Are Rask and Rusk planets

[Omni122]

On 1/29/2022 at 1:48 AM, intelliCom said:

A ratio? What's the ratio?
Who gets to decide what the ratio is?
Is it specific, is it a range?
If it's specific, what happens if we find a planet that perfectly sits on that exact ratio?
Why does the presense of debris decide a planet's status? If we found a planet the size of Earth, but it's 'neighbourhood' resulted in a dwarf planet's ratio, is it a dwarf planet- not because it's undersized- but because it's "not big enough"?

While no specific ratio is defined, the ratio between the 8 planets and the objects that share their orbits is orders of magnitude larger than all other objects that orbit the sun.

We wouldn't find a planet the size of Earth that's neighborhood had not been cleared unless the debris had been generated very recently. If an object is not a dwarf planet, it will clear it's neighborhood in time.

On 1/30/2022 at 3:08 AM, K^2 said:

In what possible world is the Moon not orbiting the Sun? It's trajectory around the Sun is almost a perfect circle. 

If something's path around another object is not perfectly elliptical, circular, parabolic, or hyperbolic, it doesn't orbit the object. Full stop, end of story.

[K^2]

2 hours ago, Omni122 said:

If something's path around another object is not perfectly elliptical, circular, parabolic, or hyperbolic, it doesn't orbit the object. Full stop, end of story.

So Earth doesn't orbit the Sun? Seeing how it's center is deflected from orbit by nearly 5,000km by the Moon's gravity. And neither do any of the other planets. Not a single one of them is in perfectly elliptical orbit, after all. Some perturbations always exist, even for Venus who does not have a moon. And then there was even that big shebang about Mercury's orbit precessing, which was eventually shown to be in part due to General Relativity preventing nice elliptical orbits in the first place.

Seems like a crap definition, if you ask me, if it doesn't apply to a single body in all of Solar System. Nothing's orbiting anything else. That doesn't seem great.

[SSTO Crasher]

The moon orbits the earth, the gravity from the earth is strong enough to keep it in orbit. The earth is the dominant object in its orbital path

[Omni122]

14 hours ago, K^2 said:

So Earth doesn't orbit the Sun? Seeing how it's center is deflected from orbit by nearly 5,000km by the Moon's gravity. And neither do any of the other planets. Not a single one of them is in perfectly elliptical orbit, after all. Some perturbations always exist, even for Venus who does not have a moon. And then there was even that big shebang about Mercury's orbit precessing, which was eventually shown to be in part due to General Relativity preventing nice elliptical orbits in the first place.

Seems like a crap definition, if you ask me, if it doesn't apply to a single body in all of Solar System. Nothing's orbiting anything else. That doesn't seem great.

The center of mass in the Earth-Moon system orbits the Sun. The Earth is by an extremely wide margin the dominant body. I guess both orbit a barycenter which goes around the Sun, but then that means every single satellite in the solar system is a planet by your definition.

[K^2]

Just now, Omni122 said:

The center of mass in the Earth-Moon system orbits the Sun. The Earth is by an extremely wide margin the dominant body. I guess both orbit a barycenter which goes around the Sun, but then that means every single satellite in the solar system is a planet by your definition.

No. By the definition I am proposing here, the body has to pass other tests for planethood, including hydrostatic equilibrium and clearing its neighborhood, and  always accelerate towards the star in order to be considered a planet. Moon is the only body in the Solar system that is not already a planet that passes this test. All other moons of all other bodies accelerate away from the Sun during part of their cycle. And this is specifically because their parent planet is the dominant body in their neighborhood. Moon's trajectory is dominated by the Sun's gravity, and it is the only such object.

[Admiral Fluffy]

It’s a planet if most people think it’s a planet.

[SSTO Crasher]

But it clearly orbits the earth, which orbits the sun, therefor making it a moon

[Jacke]

2 hours ago, Omni122 said:

The center of mass in the Earth-Moon system orbits the Sun. The Earth is by an extremely wide margin the dominant body. I guess both orbit a barycenter which goes around the Sun, but then that means every single satellite in the solar system is a planet by your definition.

Not quite, as a centre of mass is a calculated position useful in analysing, not something that actually exists.

All bodies move to follow the least-action path in the space-time manifold which is the sum of all the masses' effects on space-time as determined by General Relativity.

In KSP, this is simulated using all system bodies on rails with their own moving SoIs to determine the effect on spacecraft.

And @K^2 is quite right, the Moon is the only natural satellite of a planet who's orbit is always convex towards the Sun.

[SSTO Crasher]

But the moon is still a natural satellite of the earth, and therefore a moon

[K^2]

1 hour ago, SSTO Crasher said:

But the moon is still a natural satellite of the earth, and therefore a moon

Why isn't the Earth a natural satellite of the Moon?

[t_v]

1 hour ago, K^2 said:

Why isn't the Earth a natural satellite of the Moon?

That one has sort of been answered already, the barycenter of the earth and the moon is within the earth. I think there are some interesting points on both sides, but it is true that the current definition needs serious revision. Especially with the earth-moon case, there needs to be a definition that doesn’t take into account the acceleration towards a star since that would make the same system be classified differently when orbiting from farther away, where the acceleration towards the star is less. Maybe have a moon be a satellite of a planet, and have a planet be something that cannot be defined as a satellite of something other than a star? And then apply the other definitions to rule out space dust?

[K^2]

2 minutes ago, t_v said:

That one has sort of been answered already, the barycenter of the earth and the moon is within the earth.

That is the definition of the moon that we currently use, yes. However, if you are saying that this is what defines one object orbiting the other and not vice versa, then planets of the Solar systems don't always orbit the Sun. Or that the Sun sometimes orbits the planets, perhaps? Because barycenter of the Solar system is only sometimes contained within the Sun. To be more on point, the barycenter of the Solar System is not currently within the Sun, so the planets aren't currently orbiting it either? Or the Sun is currently orbiting the planets? I'm not sure which one you prefer to imply, but the point is, the Solar System doesn't currently pass the barycenter test. 

Now, if you do want to claim that sometimes the Sun orbits the planets, that's fine. But I think, most people would agree that the planets always orbit the Sun and not the other way around. So clearly, there is more to one body orbiting the other than just where the barycenter is. If only a definition existed by which every planet would be gravitationally bound to the Sun and most of the moons, at least, bound to their planets. Hm.... Maybe if we went with which body has the strongest gravitational influence? Perhaps? Maybe? 

To be clear, a star is certainly a different category than a planet. There is nothing fundamentally wrong with having very different definitions for planets and for moons. But if your answer to the question of why the Moon is a moon is that the Moon orbits the Earth and not the other way around, and your answer to why the Earth isn't orbiting the Moon is that the barycenter is within Earth, then, the Sun is orbiting the planets right now, and we're kind of in a bad place.

Either you have to have a better argument for why the Moon is a moon than which body orbits the other OR you have to have a different definition of one body orbiting the other than the barycenter. You just can't have it both ways if you want to keep logical consistency.

And if you really want to have a barycenter definition for moons, that's technically fine. But then it has nothing to do with which body orbits the other. It's a completely arbitrary definition which we shoehorned to make sure that we keep calling the Moon a moon and no other reason. It's a bad way to define things, but it's honest and self-consistent, at least.

[Superfluous J]

I don't like the idea of judging this based on radius of either world. If the Sun were a black hole, nothing would be orbiting it based on barycenters and radii.

I don't see why there just can't be a ratio upon which one planet is orbiting the other if their masses vary by that number. I think the number should be low. Like, 2. Or even less maybe.

Is the Earth more than 2 times the mass of the Moon? Yes? The Moon orbits the Earth. Earth is the planet and the Moon is the moon (this is not a tautology!)
Is Rask more than 2 times the mass of Rusk (or vice versa)? No? They are a dual planet system and neither is a planet by definition.

Edited February 16, 2022 by Superfluous J

[Chilkoot]

4 hours ago, K^2 said:

Why isn't the Earth a natural satellite of the Moon?

Because the Earth is a diva and won't let anyone else take centre stage.

"Just remember, it's my gravity keeping you here, and all y'all orbit me, got it?  Don't make me go all Thea on your asses."  -Earth, probably.

[K^2]

7 hours ago, Superfluous J said:

I don't see why there just can't be a ratio upon which one planet is orbiting the other if their masses vary by that number. I think the number should be low. Like, 2. Or even less maybe.

By this definition, aren't the Earth and the Moon both moons of Jupiter? Yes, I'm being a bit pedantic, but my point is that the distances have to factor in somehow. And some examples aren't nearly as clear cut.

Consider these two moons of Saturn: Janus and Epimetheus. They play an unusual game of tag. The one in slightly lower orbit catches up to the one in the higher orbit and they start falling onto each other. As they pick up speed, the one in the lower orbit moves higher, and the one in higher orbit ends up lower, and they start drifting apart again until they go all the way around Saturn relative to each other and repeat the process all over.

It so happens that Janus is a little over 3 times as massive as Epimetheus. So by the mass ratio definition, should we consider Epimetheus to be a moon of Janus? They do get pretty close, closer than they ever get to Saturn. And their gravitational interaction clearly matters to their orbits. But if we say that Epimetheus is a moon of Janus, then Epimetheus completes an "orbit" "around" Janus by going all the way around Saturn and then around Saturn again in the opposite direction.

So we really, really have to consider other factor besides mass ratio. You have to establish the hierarchy before you start applying the ratio test, and once you've established what orbits what, it's a moot point.

If in order to say that Earth doesn't orbit Jupiter, we first must establish that both are clearly orbiting the Sun, because the Sun is the gravitationally dominant body among the three, and therefore, the Earth can't be orbiting Jupiter. But then I simply go and apply the same logic to the Moon, and end up with the same answer.

[t_v]

8 hours ago, K^2 said:

the barycenter of the Solar System is not currently within the Sun, so the planets aren't currently orbiting it either?

Quick question, and thank you for this new tidbit of information. Are you looking at all the planets here? Is there any planet (probably Jupiter) where the barycenter of only that planet and the sun is outside of the sun? When talking about the barycenter definition, I would like to only look at two bodies, not the sum of many bodies. I would be fine with calling the solar system a binary sun-(planet?) system with the sun and Jupiter. 

8 hours ago, Superfluous J said:

I don't like the idea of judging this based on radius of either world. If the Sun were a black hole, nothing would be orbiting it based on barycenters and radii.

This is a good counterpoint which invalidates this definition. 

[K^2]

4 hours ago, t_v said:

Is there any planet (probably Jupiter) where the barycenter of only that planet and the sun is outside of the sun?

This is a very simple calculation.

[Catto]

Possibly protoplanets.

[t_v]

Got it. My one objection to the gravitational attraction definition is that it depends on distance from the star, and ideally the same two bodies would be classified the same way if their orbit around each other looked the same, no matter what their orbit around the host body is. 

[Akagi]

Cool math

[K^2]

3 hours ago, t_v said:

Got it. My one objection to the gravitational attraction definition is that it depends on distance from the star, and ideally the same two bodies would be classified the same way if their orbit around each other looked the same, no matter what their orbit around the host body is. 

I think there are going to be edge cases that are bad with any definition. N-body dynamics is just, well, chaotic. I obviously have a preferred definition, but ultimately, it is semantics. What I hope people will get out of such discussion is why it gets complicated. It's just too easy to get into the rut of thinking that things are obviously certain way without examining it deeper.

[Superfluous J]

13 hours ago, K^2 said:

By this definition, aren't the Earth and the Moon both moons of Jupiter?

Sorry I thought it was obvious in my explanation that the two things were actually nearby each other. Let me say it explicitly. Oh wait I just did.

To be clear, I'm talking about distance from each other not size of the planet. The fact that Earth and Jupiter both orbit the Sun is why Earth is not Jupiter's moon, not that the center of gravity of the two is inside or outside either planet's surface.

13 hours ago, K^2 said:

It so happens that Janus is a little over 3 times as massive as Epimetheus. So by the mass ratio definition, should we consider Epimetheus to be a moon of Janus? They do get pretty close, closer than they ever get to Saturn. And their gravitational interaction clearly matters to their orbits. But if we say that Epimetheus is a moon of Janus, then Epimetheus completes an "orbit" "around" Janus by going all the way around Saturn and then around Saturn again in the opposite direction.

They are both moons of Saturn.

Their interaction is interesting and in our solar system unique, enough that it deserves note. But there is no need to call one a moon of the other. If it turns out this is common enough to need discussed I think a name should be thought up at that time for this sort of pair. No need to try to shoehorn them into categories they don't belong in.

13 hours ago, K^2 said:

If in order to say that Earth doesn't orbit Jupiter, we first must establish that both are clearly orbiting the Sun, because the Sun is the gravitationally dominant body among the three, and therefore, the Earth can't be orbiting Jupiter. But then I simply go and apply the same logic to the Moon, and end up with the same answer.

The Earth-Moon system orbits the Sun. The Jupiter-and-all-its-satellites system orbits the Sun. Neither orbit each other. In these two systems, Earth and Jupiter are the planets, and the Moon and all of Jupiter's satellites are the moons.

If the Moon was over 0.5 Earth masses but less than 2 Earth masses, I think the Earth-Moon system should be considered a different thing entirely (assuming my pulled-from-nowhere number "2" above is a good number) and NEITHER is a "Planet" but both are in a pair of dual worlds that would get a new name. Rausks, or whatever.

Edited February 17, 2022 by Superfluous J

[K^2]

5 minutes ago, Superfluous J said:

The fact that Earth and Jupiter both orbit the Sun is why Earth is not Jupiter's moon, not that the center of gravity of the two is inside or outside either planet's surface.

5 minutes ago, Superfluous J said:

They are both moons of Saturn.

We are trying to define what it means to be a planet orbiting the Sun vs a moon orbiting a planet. You can't use the fact that a moon is a moon and planet is a planet to argue for the thing you are trying to define. It's called circular reasoning and is a fallacy.

You say Earth and Jupiter both orbit the Sun, therefore, Earth is not a moon of Jupiter. Earth and Moon both orbit the Sun, therefore, Moon is not a moon of Earth. See the problem?

If you want to prove to me that the Moon orbits the Earth and not the Sun, you'll have to do so without relying on which object orbits which in your definition of orbiting.

So again, from the top, give me a clear definition that tells me that object A orbits object B and not object C whenever object B orbits object C. You can use any sizes, distances, and complicated formulas if you wish. But I'm going to tell you right now, that I will find numerous exceptions in the Solar system that you'll find contradictory.

[SingABrightSong]

Pluto and Charon are considered dwarf planets because they have not "cleared their neighbourhood".  I understand there is a distinction. I am uncertain why this distinction exists₁. My own amateur set of criteria would be that a "Planet" is a body that a) Formed from a star's protoplanetary disc and b) has sufficient gravity to hold a spheroidal shape, while a "Moon" is a body that formed from a *circumplanetary* disc and has sufficient gravity to form an *ellipsoid* rather than necessarily a spheroid.  Under these criteria, Pluton and Charon would be planets, along with Ceres, Eris, Makemake, and Luna, but would fail to categorize Haumea so nevermind.




_{1. bEcAuSe iT's cOmPaNy pOlIcY}

[JoeSchmuckatelli]

On 2/16/2022 at 12:27 PM, K^2 said:

This is a very simple calculation.

Neanderthal question time: if the barycenter of Jupiter and Sun is outside the sun (hope I'm reading that equation correctly), the sun is making tiny circles in time with Jupiter in its orbit of SagA... correct?

So - what's happening to the orbit of the other planets when Jupiter is at right angles from the planet's eliptical length?

That movement has to have some effect over time, doesn't it?

Edited February 22, 2022 by JoeSchmuckatelli