Hello!

I searched for a thread about this topic, but I couldn't find one. If there is one already, then I command you to accept my apology! *Lord Vader voice*

I wondered why people still feel the need to distinguish between planets and moons. We don't make a distinction based on the question whether a star is primarily orbiting the galactic center or primarily orbiting a more massive star. So why make a distinction between a planet that is primarily orbiting a star and a planet that is primarily orbiting a more massive planet? Or a distinction based on the location of the barycenter of the two planets? If Charon is listed as a planet, then I would like to call every big spherical object a planet. Luna, Io, Europa, Ganymede, Callisto, Titan, Triton and a lot of smaller ones around Saturn and Uranus.

Hum,
i have difficulty with using the search function too.

But see this thread

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`Irony` actually does mean `metal like`...

I tried to start that debate in my "What is a moon?" thread here. But the thread got moved to Q and A.

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"What you think you thought you saw you did not see." Agent J, MiB - Manhatten Bureau

Thank you, guys! Do you think we should use this thread here? If anyone is interested at all...

The definition for a moon can be very similar to a planet:

Planet:
1. orbits a star
2. large enough to be round
3. orbitally dominant

Moon
1. orbits a planet (instead of a star)
2. large enough to be round
3. orbitally dominant (around the planet)

Just like for planets, rule #3 would help distinguish between dominant objects and belt objects.

I have no idea what the orbital data of moons around planets is like, but I expect that it's a very similar dynamic. The "cleared the orbit" phrasing of the IAU would seem especially apt for the tiny shepherd moons of Saturn's rings. In those cases, we can see the "clearing" action is still in progress! Of course, those moons would fail rule #2, so they might be better named "moonlets" or something similarly descriptive.

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

Is it possible for a moon to have a natural satellite? What would it be called?

Well, if the moon orbits far enough from its planet and is large enough, the answer is probably yes. However, in our Solar system that is not possible. The so-called Hill spheres¹ for the largest moons are only a few moon radii. Any object orbiting that close will be unstable, not to mention additional disturbances caused for example by other moons in the system.

It is not far-fetched to suggest that some of the distant irregular satellites orbiting giant planets may actually be binaries. The irregular satellites form families which suggests that they are fragments from larger objects.

¹) Hill sphere = the region where an object dominates its parent body. For example, our Moon is located in Earth's Hill sphere. It is only an approximation; there are no known moons that are near the border of a Hill sphere because their orbits become unstable well before that.

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Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.
-- Richard Feynman

Interesting. By that definition Mars wouldn't have moons at all, because Phobos and Deimos are way too small. And the four giants would have only a few moons instead of an armada.

I had a rather radical idea: Why not say that a moon can be a planet? Right now we say that something like Luna or Charon is either a moon or a planet. But we don't even have an official definition of the term "moon". What if we said: The term "planet" is referring to the physical nature of the object itself, and the term "moon" is referring to its role within a system of interacting bodies? This would have the following consequences, for example:

1) Luna is a terrestrial planet, because of its physical composition and mass. And it is a moon, because it has the terrestrial planet Earth as a planetary senior-partner.

2) Io, Europa, Ganymede, Callisto are terrestrial planets. And they are moons of the jovian planet Jupiter.

3) Triton is a dwarf planet. And it is a moon of the neptunian planet Neptune.

4) Charon is a dwarf planet. And it is a moon of the dwarf planet Pluto.

Okay, now I have an unrelated question: The gravitational pull of the sun regarding the orbit of Luna is bigger than the gravitational pull of the Earth, which is why Luna's orbit is always concave towards the sun. But why are the tidal forces caused by Luna regarding the oceans on Earth bigger than the tidal forces caused by the sun?

Because tidal forces decrease with the fourth power of distance while gravity decreases with the 2nd power.

I'd be perfectly fine if we use the term planet to define any object that has pulled itself into hydrostatic equilibrium shape and is not, or never was, a star.

It can be divided based on orbit...

Primary planet: orbits star
Secondary planet: orbits planet
Rogue planet: drifts freely in space

And also divided based on mass/size...

Giant planet: massive enough to be gaseous
Terrestrial planet: solid, massive enough for gases in its hill sphere to form atmosphere, not massive enough to be a giant planet
Dwarf planet: solid and round, but not massive enough to be a terrestrial planet

So Earth is primary terrestrial planet, Enceladus is a secondary dwarf planet, etc. But like you said, we can still use "satellite" when referring to all objects orbiting a common body.

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This space is for rent.

Interesting, and what then of the numerous co-orbital moons around Saturn and Jupiter?

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I'm not completely heartless, the doctor who removed it told me he'd never be able to get it all.

Those are indeed interesting. If I'm not mistaken, they are also very similar in mass, so there's not really an argument to be made about gravitational dominance.

thinking.....

They are in a reasonance, but neither is submissive. I'm not sure what to call it. Co-dominance? (lol)

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

The interesting day will come when we find co-orbital planets. Man, I hope I live to see it.

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I'm not completely heartless, the doctor who removed it told me he'd never be able to get it all.

I look forward to a binary planet myself.

I wonder what the Roche limit on gas giants like Neptune or Jupiter would be? For binary gas giants, it would have to be a pretty open system to be stable for any length of time.

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I'm not completely heartless, the doctor who removed it told me he'd never be able to get it all.

I think it will be anti-climactic. Based on our ability to detect larger bodies first, they will likely be super-Jovian planets (almost stars) orbiting each other at several AU, with no intermediate star. Basically, a failed two-star system.

Everyone needs to remember that classification schemes are devised by men to help men simplify and understand a complex universe. We will always find exceptions and eventually be forced to reclassify.

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

If that's the case, we might already have cross that threshold (which makes you correct, it was rather anti-climatic, if you pardon the fact that it was discovered smack in the middle of the debate over the definition of a planet )

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I'm not completely heartless, the doctor who removed it told me he'd never be able to get it all.

I wasn't trying to spoil your fun... honest!

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

Hehe, don't worry about me, new exoplanet discoveries are like a year round Christmas for me. Every time one of these new beauties throws planet formation theory for a loop, I get the giggles.

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I'm not completely heartless, the doctor who removed it told me he'd never be able to get it all.