Bad astronomy | Can moons have moons? Yes! Well, sometimes.

I am asked many questions from people curious about the Universe. Some of them are more common than others. Are black holes real? (yes) Do I believe in extraterrestrials and flying saucers? (Yes and no).

Another question I asked myself was: can moons have moons?

The question was raised recently for two reasons: one is the possible recent discovery of a moon orbiting an exoplanet. The other is that a newspaper has published quite interesting calculations on all this to see if a moon can really have its own moon.

Ironically, a few days before the publication of this document, I've speculated about that too:

In this thread, I briefly review some concepts, but there is a lot more going on here. So let's take a look.

Can a moon have a moon? It turns out that the answer to this question is simple and more complex (but still cool). The simplest is: yes! And you already knew that, if you thought about it correctly. After all, several Apollo missions have circled the Moon, and we have other probes that have done it too, like the marvelous Moon Reconnaissance Orbiter (I mean, c'mon, that's right in the name) ).

But that's not really what you mean, is it? Want to know if a Natural the moon can orbit another moon.

The answer to this question is different. It's a yes! But only sometimes. And we have never seen one. At least not yet.

First of all, what is a moon? In a nutshell, it is an object that gravitates around a planet (what is a planet? It is more difficult to define, and also the IMO is not very important). Thus, a planet orbits a star (usually) and a moon orbits a planet.

So what is going on around the moon? There have been discussions about what to call such a thing, and even though some people like 'moonmoon', I do not care about that. Not because it's silly – God knows we have a lot of silly names for classes of things in space – but because it's hard to say and is not as descriptive as it could be.

I prefer "submoon". Easier to say, and the meaning is clear *.

So, submarines. It turns out that they can exist, but only under certain conditions. And for that, we must talk about gravity.

Look at the Earth and the Moon, for example. The Earth is much more massive, by a factor of about 80, than the Moon. This means that if you place something on a large orbit around the moon, the gravity of the Earth may be enough to drive it away. The more a submarine gravitates around the moon, the greater the influence of the Earth. Even if the Earth does not simply pull it out of its orbit, its gravity could be enough to disrupt subtly change the orbit, then each time it passes between the Moon and the Earth, the orbit changes a little. These add up and the submarine ends up being lost, thrown into space or sent crashing on the moon or the earth.

It turns out that you can model this effect using physics and mathematics. When you do, you find that there is a volume around a moon where the influence of the moon is much stronger than that of the planet and where a submarine can be stable the maths are the same). This volume of space is called the Hill sphere. ^†

The size of the sphere of a lunar hill depends on three factors: the mass of the planet it orbits, the mass of the moon and the distance that separates it from the planet. It's logical; a more massive moon can better hold submarines, but a more massive planet makes it more difficult. And if you're too close to your planet, she's taking the submarine off.

The calculation is not so difficult (you can connect and chug in most cases). For example, using the Earth and the Sun, the sphere of the earth hill is about 1.5 million kilometers in radius, well beyond the distance of 385,000 km from the moon.

It is interesting to note that the sphere of the Moon Hill is about 58,000 km from its center (or about 56,000 km from its surface, which is its radius). It's pretty big! So yes, the moon can have a sub-moon.

To be picky, it turns out that even if you are inside a sphere of the Moon Hill, the gravity of the planet can still hit you and push you, destabilizing you with time. It's hard to calculate, but a decent estimate is that you're stable for a long time (for example, billions of years) if you're in the half-sphere of Hill's sphere. So, for the Moon, less than 28,000 km from the surface.

But there is a problem if you get too much To close to a moon, too. I mean, obviously, you have to be out of the moon's surface to orbit, otherwise Kapow. Impact. But there is also the problem of the tides.

In a word, tides are an effect of gravity. Since gravity weakens with distance, a large object close to a big one feels different amounts of gravity over its width. If this difference becomes too important, it can tear the object!

This distance where this occurs is called the Roche limit. It depends on a lot of things, including the mass of the two objects and the size of the smaller one. You can also calculate it according to the ratio between the two objects. densities and the size of the bigger a. But in the end, that means you can not get a submoon too big and too close to its parent moon, otherwise it will break.

And now, finally, we can look at what the astronomers discovered and did the calculations. They asked a very specific question: can you place a submarine in orbit around the moon in our solar system and that the orbit is stable throughout the life of the solar system, or 4.6 billion years?

It turns out that few moons in the solar system can accommodate a sub-fleet of decent size (say 10 km). For some, Hill's sphere is inside the moon (for example, with a moon of very low mass orbiting a gigantic planet), or so small that you can not get a stable orbit.

But they have found many, and the list is interesting: Our Moon (which we knew from what I showed above), Titan and Iapetus, the moons of Saturn, and Callisto, the moon of Jupiter. Titan is huge and can resist Saturn, while Iapetus and Callisto are far enough away from their planets that their spheres of the hill are relatively large. Our moon is a mixture of both reasons.

As soon as I read that part of their paper, the hair on my neck stood up. Iapetus has a huge mountain ridge that literally extends all around the moon to its equator. It is unclear how this was formed, but it may be due to a low-speed collision with a smaller object that tore apart; the debris then circled the moon and collapsed along the equator. To do this, the simplest way would be that, if Iapetus has a little moon herself and the influence of Saturn (and / or that of the other moons in orbit around the planet) ends up making it too near Iapetus. Then Roche took over.

Hmmmmm. It's not at all a proof, but it's really interesting.

So, it is certainly possible for a moon to have a submoon. Cool! But if so, why do not we see it?

Well, some may be too small to see well. Something, say, ten meters wide would be pretty hard to spot in orbit around our own Moon, let alone Titan. But we certainly do not see bigger should to be visible, and that's interesting.

The problem may be that forming with the moon and the planet is not possible in the first place; the disc of gas and dust that swirls around a star can be too chaotic for a sub-boat to condense easily. His possible capture an asteroid and turn it into a submarine, but it's very difficult and rare.

In other words, if there is a region around a moon where a submunition is stable, it may be too difficult to obtain a submunition in the region.

But there is more. Over time, the tides of a planet affect the orbit of the moon. The details are complex, but for example, our Moon was probably formed very close to the Earth and, over the course of billions of years, it has retreated to its present distance. It is still down by about 4 centimeters a year.

When it was really close to Earth, the Moon's sphere would have been very small, even inside the Moon. If this is the case, a submarine could not have formed! By the time he moved far enough away from the Earth to have a substantial sphere of the hill, it was too late.

In the case of Saturn and Jupiter, the other moons in orbit around these planets are quite huge (Titan and Ganymede are both the size of Mercury!). They can also affect submarines and destabilize them. It may be for this reason that we do not see any submoon in our solar system.

But that does not mean they do not exist! Sean Raymond, one of the two astronomers of the newspaper, shows that the eventual Exomoon Kepler-1625b-I could have an exosoumoon, including a decent sized one. It is not difficult to imagine situations in other stellar systems where this could also happen.

It's poetic! An extraterrestrial star, surrounded by an extraterrestrial planet, surrounded by an extraterrestrial moon, itself encircled by an alien submarine much smaller but perhaps substantive. It's too bad we do not have something like that here, but it's a big universe.

And a fun to think.

My thanks to Sean Raymond and the main author of the article Juna Kollmeier for their enthusiastic help with some questions I had about the stability of the submoon.

^*Note that in this Twitter feed, I am guilty myself of using "moonmoon" (although I actually use "exomoonmoon"). But it was for Twitter, where I sometimes let myself be more stupid, as opposed to here on the blog where it's just dignity.

^† I have already talked about it, especially with regard to some people who claim that technically the Moon does not turn around the Earth (This is done), and also with reference to Pluto being a planet or not.