faster than a smaller one.

Sorry, I lost track here. What tidelocks to what, faster? A large planet to its star? Or to its moon? Or the moon to the planet?

Neverfly
2007-Sep-20, 04:52 PM
Sorry, I lost track here. What tidelocks to what, faster? A large planet to its star? Or to its moon? Or the moon to the planet?

EDG corrected Noclevername in pointing out that a larger planet tidelocks faster not slower. A larger planet means more mass affected by gravity.

EDG
2007-Sep-20, 05:44 PM
Sorry, I lost track here. What tidelocks to what, faster? A large planet to its star? Or to its moon? Or the moon to the planet?

Sorry - I know a larger planet will tidelock to the star faster than a smaller one. I'll have to check on the planet/moon situation.