Moons of tidally locked planets

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Moons of tidally locked planets

Post by NuclearVacuum on 8th October 2008, 9:22 pm

After working on Celestia for some time now, I could not agree on how to make moons of tidally locked planets. Would they be also locked to the star (not rotating the planet at all)? Would they be tidally locked to the star as well?

I really hope you guys don't mind me asking all these questions, but I can't find any other place better to ask them pale
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Re: Moons of tidally locked planets

Post by Sirius_Alpha on 9th October 2008, 6:30 pm

For a tidally locked planet, I think the moons would be tidally locked to the planet as well. I'm not going to swear by it, though, I'm still trying to figure out tidal effects.

For a tidally locked planet, though, it's likely that it's hill sphere would be within it's roche limit, disability the existence of moons. Unless the planet orbits significantly farther out and has had a lot of time to slow down.

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Re: Moons of tidally locked planets

Post by NuclearVacuum on 10th October 2008, 9:36 am

Sirius_Alpha wrote:For a tidally locked planet, though, it's likely that it's hill sphere would be within it's roche limit, disability the existence of moons.

Does that mean 51 Pegasi b would not have any moons (pity)?
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Re: Moons of tidally locked planets

Post by Sirius_Alpha on 10th October 2008, 3:22 pm

NuclearVacuum wrote:Does that mean 51 Pegasi b would not have any moons (pity)?

Correct. For any moons to exist in a stable orbit, it has to be really, really close to the planet, since the star is so close (and thus it's gravity so much more competitive than the planet's). If the moon were really close to the planet, it's orbit would drop into the planet (like at Phobos) or if it were large enough, it would be tidally disrupted.

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Re: Moons of tidally locked planets

Post by NuclearVacuum on 10th October 2008, 8:59 pm

What does "tidally disrupted" mean?

Since we are on the subject, I am working on a plausible alien planet, and want to know if this could work or not. Lets just say Aldebaran b exists; when Aldebaran was a main sequence star, planet b would have been far enough from its star to have moons. When the star evolved into a giant, what would happen to the planet's moons? Would they stay, or be "tidally disrupted"?
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Re: Moons of tidally locked planets

Post by Sirius_Alpha on 11th October 2008, 7:50 am

NuclearVacuum wrote:What does "tidally disrupted" mean?

You know those shows about black holes that tell you that when you're so close to a black hole, the gravity pulling on your feet is stronger than the gravity pulling on your head, causing you to be torn apart?

It's pretty much like that, though not quite as extreme. The gravitational pull of the planet on the side of the moon that is facing the planet is stronger than the gravitational pull on the side of the moon that is facing away. This is because gravitational attraction diminishes with distance, and the amount of distance that the moon takes up (i.e. it's physical size) allows for gravity to pull on the moon moreso on the planet-facing side.

Note that the larger the moon, the more severely it will be affected. This is because with a large moon, there's more difference between the gravitational pulls from the planet on the planet-facing/away-facing sides of the moon. This is why Jupiter's four inner moons, Metis, Adrastea, Amalthea, and Thebe can survive for the moment. They're really close to Jupiter, but they're small and thus the difference in gravitational pulls is very small.

The distance from the planet that an object can be tidally disrupted depends on many factors,


You were asking specifically about 51 Pegasi b. The region in which a planet is gravitationally dominant is called it's "Hill Sphere". It's an area of space around the planet where, if I dropped a marble, it would fall toward the planet instead of the star. It is calculated as follows:

where r is the radius of the Hill Sphere, a is the semi-major axis, e is the eccentricity, m is the mass of the secondary (or the planet), and M is the mass of the primary (usually the star).

Wikipedia wrote:The Hill sphere is but an approximation, and other forces (such as radiation pressure or the Yarkovsky effect) can eventually perturb an object out of the sphere. This third object should also be of small enough mass that it introduces no additional complications through its own gravity. Detailed numerical calculations show that orbits at or just within the Hill sphere are not stable in the long term; it appears that stable satellite orbits exist only inside 1/2 to 1/3 of the Hill radius (with retrograde orbits being more stable than prograde orbits).

So, calculating the hill sphere for 51 Pegasi, I found that it's radius is ~397,111 km. As stated in the Wikipedia quote, we can expect stable orbits to be at half, to 1/3 of this value (~198,556 km, and ~132,370 km, respectively). All of these altitudes are from the centre of the planet. Assuming a radius of 1 Jupiter-radii, the Roche limit (in kilometres from the surface) is 128,556 km, and 62,370 km, respectively.


Last edited by Sirius_Alpha on 4th September 2010, 9:48 am; edited 1 time in total (Reason for editing : Fixed a quote.)

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Re: Moons of tidally locked planets

Post by NuclearVacuum on 4th November 2008, 9:26 pm

Another question on the same note. Would this mean that earth like moons could not form around a jovian planet around a red dwarf star?
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Re: Moons of tidally locked planets

Post by marasama on 5th November 2008, 3:12 pm

NuclearVacuum wrote:Another question on the same note. Would this mean that earth like moons could not form around a jovian planet around a red dwarf star?
Answer is probably a no, since EGPs can form beyond the tidal lock area of a red dwarf star.

I'm assuming your original question was referring to EGP's that are in the so called, "habitable zone". Then, that I am not sure. It is still too early to tell.

EGP = Extrasolar Giant Planet.

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Re: Moons of tidally locked planets

Post by Sirius_Alpha on 5th November 2008, 9:17 pm

Indeed, too early to tell, but I would suspect that if an Earth-like planet were to be in a stable orbit around a gas planet in the habitable zone of a red dwarf, the orbital motion of the moon around the planet, and thus toward or away from the star, might be significant for global temperatures in an Earth-like atmosphere*. (I'm assuming the planet is sufficiently far from the gas planet to avoid being rendered uninhabitable by its radiation).

I don't know if the planet could orbit close enough to the gas planet to both avoid being inhabitable from the planet's radiation, as well as being within the hill sphere of the planet.

Of course it may be that a Neptune-mass planet might somehow succeed in capturing a large moon (i.e. Neptune/Triton). The large moon may be habitable. A Neptune-mass planet would have a lower hill sphere, so the moon would have to orbit the planet a bit closer. Also I don't know much about the radiation environments of ice giants like Uranus or Neptune.

*Edit (Sep 4, 2010): improved understanding of oceans as thermal buffers since I wrote that.


Last edited by Sirius_Alpha on 4th September 2010, 9:50 am; edited 2 times in total

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Re: Moons of tidally locked planets

Post by Darkness nova on 7th November 2008, 9:20 pm

Something tells me that that might render said planet uninhabitable.

Were talking about a neptune or uranus sized planet that is tidally locked to a read dwarf witha moon that wants to tidally lock to BOTH objects.

Something tells me that one of two things happens.

A: the spin increases or happens in an odd way due to the tidal locking force of the star and the planet acting on the moon during it's rotation around the EGP. Meh I can't explain what i'm thinking very well in words....I'll find a way to make a pic of it.

OR

B: The tidal stresses turn the world into another io or something else that I cannot think of right now.

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Re: Moons of tidally locked planets

Post by EDG on 28th March 2010, 4:40 pm

You can't have moons around planets that are tide-locked to their stars. The solar tides that are slowing down the planet's rotation remove angular momentum from the planet-moon system, causing any moons of the planet to spiral in and collide with the planet (or break up around it to form a ring).

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Re: Moons of tidally locked planets

Post by Sirius_Alpha on 28th March 2010, 5:59 pm

I think I remember reading a paper that postulated that very small moons, like Jupiter four innermost, could survive for a long period of time around a tidally locked Jupiter. These moons wouldn't have a lot of mass and so would not make a significant tide on the planet.

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Re: Moons of tidally locked planets

Post by Parhelion on 4th September 2010, 5:53 am

Not to necropost, but I ran across this site and I think it would help me out a GREAT deal if someone could explore this with me just a little further.

I'm working on a bit of fiction and a game world design, and I'm trying to determine if my ideal setting is even feasible. I'm more of a biology person, so this is all-new territory to me.

Essentially, the habitable planet I need is orbiting a gas giant near a red dwarf. Is this even possible? What would be required for the planet/moon to maintain an atmosphere breathable to humans (besides plantlife - in terms of mass, size, gravity, etc)? Assuming the habitable moon is tidally locked to the gas giant, how might this effect the way that the star rises and falls, and how might it effect the seasons?

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Re: Moons of tidally locked planets

Post by Sirius_Alpha on 4th September 2010, 9:43 am

Parhelion wrote:Essentially, the habitable planet I need is orbiting a gas giant near a red dwarf. Is this even possible?

Because such stars are colder and dimmer, a red dwarf's habitable zone is going to be much closer to the star than it would be for a hotter, sun-like star. This means your gas giant will need to be much closer to the star than even Mercury is from our sun.

Being close to a star presents problems for having stable moon systems. A planet's gravitational sphere of influence (it's Hill sphere) shrinks as the planet gets closer to the star, so there isn't quite as much room for a moon for a close-in gas giant. Interactions with the star would destabilise the moons orbit unless it is really close to the gas giant (so the plant can have a firm "grip" on the moon). Then you have tidal effects to contend with (see Io).

Being close to a star presents tidal challenges for the moon. With the planet being so close to the star, it will become tidally locked with it (similar to how Luna is to Earth). The planet will rotate ~once per orbit and so the moon will orbit the planet faster than the planet is rotating. The moon will raise tides on the planet, much like on Earth, but since the tides will lag behind the moon (as opposed to in front of as with Earth-Luna), the moon will lose energy, and its orbit will decay into the planet.

Not to say it's impossible, but there's numerous challenges associated with it.

Parhelion wrote:What would be required for the planet/moon to maintain an atmosphere breathable to humans (besides plantlife - in terms of mass, size, gravity, etc)?
The moon will need a significant amount of mass, which will increase its size and gravity. It will need to have nearly planetary mass to hold onto a significant atmosphere, otherwise it will be lost through atmospheric escape mechanisms (photons strike air molecules, providing them with kinetic energy. If they have enough to achieve escape velocity, they will leave the planet ... note this is not the same as direct erosion). While Titan has a sub-planetary mass, it is also much further away from the sun, helping it hold onto its atmosphere.

Formation of moons of such mass is an extreme challenge with your typical gas giant. The best we've figured out from modelling and from the examples in our solar system, gas giants form moon systems that have a mass totalling around ~0.02% the mass of the planet. So you might imagine scaling up the mass of the planet so that the ~0.02% of its mass is comparable to an Earth-mass planet... however one finds that their gas giant is needed to be quite hefty, 6, 7, or more Jupiter-masses (MJ).

While having more mass may aid the moon in holding in atmosphere, it also aids the moon in delivering it to its tidal destruction. A higher mass (and thus gravitational pull) will exagerate the tides on the planet, quickening its tidal in-spiral timescale. The moon falls into the planet in a shorter amount of time. Having a large radius of the moon also affects the amount of tidal heating it will receive from the planet as well (Note that Amalthea is much smaller than Io, so avoids the excessive tidal heating we see there despite being closer to Jupiter).

Parhelion wrote:Assuming the habitable moon is tidally locked to the gas giant, how might this effect the way that the star rises and falls, and how might it effect the seasons?
The sun would rise and set just like on Earth, the rate at which depending on the moon's rotation/orbital period. Though being close to the red dwarf, the apparent size of the star would vary considerably throughout the course of the moon's orbit.

Tidal heating from the planet aside, the seasons would likely be dominated by its distance from the star (axial tilt will tend to zero due to tidal synchronisation). Given your average red dwarf, a habitable planet may have an orbital period of 20 days or so, thus a different 'season' every few days. If your moon has a significant amount of water on its surface like Earth, the heat capacity of the oceans will provide a thermal buffer: in summer, the oceans absorb some heat, in winter, they release it. The cycle occurs too fast for the biosphere to really cool or heat significantly. So the residents on your moon may not actually notice the seasons changing.

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Re: Moons of tidally locked planets

Post by Lazarus on 4th September 2010, 12:36 pm

One thing to think about regarding habitable moons around M dwarfs is the effect of the tides induced by the star on the moon. Remember these are going to be strong enough that without the influence of the planet, the habitable world would have ended up locked to the star. The moon will end up locked to the gas giant instead, which means it will be rotating relative to the star.

How severe are the tides going to be? Well since the tidal force is a result of the gradient of the gravitational force, it scales as Mprimary / d3. I posted some numbers for the case of HIP 57050 in this thread, which suggests they will be roughly 35 times the strength of the lunar tide on Earth. Not sure how these numbers would translate into heat flux though. The nearby star would also presumably act to pump the eccentricity of the moon's orbit around the planet, leading to planet-induced tidal heating perhaps similar to that of Io.
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Re: Moons of tidally locked planets

Post by Parhelion on 7th September 2010, 6:46 pm

Thank you for the insight! This has helped a lot and I've gone back over my original ideas and tweaked them accordingly to try and make them a bit more plausible.

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Re: Moons of tidally locked planets

Post by Diakonov on 4th April 2011, 8:23 pm

So this may be the relation: The more the planet is near the star and the smaller is it's mass, it's moon need to be closer of it, right? After some point, it's impossible to have moons because of the roche limit of the planet itself. So due to this, due to the proximity of rocky habitable planets in M dwarf stars, it's improbable that they have at least big moons... maybe some asteroids orbiting them!

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Re: Moons of tidally locked planets

Post by Sirius_Alpha on 5th April 2011, 2:18 am

That is essentially correct.

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Re: Moons of tidally locked planets

Post by marasama on 5th April 2011, 10:23 am

But what about a binary planet setup?

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Re: Moons of tidally locked planets

Post by ciceron on 6th April 2011, 3:08 pm

How could you possibly have a binary planet locked to a star? The only way out i see is if they orbit each other perpendicular to their orbit to the star , but such thing i don't see viable or stable.

Of course it's feasible if each of them is locked to the other but not to the star. You could even attach some circumbinary moons for effect. Can't see how the stability of that system could go on the Gy scale , but sould be pretty to see.

If that kind of complex exists in far out than the frost line of a M star , the circumbinary moons could even be habitable , due to the geotermical heat similar but on a lesser degree that the one we see in Europe and Io. Heat from the planets , ligth (not much) from the star and two big brothers to fend-off life crushing asteroid bombardmend.

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Re: Moons of tidally locked planets

Post by Sirius_Alpha on 6th April 2011, 4:08 pm

You can consider a binary planet just an extension of a planet+moon set up. Still unstable close to the star.

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Re: Moons of tidally locked planets

Post by Diakonov on 9th April 2011, 7:51 am

Could you have langrarian points in planets close to M star? Imagine a Jupiter-like planet close to the star, in the habitable zone, and in one or two langrarian points you have a terrestrial planet. Is that viable?

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Re: Moons of tidally locked planets

Post by Sirius_Alpha on 9th April 2011, 11:36 am

I don't see why it wouldn't be stable.

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Re: Moons of tidally locked planets

Post by Diakonov on 9th April 2011, 7:42 pm

It's because in an orbit near the star, the Jupiter-like planet would be near the planet in the langrarian point, due to the small orbit size. An M star system would have planets near each other, and that would cause disturbance in their orbits. Is this correct?

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Re: Moons of tidally locked planets

Post by Sirius_Alpha on 9th April 2011, 9:12 pm

Diakonov wrote:It's because in an orbit near the star, the Jupiter-like planet would be near the planet in the langrarian point, due to the small orbit size. An M star system would have planets near each other, and that would cause disturbance in their orbits. Is this correct?

I thought we were talking about a simple three-body system with a star, a Jovian planet, and a terrestrial trojan planet. If you add destabilizing influences (such as additional planets), then the system will likely destabilize. I would imagine, however, that if a planet is near enough to destabilize the trojan, then the Jovian is near enough to it to destabilize this third planet. If anything, the Jovian may act as a sort of perturbation shield. A third planet may not be able to get close enough to perturb the trojan significantly without itself being perturbed by the Jovian.

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