Order of Kilopi
Since there is an interesting thread about Pluto's planetary status I thought I'd throw this one out for fun. Isaac Asimov once wrote an essay called "Just Mooning Around" in which he demonstrated that the gravitational pull of the Sun on the Moon is twice the pull the Earth exerts on the Moon. He poses the question - Since the Sun is the primary gravitational attractor for the Moon, is the Earth-Moon system really a double planet?
What do you think?
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Depends on what you mean by planet. If you use this definition, the answer is no, the moon is not a planet.
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I saw an Eddie Izzard stand-up sketch where he refers to the moon as a "boring planet". I almost submitted it to the "Bad Movies/Bad Science".
In any case, I think the definition of the moon is not whether it gets its primary gravitational attraction from a planet, but rather whether it is orbitting more than one body. For example, Dactyl would be considered a moon of Ida.
Order of Kilopi
Kilopi,
I'll agree with your assessment. Item 4 on that definition: "4) Center of gravity of two-body system is not inside another planet" clearly eliminates the Moon from consideration.
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Short or long answer (you decide): No.is the Earth-Moon system really a double planet?
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I think I should make a small disclaimer.
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What qualifies as double planet will depend somewhat on how clearly we can define both “what is a planet” and “what is a moon”. If we constrain the definition of double planet too tightly, it then becomes a category in search of a candidate.
One reason we might consider calling Terra-Luna a double planet is the contrast and comparison of Luna with the other moons of the solar system. In terms of it’s mass and radius in relation to it’s primary, Luna stands apart. We can safely say “natural”, but not “regular”.
That is, if we define “regular” by the examples set by the moons of the gas giants (formed in circum-planetary accretion disks) then Luna is not “regular”. Because it’s not a captured object, we can’t call it “irregular” either. We can’t call it a co-orbital sibling because it did not form independently from Earth, but from the mantle of the Earth itself (with contributions from the unknown impacter).
Of course, we need greater imaging capabilities to know for sure, but Luna’s method of formation may be extremely rare. As a general rule, terrestrial planets probably don’t have regular moons or rings due to insufficient mass. I think we need more descriptive categories like “spallation satellite” for asteroid moons. Personally, I could live with Terra-Luna as a double planet. Now, it’s up to the recognized experts to make a determination or ruling.
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The moon has 1/81 the mass of the Earth. It was the original "moon"--it's about as regular of a moon as you can get.On 2003-02-27 10:56, Heathen wrote:
One reason we might consider calling Terra-Luna a double planet is the contrast and comparison of Luna with the other moons of the solar system. In terms of it’s mass and radius in relation to it’s primary, Luna stands apart.
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kilopi.
“The moon has 1/81 the mass of the Earth. It was the original "moon"--it's about as regular of a moon as you can get.”
If we use the generic definition of “regular”, then, I would agree. However, I was using a conditional definition.
From my previous post;
“That is, if we define “regular” by the examples set by the moons of the gas giants (formed in circum-planetary accretion disks) then Luna is not “regular”.”
So, let’s examine the ratios of the masses of secondary bodies to their primaries and see if a pattern emerges.
Luna = 1/81 mass of Terra.
Ganymede = 1/12,800 mass of Jupiter.
Titan = 1/4,225 mass of Saturn.
Oberon = 1/14,500 mass of Uranus.
In the interests of fairness I used the most massive moon of each planet for comparison. I’ve omitted Triton-Neptune from the comparison because Triton is retrograde, therefore irregular. I’m confident that you would agree that without the capture of Triton, Neptune would have almost certainly possessed 3 or 4 regular moons. I’ve also omitted Charon-Pluto because of their contentious status in at least two current threads.
Info used found here; http://users.skynet.be/hermandw/weralone/planets.html
There is a pattern. Luna is far more massive in relation to its primary than the rest of the moons of the solar system. I submit that this fact plus the fact that Luna formed by catastrophic impact rather than within a circum-planetary accretion disk qualify as justification for re-evaluating its status as a “regular” moon. Regarding the double-planet issue, I can go either way.
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Surely you're not suggesting that Pluto and Charon are double planets? [img]/phpBB/images/smiles/icon_smile.gif[/img]On 2003-02-28 22:20, Heathen wrote:
I’ve also omitted Charon-Pluto because of their contentious status in at least two current threads.
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kilopi.
“Surely you're not suggesting that Pluto and Charon are double planets?”
Ironic isn’t it? The one clear candidate for double planet is mired in designation limbo. “Double KBO” perhaps? Something else that I find curious is that I can’t seem to find computer simulations for the origin of the Pluto-Charon pair. Nor can I find any clear consensus on the Triton capture event(s). Maybe they’re only publishing in Icarus. Let’s see… about $900 a year? Guess I wont be subscribing anytime soon.
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Is the Moon a Planet?
BTW today in the scifi section will be discussed
What makes a planet a planet?
Where? at
http://www.nakednews.com/
sorry
got to have a player and be a member.
Established Member
That might be because we hardly know anything about them. We haven't even been to Pluto yet, the "last planet". -ColtOn 2003-03-01 10:32, Heathen wrote:
kilopi.
“Surely you're not suggesting that Pluto and Charon are double planets?”
Ironic isn’t it? The one clear candidate for double planet is mired in designation limbo. “Double KBO” perhaps? Something else that I find curious is that I can’t seem to find computer simulations for the origin of the Pluto-Charon pair. Nor can I find any clear consensus on the Triton capture event(s). Maybe they’re only publishing in Icarus. Let’s see… about $900 a year? Guess I wont be subscribing anytime soon.
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Chill out here. This can be pushed into the extreme of every object in the solar system falling into its own category and that would be very boring for the public.
What angle of orbital inclination divides regular from irregular? Don't even think of saying 90 degrees because then we must agree that an inclination of 89 degrees is more regular than "retrograde"?Triton is retrograde, therefore irregular.
Degrees of regularity? Just give the angle and be done with it. Also, as we may never know the physical origins of all moons, now basing their classification labels on their unkown origins sounds like an act of religious faithe.
Try to explain to the public we want money to send a spaceprobe to the Tweedle of Neptune. Just say Moon of Neptune, take your money, and run like hell. [img]/phpBB/images/smiles/icon_biggrin.gif[/img]
Hell, the public does not even now what a planetary moon is and its not because of the classification scheme.
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Colt.
“That might be because we hardly know anything about them. We haven't even been to Pluto yet, the "last planet". –Colt”
This is true. The point I wish to emphasize is not so much the paucity of data in hand but how far we can go with the data we do have. The following link suggests that the dynamic characteristics of a number of “plutinos” roughly resemble those of asteroid collision “families”. If this is the case, then we can at least narrow our search for possible causal mechanisms for the origin of Pluto.
http://www.lpi.usra.edu/meetings/LPSC99/pdf/1213.pdf
This next link carries it a step further by stating that the collision must have occurred prior to Neptune’s migration out to it’s current orbit, thereby “trapping” Pluto-Charon and the plutinos in a 2:3 resonance. They also conclude that we should find more Pluto mass objects within the Kuiper belt.
Warning; 26 pages. Allow a few minutes to load. It’s worth the wait.
http://www.ifa.hawaii.edu/faculty/je...EPS/annrev.pdf
Lexx_Luthor.
“Chill out here.”
If I were any more chilled than I am now, I might get frostbite.
“What angle of orbital inclination divides regular from irregular?”
Orbital inclination is not the most significant factor here. What makes Triton irregular is the virtual certainty that it is a captured object. Perhaps an amendment to my earlier statement will help to clarify what I was trying to say.
“Triton is retrograde, therefore irregular.” And is therefore a captured object.
“Degrees of regularity?”
None that I’m aware of. Here are the definitions for “regular” and “irregular” which I have been using in this thread.
“Regular satellite is a satellite within a miniature "solar system" orbiting about the three largest gas-giant planets. They include the four Galilean satellites of Jupiter, eight satellites of Saturn, and five satellites of Uranus. Orbits of regular satellites, as a rule, are nearly circular and lie near the equator of their central planet. It is believed that the systems of regular satellites formed under processes similar to those that resulted in the formation of the Solar System as a whole.”
“Irregular satellite is a satellite with an eccentric (non-circular), often inclined orbit, which indicates that it was captured into its present orbit. Orbits of irregular satellites usually have the objects relatively far from their central planet. At Jupiter, the irregular satellites form two groups. The first (internal) group is within a distance of 160 RJ (radii of Jupiter) and is characterized by an eccentricity e > 0.15 and orbital inclination i > 28°. The second (external) group is outside a distance of 360 RJ, with e > 0.25 and i > 150°. Some satellites of Saturn (such as Phoebe) and one satellite of Neptune (Nereid) are also irregular satellites.”
From here: http://www.nasm.si.edu/ceps/ica/glossary.htm
Regarding the classification of objects, I have no intention of re-inventing the wheel here. Instead, I would prefer that we add qualifiers or descriptors to the existing designations for clarity.
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Thanks. It does make some sense.
However::Are we all agreed on this? Enough to make it permanent through practical classification?I submit that this fact plus the fact that Luna formed by catastrophic impact rather than within a circum-planetary accretion disk qualify as justification for re-evaluating its status as a "regular" moon.
Keep in mind that the "irregular" satellites may far outnumber the "regulars." Indeed, an "irregular" satellite may be a more natural occurence than "regular" satellite.
Maybe "native" satellite as opposed to one that is captured (can't think of a good name now--"snatched" ? ). I know its hard not to, but don't laugh at me. You are suggesting (quote below) we classify on the basis of presumed origin. "Native" is certainly far more descriptive. "Irregular" could be used to describe any number of properties of any satellite.
Virtual certainty? That is *one* problem I am having here.Orbital inclination is not the most significant factor here. What makes Triton irregular is the virtual certainty that it is a captured object.
The other is what makes Triton's capture virtually certain? The "irregular" orbital inclination? Are we are going both ways here?. Again, I ask what degree of orbital inclination (and eccentricity) constitutes irregularity. Granted, looking at these numbers as well as the satellite's physical nature may help us decide if it is a "snatched" object. [img]/phpBB/images/smiles/icon_biggrin.gif[/img] But what are the numbers dividing regular from irregular? I believe Mimas has a 5 degree inclination. Wozop with that?
If through common use the classification comes to imply origin, and if by chance a satellite was captured yet now has an orbit that looks "regular," (or is this not possible?) the classification scheme would fail to associate origin with human label.
Again, makes sense. I strongly suggest not associating the classification too strongly with ideas of origin but on the orbits and physical nature of the satellite--something all can agree on and hence can exchange ideas about (if they want to). In most cases you and I will probably agree on any particular moon.I would prefer that we add qualifiers or descriptors to the existing designations for clarity.
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Lexx_Luthor.
“Are we all agreed on this? Enough to make it permanent through practical classification?”
That my suggestion met any resistance at all indicates the majority does not yet agree. I attribute this more to the fact that most simply hadn’t given it a lot of thought. It is a minor detail, after all. Like the “Whisper Campaigns” of politicians or advertisers, if the idea or suggestion is memetic, it will eventually percolate up through the ranks and reach the people who have been tasked with ruling over such things.
“Keep in mind that the "irregular" satellites may far outnumber the "regulars."”
I agree. The irregulars do outnumber the regulars. From the above definition for regular:
“They include the four Galilean satellites of Jupiter, eight satellites of Saturn, and five satellites of Uranus.”
By this count, the regulars are greatly outnumbered by the irregulars.
“Maybe "native" satellite as opposed to one that is captured”
Works for me. Any thing that helps clarify would be a slight improvement over the current situation.
Regarding the capture of Triton by Neptune;
“Virtual certainty? That is *one* problem I am having here.”
If we google up “Triton-Neptune” we find a majority consensus that it is a captured object. The hair-splitting which remains concerns when and how. Triton appears to be a huge proto-KBO which has been captured by Neptune. Easy to say, difficult to prove. So, instead of saying “absolutely certain”, I say virtually certain. Subtle, yet meaningful distinction.
“what makes Triton's capture virtually certain? The "irregular" orbital inclination?”
No. Its retrograde motion and its composition are what make it virtually certain. I view the inclination and eccentricity as more suggestive than indicative. I do not view retrograde motion as an extreme function of inclination.
“But what are the numbers dividing regular from irregular?”
I’m not sure there are any. In the definition, they use the word “often” as opposed to always. (regarding inclination and eccentricity) This implies that retrograde motion may be viewed as a dynamic characteristic in and of itself. Oh yeah, I do consider Mimas to be natural and regular.
“If through common use the classification comes to imply origin, and if by chance a satellite was captured yet now has an orbit that looks "regular," (or is this not possible?) the classification scheme would fail to associate origin with human label.”
Now that you mention it, I’m not sure that Titan is “native” to Saturn. And the details of a prograde capture give me a headache. Titan appears to be too massive. Anomalously so. But is that enough to prove it? Nope. I patiently await the Cassini encounter with Saturn and fresh data on Titan.
“In most cases you and I will probably agree on any particular moon.”
I’m confident that we would.
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If Titan is indeed a captured object, that means that Hyperion, Iapetus, and Phoebe were likely captured as well, since they orbit further out. I can easily see Phoebe being a captured object, with its similarities to many stray asteroids. Iapetus...ehhhh...well its inclined 17º that may hint in a capture. As for Hyperion there isn't any thing about this moon that suggests its not native. Maybe I'm thinking too hard; is it possible to have 2-3 native moons orbiting beyond a captured one?On 2003-03-03 11:46, Heathen wrote:
Now that you mention it, I’m not sure that Titan is “native” to Saturn. And the details of a prograde capture give me a headache. Titan appears to be too massive. Anomalously so. But is that enough to prove it? Nope. I patiently await the Cassini encounter with Saturn and fresh data on Titan.
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Zap.
“Maybe I'm thinking too hard; is it possible to have 2-3 native moons orbiting beyond a captured one?”
It’s good to know I’m not alone in this regard. Maybe I’ve been thinking too hard in directions that may prove fruitless. If the capture occurs late (3 bya or less) then a Titan capture is not merely problematic, it’s all but unworkable. For this reason I’m considering a very early capture (4 bya or more). This raises problems of a different nature. For example, how closely did the circum-planetary accretion disk resemble the proto-solar accretion disk? Are the satellite systems really just miniature solar systems or is the resemblance superficial? Specifically, are we over estimating the gaseous component of the circum-planetary accretion disks? If so, then we must adjust the effects of gas drag on an early interloper (Titan) into the disk of proto-satellites.
If the gaseous component is high, then the object falls into Saturn. But what would happen if little gas is present? As I do not possess a computer capable of running such a simulation, I am reliant upon pillaging the scraps of those who do. I’ll have to defer judgment until someone models just such a scenario. Although I’m not “married” to this model, I do see it as worthy of exploration, if only to discount it.
Here are a few assumptions I begin with.
A. With the exception of Jupiter, the outer planets and their moons formed more gradually than the planets of the inner solar system.
B. The gas component of the circum-planetary accretion disks was low.
C. Titan may be viewed as a basic building block which was captured at the approximate helio-centric distance of its formation and never made the journey out to the Kuiper belt. In other words, Titan may differ from Centaur objects only by virtue of being tidally processed as a result of being hijacked by Saturn. (which relies on the assumption that Centaur objects are really KBOs which have been kicked in system, returning to the zone of their origin)
D. That Jupiter migrated inward ~1 AU while Saturn, Uranus and Neptune migrated outward by several AUs.
Let’s return to your question; “it possible to have 2-3 native moons orbiting beyond a captured one?”
I don’t know. Certainly an object the mass of Titan would disrupt and rearrange a nascent satellite system. The recent discoveries of small objects bound to Saturn fall into several categories; irregular captures, collision fragments of formerly larger moons and ring shepards which may prove to be fragments of former moons as well. I’ll take the easy out with a common and unsatisfying response; Inconclusive. Need more data.
Order of Kilopi
My understanding of planetary formation models is that the planets should migrate inwards. What is the reason you assume that Jupiter would, but that Saturn/Uranus/Neptune migrate outward?Heathen Wrote: That Jupiter migrated inward ~1 AU while Saturn, Uranus and Neptune migrated outward by several AUs.
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Current accretion models of the formation of the four gas giants show that there probably was not enough material around the current orbits of Uranus and Neptune to build such large giants. So it is only logical to think that these two planets formed further inward where there was enough matter; perhaps they were eventually shoved outward by more massive Jupiter and Saturn.On 2003-03-04 12:58, dgruss23 wrote:
My understanding of planetary formation models is that the planets should migrate inwards. What is the reason you assume that Jupiter would, but that Saturn/Uranus/Neptune migrate outward?Heathen Wrote: That Jupiter migrated inward ~1 AU while Saturn, Uranus and Neptune migrated outward by several AUs.
Order of Kilopi
I've never heard that before. Do you have a specific reference where that result is found? The images I've seen of protoplanetary disks indicate that they are larger than the dimensions of the solar system which suggests and that a general inward migration of planets is part of the process.Zap Wrote: Current accretion models of the formation of the four gas giants show that there probably was not enough material around the current orbits of Uranus and Neptune to build such large giants.
You do need a mechanism to explain why Jupiter migrates one way and the other planets migrate the other way. Are you saying that is gravitational? How do you explain the methane content of the atmospheres of Neptune and Uranus?So it is only logical to think that these two planets formed further inward where there was enough matter; perhaps they were eventually shoved outward by more massive Jupiter and Saturn.
<font size=-1>[ This Message was edited by: dgruss23 on 2003-03-04 18:40 ]</font>
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Here is a documentary on the possibility of Uranus and Npetune forming around the orbits of Jupiter and Saturn. Basically all four gas giants may have formed in the same region, but Jupiter and Saturn were able to become more massive. Thus, by gravitational interactions, Uranus and Neptune migrated outward to their present orbits (you'll find all the juicy details in the link given)
As for the theory of Jupiter migrating inward, it could have had a repelling reaction with the outward movement of Uranus and Neptune. Now if you're talking about Jupiter being captured by the Sun from afar, that's a whole other story. Frankly, I don't buy the capture theory right now as there really isn't much evidence to support it.
_________________
-Zap
If you didn't like the opinions expressed in this post, get over it!
<font size=-1>[ This Message was edited by: Zap on 2003-03-04 19:38 ]</font>
Established Member
Zap, the link does not work. It just consists of "Http://" and nuthing else.
<font size=-1>[ This Message was edited by: g99 on 2003-03-04 19:36 ]</font>
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Oops my bad. Try now.
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dgruss23.
“My understanding of planetary formation models is that the planets should migrate inwards. What is the reason you assume that Jupiter would, but that Saturn/Uranus/Neptune migrate outward?”
First, a couple links for background on why Jupiter should migrate inward.
http://www.mpia-hd.mpg.de/THEORY/gal...-01/00-01.html
http://www.aas.org/publications/baas.../dps98/357.htm
And a link not directly related to migration, but interesting anyway.
http://www.physics.vanderbilt.edu/as.../snowline.html
And a couple links for the outward migration of Saturn-Uranus-Neptune.
http://www.nature.com/cgi-taf/DynaPa...02635a0_r.html
http://www.fiz.huji.ac.il/~lazar/PLUTINO.HTML
Order of Kilopi
Zap and Heathen,
Thanks for the links. Great information! A couple of questions. If you look at the link below, you will see that for this star the planet indicated is at a much larger radius than the current orbits of the outer planets. Now when we consider that the newly discovered Jupiter mass planets around other stars are often well inside the distance that Mars is from our Sun, it seems clear that there must be major inward migration of planets.
http://antwrp.gsfc.nasa.gov/apod/ap980122.html
So my first question is: In the models you have noted, would it be correct to say that they are modeling an advanced point in the evolution of the solar system such that the major inward migration has already ceased and now orbital stabilization is what is happening - which would explain outward migration?
My concern here is that the models discuss evolution of the planetary orbits in the range of 4-10 AU. But the link above shows that planets may form much further out and the new planets discovered around other stars show that massive planets usually end up much further in. So is it valid for these researchers to model Jupiter forming about where it actually happens to be today?
Second question: Is it assumed in these models that the methane in the atmospheres of Uranus and Neptune is picked up during outward migration? I would think that if these planets were formed in the same zone as Jupiter/Saturn that they should have similar composition (I'm aware that all of these planets have mostly H and He in their atmospheres).
Established Member
Their is no hard and fast distinction
between a moon and a double planet.
Strictly speaking, no object ever orbits another one. "Planets" don't really orbit "moons". A "moon" and a "planet" really orbit a central point lying somewhere between the two. The central point will be closer to the more massive object. The greater the difference in mass, the closer the central point will be to the more massive object.
For the Earth-moon system, this central point is fairly close to the earth's core. However the Earth also orbits arround this central point, and we can measure this motion. Since the point is way bellow the Earth's surface, it looks more like a "wobble" than an orbit. This is one way to detect extra-solar planets - we can detect the parent star's motion as it orbits arround the central point in the planet-star system.
When the planet and moon are pretty close in mass (as in Pluto - Charon), it becomes more obvious that both objects are orbiting a central point. This is because the central point is then much further away from the more massive body (though it will ALWAYS be closer to the more massive body than to the lighter one). It is then that we call the system a "double planet".
But you cannot get a hard and fast rule based on what orbits what, because NO OBJECT EVER ORBITS ANOTHER. It always orbits a central point - the location of which is determined by the masses of the bodies involved.
When we say a moon orbits a planet, we are using what linguists call "phenomenological language". It is a kind of shorthand that describes how things APPEAR to an observer - without having to go into detail about how things really are. The reader might want to keep this concept in mind when you look at some of the theological and philosophical debates on this forum. It is a concept that many people seem to oppose, without realizing how often we use it even in the most literal and scientific of discussions.
<font size=-1>[ This Message was edited by: Cloudy on 2003-03-05 08:37 ]</font>
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Hold up. As I mentioned to Heathen earlier in this thread, it's probably a little hasty to call Pluto/Charon a double planet, since so many people have a problem even calling Pluto a planet. I personally don't, but still...On 2003-03-05 08:33, Cloudy wrote:
When the planet and moon are pretty close in mass (as in Pluto - Charon), it becomes more obvious that both objects are orbiting a central point. This is because the central point is then much further away from the more massive body (though it will ALWAYS be closer to the more massive body than to the lighter one). It is then that we call the system a "double planet".
And another thing, the center of mass of the Sun and Jupiter is outside of the Sun. So is the Sun/Jupiter system a double planet, or a double star?? [img]/phpBB/images/smiles/icon_smile.gif[/img]
Order of Kilopi
Cloudy wrote: But you cannot get a hard and fast rule based on what orbits what, because NO OBJECT EVER ORBITS ANOTHER. It always orbits a central point - the location of which is determined by the masses of the bodies involved.
Both of these are excellent points and they seem to illustrate that orbital parameters alone cannot constrain the definition of a planet (or star). Physical dynamics must come into play (eg. the distinction between brown dwarf and star seems to surround the existence or not of sustained nuclear fusion). This whole discussion seems to be the classic problem of people trying to shoehorn messy nature into clean definitions constructed by people.Kilopi wrote: And another thing, the center of mass of the Sun and Jupiter is outside of the Sun. So is the Sun/Jupiter
system a double planet, or a double star??
Maybe the question could be asked a different way. If it is "fuzzy" as to whether or not the Earth/Moon and Pluto/Charon are double planets, then what would be the appearance of of an unambiguous double planet system?
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