# Talk:Double planet

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## (unorganized)

Ok, so here's a proposed re-write:

A double planet is a set of two planets of comparable mass orbiting one another. There is no accepted definition on where to draw the line between a double planet and a system consisting of a planet and moon, so the term "double planet" is an informal one. One example is Pluo and its moon Charon

In most cases, a moon is of very small mass relative to its host planet. However, there are some examples of moon/planet mass ratios much closer than average: particularly, the Earth and its moon Luna (mass ratio 0.01230), and Pluto with its moon Charon (mass ratio 0.147). (Ignoring the debate over whether Pluto should be considered a planet, or simply a larger Kuiper belt object.)

Several approaches have been suggested for defining a double-planet. One method, suggested by Isaac Asimov, is to compute the force of gravitation exerted by a planet on its moon (Fp), and the force exerted by the Sun on that moon (Fs). The ratio Fp/Fs is the Tug Of War (TOW) value. A TOW value less than 1 indicates that the moon is more firmly gripped by the Sun than its planet. The TOW value for Earth and Luna is approximately 0.455. Two recently discovered satellites of Jupiter, and the outermost satellite of Neptune, also have TOW values less than one.

Charon was still undiscovered at the time Asimov proposed the TOW ratio. The TOW ratio for Charon is over 600, so by Asimov’s reckoning Charon would be a moon of Pluto.

However, astronomers prefer to regard Pluto and Charon as a binary system because (a) they are so close together and so close to the same size that the point they orbit about (the barycenter) is above Pluto's surface, and (b) they orbit with the same side facing each other. By the barycenter method, Pluto/Charon is a double planet, but Earth/Luna is not.

--Gary Puckering, 2004-03-15

Some excellent work on this subject was published in 1975 by Isaac Asimov in "Time and Space and Other Things", Avon Books, New York. Asimov's approach is to consider the tug-of-war (TOW) ratio between the satellite and it's primary, and the satellite and the sun. One would expect a satellite to be more firmly grasped (gravitationally speaking) by it's primary than by the sun, and such is the case for all the satellites in our solar system, including Charon. But is not the case for our Moon. For a good accounting of Asimov's treatise on the web, see [1]

Asimov suggests that the appropriate cutoff points for "moon" status are the TOW distance and the Roche Limit. A true "moon" must lie between these two points. It should be fairly obvious that the Roche Limit is a more reliable minimum distance than the surface of the primary -- regardless of the location of the barycenter -- since it cannot exist inside the limit without being destroyed by tidal forces.

As for the maximum distance, I'm with Asimov -- the TOW distance makes the most sense. And by this line of reasoning, Earth and Moon would be classified as a double-planet, whereas Charon would be classified as a satellite of Pluto -- a conclusion that is quite the opposite of that drawn by Bryan Erickson (below).

--Gary Puckering, 2004-03-15

Not having actually read this article, I see that two instances of the phrase "I propose" occur and that it's signed. Wikipedia isn't the place to make proposals (i.e., to do original research). To do so is to ask the people here to act as peer reviewers and, worse, to pass off your proposals as the state-of-the-art research in the field. For all I know about you (Bryan Erickson), they might be, but I doubt it. But I feel strongly enough about this particular (proposed) rule that, on Nupedia, a very distinguished Classics editor quit when I refused to allow him to expound his latest theories on something or other. I'm not in the same position of authority on Wikipedia (because this is a wiki :-) ) but I do tend to get put out by this sort of thing nonetheless.

Beyond these concerns about the propriety of original research, there is also the very generally accepted rule about writing from the NeutralPointOfView.

So, how might we rewrite this article so as to remove idiosyncratic proposals? --Larry Sanger

Frankly, the whole thing is just an argument about the use of terminology--it belongs in a style guide for publications on Astronomy or something. I don't see that it has any purpose in an encyclopedia at all. An article entitled "double planet" should simply mention that the definition is necessarily vague, and then list a few and describe them. I want to learn something about double planets, not something about what astronomers argue about. --LDC

Following are the problematic paragraphs:

I propose two different quantitative analyses that could draw a sharp distinction between double planets and planet-moon systems. The first is the Center of Mass Method. If the center of mass of the two bodies lies under the surface of one body, then that body will be defined as a planet and the other body will be defined as a moon. This method has the following advantages: it's relatively easy to perform when the masses and volumes of the two planets are observable; it is logically sensible; and it provides a definite demarcation between the two classes of two-body systems in almost all cases. The disadvantages are: in a few hypothetical cases, the center of mass might lie almost exactly on the surface of the more massive body, so that the center of mass is below the surface when the maybe-moon happens to be passing over mountain ranges but above the surface when the maybe-moon is above a valley; this would require further definition of exactly where the surface is, is it sea level, or if the planet is among the great majority that don't have seas, is it above the "datum" or average radius; secondly, this method requires observation not only of the masses of the two bodies but also of their radii; in the near future, as observations of planets in other star systems improve, we will reach the point of measuring masses and tracing orbital paths of planets but remaining capable of imaging them only as individual pixels. This would be under the level of technology embodied in NASA's Terrestrial Planet Finder telescope. It will be a long time after that before our data gathering technology achieves the ability to make reliable measurements of the radii of these planets. Therefore, the Center of Mass Method for distinguishing double planet systems and planet-moon systems, while working fine for our own Solar System, would be face more difficulty for other star systems.

Which leads to the second analysis, the Lagrange Method. If the mass ratio between the two bodies is at least about 0.081, the 4th and 5th Lagrange points of the two-body system do not provide a stable equilibrium. If the ratio is less than that amount, Lagrange 4 and 5 do provide stable equilibrium. This provides a distinction that could be used to define the cutoff between double planet systems and planet-moon systems. It has the advantage that it requires only determination of the masses of the two bodies, which could be accomplished for extra-solar planets at a much lower level of technology; we already measure the minimum mass of extrasolar bodies, and with observations of their orbital paths, even with only single-pixel resolution, as with the proposed NASA Terrestrial Planet Finder orbiting telescope, we will be able to define their masses pretty well. This method also provides a definite cutoff between the two categories. However, it has the drawback of providing a less logically compelling definition. Therefore, I propose that the Center of Mass Method be adapted as the official analysis. It could still be applied, albeit with less precision, to extrasolar planets under the Terrestrial Planet Finder level of technology, if we use a model of a planet's density based on its mass and the semimajor axis - average distance - from its host star. This density model would provide a good prediction of the planet's radius, which could be used together with measured mass to determine whether the center of mass is beneath the surface of the larger mass or is in space between the two bodies.

Coming back to our solar system, under the preferred Center of Mass method, Pluto and Charon are clearly double planets, but Earth and its Moon are not; the center of mass is almost 2000 kilometers, or about 30%, of the way to the center of the Earth.

Must the terms be mutually exclusive? I'd been fine with "planet-moon" being a broader term (one larger than the other), and "double planet" being a more specific term (barycenter) CobraA1 09:02, 7 April 2007 (UTC)

## Double planets

Don't forget that the moon is slowly moving out in its orbit...in six or seven billion years, when the sun becomes a red giant, the center of mass of the Earth/Moon system should be outside of the Earth's surface, due to tidal evolution.

Tom Mazanec

### Agreed

Tom hopes the facts speak for themselves. They don't.

I was disappointed that the new planet definition sidestepped the double planet question. The arguments to try and keep the moon a satellite are simply because of social inertia. If a body has the sun as it's primary it is a planet. If it has a planet as it's primary then it is a satellite.

I think copericus would agree.

Sit outside when you can see multiple planets. The moon travels in the planetary arc just like any other planet. The longer you look the more obvious it becomes. It's quite an emotional feeling. The moon orbits the sun. It doesn't orbit the equator like a good long term satellite should. It doesn't have a weird captured satellite orbit either.

I did the math in high school in the 60s and have discussed whether the moon is a satellite or planet with about a dozen astronomers and the truth is none of them cared. The common reply is "it's just a three body problem anyway." Orbits are fasinating to them but none felt any need to try to start explaining to the general population why they should turn their concept of the solar system around. Why get in an argument about what orbits what when to that astromer who sees the bigger picture everything dances around everything else. It is my opinion that the definition avoided clarifing double planet in order to avoid social turmoil.

That said, to me it is clear that this is an issue for wikipedia because the issue concerns general people more than astronomers.

To try and justify the moon as a satellite when it is not captured by the earth and is drifting away on it's own path around it's primary is really untenable no matter where the center of mass is.

I picture a cartoon of a man, a dog and an animal control officer. The dog has a leash around it's neck but the other end is lying on the ground, albeit between his legs. The man is telling the officer than the dog is under his control because it is on a leash.

re: Two recently discovered satellites of Jupiter, and the outermost satellite of Neptune, also have TOW values less than one.

Hmmm. As the attraction from the planet decreases with time and distance, will these satellites remain in a stable orbit of the sun?

--Strider22 04:09, 12 September 2006 (UTC)

## A couple of notes on today's major edit of the Double Planet entry:

A couple of notes on today's major edit of the Double Planet entry:

1. Mr. Puckering's proposed definition forces a conclusion that is quite the opposite of that found in the professional literature by the scientists in the field as sampled in the references provided, since he concedes that his definition would exclude Pluto and Charon from double planet status while including tiny moons of Jupiter and Neptune as new double "planets", along with Earth's moon. He might also pay attention to the fact that Asimov was not defining limits for a double planet at all, but for a moon. The Roche Limit has no effect on the question of defining whether a moon qualifies as a double planet, rather than of how close a moon (or double planet) can get to the host planet before shredding apart. A double planet can get along just fine with its barycenter well under the Roche limit just as a moon get get along just fine with its planet-relative barycenter under the planet's surface.

Comment by Strider22: As you say the Azimov proposal defines a moon. If the object exists beyond the TOW limit then it is not a moon. Asimov asks "What is our moon doing way out there?". He has concluded that the moon does not have earth as it's primary but rather has the sun as it's primary. If the moon is not a satellite, travels around it's primary along with another planet, what is it?

2. The first, relatively minor edit corrected a factual error, consiting of a definition so broad it applied to all moons rather than double planet-systems as opposed to planet-moon systems. It said that:

"Hypothetically, the moon/planet mass ratios between two bodies could vary anywhere between zero through one, where two planets of exactly the same mass orbit each other..."

But a mass ratio approaching zero would include every conceivable planet-moon system, eliminating any possible difference between a double planet system and a planet-moon system. It would define literally thousands of orbiting debris objects to be in "double planet" systems with the Earth.

I'd like to point out that Asimov's satellite definition is problematic for another reason. Not only would Charon be a satellite of Pluto by this definition, but Pluto would be a satellite of Charon.129.6.54.5 (talk) 20:48, 13 March 2009 (UTC)

The "TOW definition" or "Asimov definition" is intended to define what counts as a satellite, a dividing line between a "double-planet system" and a "planet-moon system". If we use that definition, then Luna and these thousands of orbiting debris are not satellites. "not a satellite, ... what is it?" -- If everyone we to switch to this definition, then what are those "tiny moons of Jupiter and Neptune" and "orbiting debris objects"? I presume they would be considered asteroids (not planets), similar to the Trojan asteroids and 3753 Cruithne. My only question is: would Luna be promoted all the way to planet, or would be considered a "dwarf planet" like the much smaller Ceres (dwarf planet)? --68.0.124.33 (talk) 22:00, 17 March 2008 (UTC)

## Corrected mismatch of terms.

One sentence in the article read, "In particular, the Earth-Moon and Pluto-Charon systems are the only examples in our present Solar System where the mass of a moon is larger than one fortieth of one percent of the volume of the host planet (i.e. mass ratio of 0.00025 or less)." (italics added)

I changed this to read, "In particular, the Earth-Moon and Pluto-Charon systems are the only examples in our present Solar System where the mass of a moon is larger than one fortieth of one percent of the mass of the host planet (i.e. mass ratio of 0.00025 or less)," so that the terms would match up. Linguofreak 05:09, 19 March 2006 (UTC)

## Proposed deletion of "two planets colliding with each other"

I propose to delete the following:

"The second usage for the term double planet, in the context of the theory of the origin of the Moon (i.e. Earth's Moon), is a set of two planets of comparable mass that collide with each other - i.e. with at least transiently overlapping orbits. A double planet in this sense occurred in the very early solar system, consisting of the proto-Earth and a second, Mars-sized planet that collided with it at an oblique angle, in the consensus hypothesis of the formation of the Earth-Moon system. The second body was not a proto-Moon because most of its mass was incorporated into the Earth, while the Moon formed from a small fraction of debris kicked up from the Earth by the collision. These double planet precursor bodies to the Earth-Moon system had roughly comparable mass - i.e. a mass ratio in the neighborhood of 10:1. This happens to be similar to the mass ratio of Pluto-Charon."

I have never heard anyone calling the proto-Earth and impactor (in the context of the giant impact hypothesis) a "double planet" . When were these two a double planet? At the instant of impact? The intersection of two orbits doesn't make a double planet. Moreover, this section is poorly written and confusing. Lunokhod 20:04, 4 December 2006 (UTC)

## Angular momentum as a criterion for defining a double planet

Does anyone here know if anyone has attempted to use the relative angular momentum of a satellite to define whether or not it is a double-planet or planet-satellite system? It seems to me that the only two candidates in our Solar System for being called double-planet systems are Earth-Moon and Pluto-Charon, and both of these satellites contain an abnormally high angular momentum. Indeed, it appears that the commonality between these two systems is that they formed by giant impacts. Lunokhod 09:55, 8 December 2006 (UTC)

## barometric centre

i think the definition of a double planet system as having a barometric centre outside the surface of the bigger element is unscientific, since the barometric centre of every system will move out of the radius of the bigger element as a factor of time. so this definition says, it's a double planet when it is old enough.. well if some pair of celestial bodys becomes a double planet at one point in their life or existence, i must either think it had been a pair before, or it didn't couple just after that. since that is ridiculous i think it might be an error in the article?24.132.171.225 (talk) 08:26, 7 February 2010 (UTC)

## Dubious

I read that entire BLOG reference and have found:

• At no time is the author's name mentioned, so how is it that it has been ascribed to Alan Stern?
• At no time does the author, whoever he or she might be, refer to the Pluto-Charon system as a "double planet". The author cites and quotes material that refers to the Pluto-Charon system as a double planet, but the author of the blog does not do so.

While this claim is not damaging, it appears to be false and unverifiable. Therefore it should be either clarified and verified, or removed.
—  Paine (Ellsworth's Climax)  05:34, 31 May 2010 (UTC)

The dubious claim has been removed and brought here, in case it needs to be discussed...

Though not accepted for official usage by the IAU, astronomer [[Alan Stern]] of the [[New Horizons]] mission to Pluto and the [[Kuiper belt]] refers to the Pluto-Charon system as a double planet.<ref>http://www.oneworldblueportals.com/bernardsstar/index.php?page=blog</ref>{{dubious|date=May 2010}}{{verify source|date=May 2010|This is a BLOG! At no time does it say that the author of the material is "Alan Stern", nor does it imply that the author, whoever he or she is, refers to the Pluto-Charon system as a double planet!}}

—  Paine (Ellsworth's Climax)  06:53, 30 August 2010 (UTC)

## Recent lede edits

The purpose of my recent edits was to get away from the "special" case of a planet with a moon that might be a planet instead of a natural satellite. I feel that the first part of the lede is now more "general".

To the editor that changed "requirement" to "concept", Kheider... We might get opposition to that change from those who feel that when an "official" definition of "double planet" is formed, the requirement (or concept, if you will) of a barycenter above both surfaces shall be one of the first things included in that definition. However, there is no opposition to your edit from me, as I consider that distinction to be dubious. I figure to wait to see if any other editors can come up with a reliable source, perhaps look for one myself when there is time, and then rm the "concept" sentence if no verification is forthcoming.

Also, if I'm not mistaken, the last big meeting of the IAU, when such things can be decided, was in 2009, hence the use of that date in the lede. I am not against "bringing it up to date", though.

On a separate issue, I designed the "citation needed" wrap template to include a Tooltip (hover mouse over text), and it looks like there is also underlining of the questioned text if an editor uses the Firefox or Chrome web browser (the underlining does not appear in Internet Explorer). In case you're interested in this issue, it is presently being discussed here and in other sections of that Talk page. – PIE ( CLIMAX )  05:32, 30 August 2010 (UTC)

Since there is no official definition of a double planet, we should not claim a barycenter "requirement". I also suspect that the IAU will never waste time on such a minor issue (minor from a scientific standpoint.) The whole barycenter concept is primarily a result of leftovers from the 2006 IAU draft proposal. Professional astronomers do have have more fruitful projects to focus on. -- Kheider (talk) 06:43, 30 August 2010 (UTC)
It is agreed that such a definition is presently a mundane, minor issue; however, there will soon come a time when we are able to detect exosatellites and binary exoplanets. It might be good to have a working definition already in place at that time. Also, it might keep us from calling a spade a rake, i.e., a "planet" a "satellite", as in the case of Pluto's Charon, and perhaps even Earth's Moon. – PIE ( CLIMAX )  07:11, 30 August 2010 (UTC)
The working definition is that the more massive body will be the planet and the secondary will be the moon. If we can not tell which body is more massive only then will it be a candidate double planet. As long as moons larger than Pluto are still called satellites (and not planets), there will always be grey areas. Your statement above, "it might keep us from calling a spade a rake, i.e., a "planet" a "satellite", as in the case of Pluto's Charon" almost suggests that you are biased towards Charon being a true planet. We need to avoid inadvertently enforcing such suggestions in the lede. -- Kheider (talk) 08:39, 30 August 2010 (UTC)

──────────────────────────────────────────────────────────────────────────────────────────────────── More than a year has elapsed since I placed a Citation needed template after the following sentence from the lede, so I've moved it here for now in case someone can find a reliable source for it:

– PIE ( CLIMAX )  05:33, 15 January 2012 (UTC)

One reason this may be difficult to source is that the definition is time-sensitive. In the case of Earth's Moon, for example, the Moon gets farther from Earth every moment, so every moment the Earth-Moon barycenter gets a little closer to the surface of the Earth. There will come a time when that barycenter is outside the surface of Earth. When that time comes, the Earth-Moon system will, under the above definition, cease being a planet-satellite and become a double planet. So the above definition is not very effective to describe a double planet. – PIE ( CLIMAX )  17:15, 16 January 2012 (UTC)
Note: It was pointed out on another talk page that it will take a very long time, perhaps long after our Sun goes red giant, before the Moon's orbital radius grows to the point that the Earth-Moon barycenter is outside the surface of the Earth, so the time sensitivity is, it is conceded, not all that sensitive. – PIE ( CLIMAX )  05:25, 24 January 2012 (UTC)

Since the Citatation needed templates were both over a year old, I am pulling the following quote from the first section:

– PIE ( CLIMAX )  17:13, 20 January 2012 (UTC)

## Tug-of-War formula revisited

Could someone please add Asimov's formula to the article? Unfortunately, there seems to be no online resource on this (and the link given above on this discussion page has been removed, as being typical for web resources nowadays). If there is no formula available, or at least an explanation which physical quantity the "tug of war" value refers to (mutual force? orbital period?), this section does not really make sense.--SiriusB (talk) 12:28, 1 October 2010 (UTC)

There seems to be no addition to this issue so far. However, from trial and error one finds that the formula is probably
${\displaystyle {\text{tug-of-war index}}=\left({\frac {a_{\text{planet}}}{a_{\text{satellite}}}}\right)^{2}\times {\frac {m_{\text{planet}}}{m_{\text{star}}}}}$

In fact, this is just the ratio of the gravitational accelerations of the host star (here: the Sun) and the host planet on the satellite. I do not have Asimov's book, but I guess that the formula is given there in some form and adding it to the article won't be original research. One should note that this is not equivalent to the strength of the bond (and thus the stability of the orbit) which is better described using the ratio of the satellite orbit to the Hill radius of the planet.--SiriusB (talk) 08:03, 20 August 2011 (UTC)

Definitely not OR. I revised the formula just a bit to bring it in line with Asimov's presentation of the formula in his article "Just Mooning Around". Also found the book in .pdf format on the web and added this to the ref. citation. Made a few other upgrades to that section. – PIE ( CLIMAX )  19:56, 20 January 2012 (UTC)

Just added a link to the page about the Moon's path - it shows a bit more clearly how the Sun's effect on the Moon is greater than the Earth's. NJHartley (talk) 11:50, 8 October 2011 (UTC)

## Mass Gap Between Planets and Moons

I think we are on the right track in considering the gravitational influences of the host planet and star. If you notice there is a significant "mass gap" between the largest moon and the smallest true planet. There is only one real moon-sized object inside the orbit of Neptune that hasn't been captured by a planet (Ceres). Irrespective of the method of capture, there seems to be a maximum mass for a moon, relative to the Sun and its orbiting planetary masses. Otherwise the only object in the Solar system large enough to capture it is the Sun itself. That maximum mass seems to lie somewhere between Ganymede (0.0248 Earth masses) and Mercury (0.0553 Earth masses). Under that definition, Moon is a moon, destined to be captured by some larger object, be it Earth or some other planet. I think it would also be fair to say that every planetary mass must have some maximum moon size that it can retain. Given the lack of any known planet-moon systems with mass ratios lower than 1/9, coupled with the relative instability of the Earth-Moon system over time, a "double-planet" configuration is at best only temporary if there are larger masses nearby to disturb it. — Preceding unsigned comment added by Lizardite (talkcontribs) 00:07, 17 November 2011 (UTC)

## Aslaksen

I have added the Helmer Aslaksen page in as a reference citation. His page is used in the Orbit of the Moon article, and it clearly shows the scalloped shape of the Earth's and the Moon's Solar orbits. The only difference is that he describes the Earth-Moon motion around the Sun as "convex", while Asimov uses the term "concave" to describe the exact same motion. The difference is only the perspective of the observer. Aslaksen's POV is from outside the orbits, as if from the orbit of Mars, and Asimov's POV is from inside the orbits, as if from the orbit of Venus. Aslaksen's page also shows that there are no loops in the Moon's Solar orbit, as are seen in the Solar orbits of most satellites in our Solar system. – PIE ( CLIMAX )  05:34, 24 January 2012 (UTC)

## Satellites? Dwarfs?

The double-body structure could consist of a primary planet and a satellite, a full-sized larger planet and a full-sized smaller planet, a larger dwarf planet and a smaller dwarf planet, or more generally, a primary (larger) body and a smaller body. The term "smaller body" covers all possibilities. The extensive usage of "dwarf" and "(dwarf)" detracts from the fact that this article is about Double planets. It is not only about double-dwarf planets nor is it only about planets and satellites. – Paine (Climax!)  21:39, 30 January 2013 (UTC)

Good point. Do note that there are three bodies involved in the tug-of-war, also the central star/Sun. I must also reiterate that I hate it when people undo also the parts that they don't disagree with... --JorisvS (talk) 21:58, 30 January 2013 (UTC)
Not for anything, and just out of curiosity, how do you know for sure when people don't disagree, or at least are neutral about some changes? For example, "figured" vs. "calculated" – It's not that I disagreed or didn't disagree, it's that I accepted either as viable, although "calculated" does sound more encyclopedic. – Paine (Climax!)  09:54, 31 January 2013 (UTC)
Of course, I don't for sure, but sometimes something is so straightforwardly better that I have a hard time imagining why someone would oppose the change. --JorisvS (talk) 10:06, 31 January 2013 (UTC)

Is there s.t. missing from Asimov's definition? It seems incredible that if we had a rubble pile in place of the Moon, it would still count as a double planet.

Added a couple other defs. Had these been removed? The barycenter def used by the IAU is far more common than TOW, yet TOW is the only example we used.

I suspect there are others. I wonder if anyone has considered like objects. A double terrestrial planemo would count as a double planet (assuming the primary qualifies), which would include Earth, but a terrestrial (or icy) planemo satellite of a gas giant (unlike) would not, sparing all four of the Solar giants. Also, Stern does consider "satellite planets" to be planets; don't know if he considers Jupiter to be a multiple planet, though. — kwami (talk) 23:55, 31 January 2013 (UTC)

Given what it is, I think the TOW definition is actually complete. But yeah, it's a preposterous definition if you think about it...
I'd also like to say that you're doing a great job expanding this article! --JorisvS (talk) 00:04, 1 February 2013 (UTC)
As can be seen by how long it is taking the IAU to assemble an official definition of "double planet", it will take more than one parameter, such as the TOW, to do the job. Asimov actually came up with much more than the TOW, which I have outlined here on this part of my website (that's a compilation of several UseNet posts made to the newsgroup, alt.astronomy). Like all the parameters, the barycenter definition is not an official definition. The Earth-Moon barycenter lies only a thousand miles beneath Earth's surface, which is only 1/8 the diameter of Earth. You added to the article that the barycenter will lie outside the surface of Earth in a few hundred million years. The figure I've read was more like a few billion years and actually longer than it will take for the Sun to go red giant. Do you have a reliable source for the figure you used? It would be good if you could add more reliable sources for your added material. I agree that your expansions to the article are welcome. – Paine (Climax!)  09:19, 1 February 2013 (UTC)
A cited figure would be better. The point, however, is that all of these definitions are arbitrary in an unintuitive way, when the concept of a double planet is simply two bodies of similar size. — kwami (talk) 10:20, 1 February 2013 (UTC)
Of similar size..or of similar mass? --JorisvS (talk) 10:33, 1 February 2013 (UTC)
Either. Actually, one of the defs is not arbitrary: co-accretion. Just turns out not to be relevant. — kwami (talk) 12:29, 1 February 2013 (UTC)
Well, one can come up with hypothetical situations in which the planemos' masses are similar but their sizes rather dissimilar or vice versa. --JorisvS (talk) 16:45, 1 February 2013 (UTC)

## Barycenter

Over time this article has slowly moved towards Barycentric coordinates (astronomy) being removed. This is at odds with Moon#Relative_size, which gives http://www.iau.org/public_press/news/release/iau0601/q_answers/ as a source. I suspect that this should be mentioned, but I am not sure. If it is merely because of sourcing, what type of sources are needed? 06:38, 7 January 2014 (UTC)

## Double planet image updated (The Earth-Moon size comparison)

Better version of Moon Earth Comparison.png — Preceding unsigned comment added by Jcpag2012 (talkcontribs) 11:08, 14 February 2015 (UTC)

## Terminology

I propose that this article be renamed binary planet and that double planet not be treated as a synonym. Multiple scientific sources use double planet to mean a system with two planets [2][3][4]. Scientific sources pretty nearly always use binary planet to mean two gravitationally bound bodies [5][6][7][8]. SpinningSpark 21:57, 25 December 2015 (UTC)

The term double-planet system is often used to refer to a system of two planets orbiting the same host star. This article refers to two planetary bodies orbiting one another, which is a different concept. However, I agree that binary planet and double planet should not be treated as synonyms. It may be better to write an article devoted to binary planets and to specify under which condition a binary planet can be considered a double planet. (The IAU has yet to define planets - and binary planets - generally). This structure would parallel the situation for binary asteroids, where a fraction of the binary asteroids are considered double asteroids. (Again, with no official IAU classification at this time). JeanLucMargot (talk) 00:48, 26 December 2015 (UTC)

## References

1. ^ a b Robert Roy Britt (2006-08-18). "Earth's moon could become a planet". CNN Science & Space. Retrieved 2009-11-25.