Moons of tidally locked planets

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

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.

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)?

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.

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"?

[quote="NuclearVacuum"]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.

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?

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.

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.

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.