Looks like we discovered two more moons around Jupiter

[insert_name]

Two small retrograde moons were discovered in a recent survey

http://www.skyandtelescope.com/astronomy-news/two-new-satellites-for-jupiter/

[YNM]

Yet another captured objects, or far flung moonlets ?

[Scotius]

2 hours ago, YNM said:

Yet another captured objects, or far flung moonlets ?

Article itself suggests that retrograde, elongated and highly inclined orbits indicate that most of Jupiter's micro-moons are captured bodies.

[Streetwind]

Jupiter is just too darn greedy.

"Eenie, meenie, miney, moo, I will capture you... and you!"

[NSEP]

I think we should call them dwarf moons, there are just too much of them! 

[K^2]

Definition of a moon is too broad. I would require the same of a moon as any dwarf planet. Namely, enough mass to make a tidy sphere. Everything else can be classified as a moonlet. That would, of course, mean that Mars has zero moons, and Saturn and Jupiter have way fewer.

Additionally, I stand with Issac Asimov and a handful of astronomers in that a moon is an object that orbits its planet first and foremost. As such, Moon isn't a moon at all, since it primarily orbits the Sun, merely sharing orbit with Earth, and is therefore a part of a double planet Earth-Moon system. To avoid confusion, I would rename Moon back to its Latin name Luna, and therefore, all five inner planets, Mercury, Venus, Earth, Luna, and Mars would have no moons under this classification. Nice and tidy.

I do not appear to be in the majority with this point of view, but I'll keep pushing for it.

[Streetwind]

35 minutes ago, K^2 said:

Additionally, I stand with Issac Asimov and a handful of astronomers in that a moon is an object that orbits its planet first and foremost. As such, Moon isn't a moon at all, since it primarily orbits the Sun, merely sharing orbit with Earth, and is therefore a part of a double planet Earth-Moon system.

This part confuses me. How would a satellite that "orbits the Earth first and foremost" have to look, in contrast to how the Moon looks, in order to fulfill that criterion? In my mind, any body that orbits the Earth automatically also orbits the Sun, because the Earth itself orbits the Sun.

[Green Baron]

43 minutes ago, K^2 said:

.... Moon isn't a moon at all, since it primarily orbits the Sun, merely sharing orbit with Earth, and is therefore a part of a double planet Earth-Moon system. .....

Pls. elaborate a bit on that. Why isn't the moon (Luna/Mond ...) a satellite of earth (Terra/Erde ...) ? I have absolutely no problem with that, i'd just be happy if i get a defintion of moon or satellite. Common barycentre apparently is no criteria fo you if i follow your thoughts.

The iau doesn't have a definiton of "moon" because it makes no sense. In principle you can go down from planet, moon, natural satellite of moon, satellite of satellite and so on. If a body orbits another they have a common barycentre. A body can co-orbit with another body around the same central body, you can name it a quasi-satellite if you like, but they have no common barycentre then, instead they have to be in resonance in order for that constellation to happen.

Earth/Moon have a common barycentre and orbit each other. Together they orbit the sun.

Edit: partly ninja'd by @Streetwind

Edited June 7, 2017 by Green Baron
body baby bablefish

[Urses]

3 hours ago, Streetwind said:

Jupiter is just too darn greedy.

"Eenie, meenie, miney, moo, I will capture you... and you!"

Let us hope he will stay on his Grooge manner forever... i am for sure not after get a "moonlet" on collision course with Earth....

Lovely Jupiter be as gready as you wish we love you for it

Funny Kabooms 

Urses 

35 minutes ago, Green Baron said:

/snip

AFIU this means Earth/Moon are not a Planet with a satelite but one double planet. Because they have a barycenter for Orbit they spin both around. Jupiter moons on other hand are "true" satelites because they are to smal to noticable change Jupiters orbit?

Urses

[Green Baron]

Hey, don't cite me after i liked your post ;-) (i know, the forum software merged this without being asked)

Yeah, we are looking for a definition of "moon" and "orbit". Still even Jupiter and a dust particle have a common barycenter if they orbit each other. Size doesn't matter. Or does it ? How much less mass than the bigger one must the smaller one of the bodies have in order to be called a moon of the bigger one ?

Furthermore, what defines an orbit ? It was posted above that common barycenter is no criteria. But is that so ? If the moon and earth would co-orbit the sun as independent bodies (without catching each other) other then they must be in 1:1 resonance (like 2016 HO3). But we know the moon orbits earth several times a year.

Though it gets higher and higher due to impulse preservation the moon will not leave the earth's orbit during the solar system's life time. If it had more time ....

Edit: to further complicate things i leave this link here: https://en.wikipedia.org/wiki/(469219)_2016_HO3

and kindly bid mercy for linking to wikipedia.

Edited June 7, 2017 by Green Baron

[sevenperforce]

"Common barycentre inside the larger body" seems like a good basis for what discriminates a co-orbit from a primary/secondary arrangement, until you remember that the Jupiter-Sun barycentre is (almost always) outside the Sun. Also consider that if the Sun were a white dwarf or neutron star, even Earth and Saturn and Mars and Venus would orbit a barycentre outside the primary.

I, for one, wholly endorse the idea of dividing "moons" from "moonlets", both under the umbrella of "natural satellite". If it's not gravitationally rounded, it shouldn't be considered a moon.

 

[K^2]

2 hours ago, Streetwind said:

This part confuses me. How would a satellite that "orbits the Earth first and foremost" have to look, in contrast to how the Moon looks, in order to fulfill that criterion? In my mind, any body that orbits the Earth automatically also orbits the Sun, because the Earth itself orbits the Sun.

 

2 hours ago, Green Baron said:

Pls. elaborate a bit on that. Why isn't the moon (Luna/Mond ...) a satellite of earth (Terra/Erde ...) ? I have absolutely no problem with that, i'd just be happy if i get a defintion of moon or satellite. Common barycentre apparently is no criteria fo you if i follow your thoughts.

It effectively comes down to whether star or planet pulls harder on the satellite in question. If satellite orbits primarily the planet, the pull of the planet is stronger. That would imply that at least for a portion of the orbit, the satellite is pulled towards the planet, away from the star. As such, if we look at the full orbit of the satellite around the star, it will have a few parts in its trajectory that curve away from the star. This is the case for every moon of every planet in the Solar system save for one.

Our own Moon does not behave this way. Its orbit around the Sun is almost a perfect ellipsoid, but most importantly, it is entirely convex. At every single point along this trajectory, the Moon is curving towards the Sun, even as the Moon passes between Earth and Sun. This is because gravitational pull of the Sun is stronger than that of the Earth. Moon orbits the Sun first and foremost, and gravitational perturbation from Earth are causing it to go to slightly higher or lower orbit, leading or lagging Earth, resulting in relative motion of Moon around Earth.

This might seem like a very subtle distinction, and for Earth-Moon system it is, but for an extreme case of this interaction, look at Epimetheus and Janus as they orbit Saturn. As they pass close to each other, they exchange orbits, one going slightly higher, the other slightly lower, and they separate again until the faster of the two catches up with the slower, and they swap again. You would never call that orbiting each other. But these two objects are co-orbiting Saturn together in a lot of ways the way Earth and Moon co-orbit the Sun.

Such classes of behavior prompted a suggestion that distinction between satellite and co-orbiting bodies shouldn't depend on location of barycenter, but rather on how strong the pull from the star is in comparison to mutual attraction.

As a follow up example, consider Charon as it orbits Pluto. There is currently a suggestion that it should be classified as a dwarf planet due to the fact that the barycenter is located between Pluto and Charon. However, the mutual attraction between these two bodies is far greater than pull of the Sun on either of them. And with Charon being only 12% of Pluto's mass, it is clear that Charon does in fact orbit Pluto making it a moon.

Finally, while no bodies I know of within this Solar system fall under this, there is a very serious potential problem with using barycenter definition as we start looking outside of the Solar system. Consider two planet-sized bodies orbiting each other in highly elliptic orbit. As the distance between them increases, barycenter leaves the interior of the heavier body. So according to barycenter definition, the orbiting body is only sometimes a moon? This isn't a problem when we look at trajectory around the star. If both bodies have convex trajectories around the star, it's a double planet. If at least one of them has trajectory that is not strictly convex, it's a planet-moon system. This is more general, closer to what we understand as concept of orbiting each other, and gives us cleaner classification.

The only reason we haven't switched to this definition yet is because under it, Moon isn't a moon, and that seems to scare people more than the lack of a clean definition of what actually constitutes a moon. I say screw that. Let there be five inner planets.

[Streetwind]

@K^2 Okay, that is clearly something I don't understand well enough to agree or disagree with until I look it up in greater detail. But just from gut feeling, two spontaneous questions: one, wouldn't the shape and other parameters of the satellite's orbit influence this? In other words, if our Moon had a highly elliptical orbit instead of a fairly circular one, might that not cause it to more distinctly fall away from the Sun and towards the Earth at least at one or two points per year, even though nothing about the Moon itself changed? And two, does that not implicitly make the definition of what's a satellite dependant on the distance to the next major gravity well? In other words, if Mars and Earth would swap places in the solar system, Earth's gravity would still be just as strong, and but the Sun's would be weaker, so even if the Moon had the exact same orbit around Earth, it would suddenly curve outwards here and there from the point of view of a solar orbit?

...In fact, isn't this all a matter of using different reference frames in the first place?

It might just be me, but a definition of a satellite that is dependant on the exact orbital parameters, which additionally change with the distance to the sun, and/or which reference frame you decide to look from, sounds rather impractical and flawed. If I'm wrong, please don't hesitate to correct me, though!

Edited June 7, 2017 by Streetwind

[Green Baron]

Thanks, @K^2. But this is the first time i read that the suns pulls more at the moon than earth does. All my information until now was that the moon revolves completely inside earth's hill sphere where the earth is the dominating force, not the sun. So, i have my problems accepting the moon as a satellite of the sun.

The moon follows Kepler's laws (afaik) in respect to its (imo) earth bound orbit, it's orbital plane around the earth is slightly tilted in respect to the earth's ecliptical plane and the tilt is stable (afaik). Were it in an orbit around sun but co-orbiting earth, it would perform a cycle once a year, not an odd number one. The moon stabilizes earths axis and it is forced in a bound rotation in respect to earth, not to the sun.

I would be more willing to accept the idea if you could provide me with let's say the moons orbital parameters in respect to a sun orbit ...

Edit: of course the moon is influenced by the sun as well, but afaik not as the dominating force.

Edited June 7, 2017 by Green Baron

[K^2]

@Streetwind Yes to both. That's kind of the point. Defining what is a planet and what is a moon not by their physical characteristics, but by their orbits. After all, largest of the moons of Jupiter and Saturn would make pretty believable planets were they orbiting around the Sun directly. Titan and Ganymede are both larger than Mercury, albeit, a bit lighter.

Quote

In fact, isn't this all a matter of using different reference frames in the first place?

No, because acceleration is absolute and does not depend on reference frame.

@Green Baron The math is trivial. You don't even have to do reduced mass. Computation 1 Computation 2 As for orbital parameters, they are essentially identical to Earth's. If you plot Earth and Moon's orbits around the Sun, they are indistinguishable.

Edited June 7, 2017 by K^2

[Green Baron]

*scratchbaldhead*

rejected :-) The moons orbit around the sun is a spiral movement around the common centre with the earth. Like the Ptolemaian outer planets, but several times a (edit: earth)year. I need something better ...

 

Edit: Heathen ! You want to put the Sun in the centre ? (of the 3-body-system earth-sun-moon)

:-)))

Edit: scrub it, you are right, the moons orbit around the sun is totally convex ! Nevertheless the earth is the dominating body ;-)

Edited June 7, 2017 by Green Baron

[K^2]

It's not spiral, though. That's the problem. It's not even wobbly enough to have a bend away from the Sun. It's a very slightly perturbed ellipse, same as Earth's. Other moons curve away from the Sun and some even go retrograde relative to Sun for part of the orbit. Our moon is the only one that does none of that.

[Green Baron]

True, that was a false assumption from my side. That's a geometrical thing resulting from the orbital speeds of the bodies projected on a two-dimensional plane.

Nevertheless, the moon is completely within the earths hill sphere, no way it leaves in foreseeable time and orbits the sun independently. The sun could pull it out if the system earth moon fell below the roche limit, as far as one can speak of such a thing for a two body system whose common center revolves around a central body (sun in this case). Sure, both earth and moon orbit the sun, but while doing so the moon is bound to earths gravity. It will never not so soon cross L1, like an astronaut in orbit around the moon will not fall towards earth even if earths pull is stronger than the moon's (didn't do the math).

Not ready to accept the moon as an (independent) satellite of the sun :-)

[CSE]

1 hour ago, Streetwind said:

It might just be me, but a definition of a satellite that is dependant on the exact orbital parameters, which additionally change with the distance to the sun, and/or which reference frame you decide to look from, sounds rather impractical and flawed.

True, but exactly the same can be said for the (current) definition of 'planet': the semi-major axis of the orbit has a significant effect on whether the neighbourhood will have been cleared - in some discriminants, it's a bigger effect than the planet's mass. Compound this with the very varied lifetimes of stars and all sorts of counter-intuitive results will emerge. A pluto-analogue orbiting a red dwarf might have enough time to eventually be promoted to planet or demoted to moon/moonlets/impactor Any categorization that relies on orbital dynamics is going to have some problems of this kind; even the definition of 'satellite' could be suspect for bodies in multiple resonances.

Personally, I prefer a body-only definition of planet as a round non-fusor. I think we have enough of a sample size to say some useful things about the category.

By contrast, for moons and moonlets I think we're still in the very early stages of understanding them. We have some accreted-in-place examples like mini-solar systems (probably?); we have two cases that don't necessarily fit this idea but we're short on the specifics of their histories (Luna, Triton); and we have the idea that the accretion process from dust-rock-planet leaves a fair amount of bodies stuck at 'rock', like these latest two. As nice as their individual discovery is, I'm not expecting a revelation; rather, they add a small incremental piece to the very large puzzle of the small solar system bodies population. Perhaps the best news is that the chance of a flyby target of opportunity for the next mission to or past Jupiter has just got two moonlets better.

[Urses]

All said above, would it not be a Definition for a true satelite?

A object that is able to change orbital Vector from prograde to retrograde to the main gravitational force (Sun) are satelites of the sub gravitational force (planet) otherwise those objekts are parts of a multiplanetal combines with the planet they are on same orbit....?

Urses

Edited June 7, 2017 by Urses

[Green Baron]

I don't think that categorization should not depend on orbital parameters. The orbital state of any body is not "hammered in stone", after and before all these systems are dynamic. So a moon can become a (dwarf-)planet (ours could if it had enough time), a dwarf planet can be captured and thus become a moon. Even a planet can loose its state due to dynamics.

Any categorization is just a helper, so that the other one knows what i mean when i say "planet" or "dwarf planet". These two now have a definition, we can accept it for convenience or not. Good thing, everyone now knows what an astronomer means if he says "dwarf planet" (until re-definition).

With moons this is less clear as "moon" isn't defined that hard (obviously they were clever enough to avoid the discussion :-)), same goes with orbit or pseudo orbit. We can accept the hill sphere and a barycenter as the elements or the projected orbital path, the exerted force, whatever. It's only a convention, and after all anything describes a current state in a dynamic system.

I think that one day soon someone will come up with a definition of moon or natural satellite, as we describe more and more of these systems. Not to degrade or promote anything, just for the convenience of being able ffor a description with as little misunderstandings as possible.

[Kerbart]

5 hours ago, Green Baron said:

Earth/Moon have a common barycentre and orbit each other. Together they orbit the sun.

Edit: partly ninja'd by @Streetwind

Even more accurate, the barycentre is inside Earth's surface. If it were outside (like Pluto/Charon) one can argue that it's a binary planet system, but it's not.

[kerbiloid]

Spoiler

Are we going to find that Solar System outer rim is populated by retrograde trash?

[YNM]

12 minutes ago, kerbiloid said:

  Hide contents

Are we going to find that Solar System outer rim is populated by retrograde trash?

The case for Jupiter and the Sun is completely different, I think. Though, well, hello interplanetary rouge bodies...

[Green Baron]

16 minutes ago, kerbiloid said:

Are we going to find that Solar System outer rim is populated by retrograde trash?

Is that real or hypothetical ? If real, source pls. Because afaik only a few asteroids / kbos go withershins.

May be caught from outside or exchanged with neighbours (hypothetical) or forced by encounters with inner planets. These objects don't need much to change their path.

Edit.: oh, the picture does not show the solar system, just Jupiter and its kin. So, option 3, forced by big daddy :-)

Edited June 7, 2017 by Green Baron