Let's assume for a moment that two planets formed independantly in a system, one with an orbit significantly inclined from the ecliptic due to earlier chaotic interactions. Both planets are of near-equal mass. Both planets have substantial moons (relative to the Earth-Moon mass ratio).

Is it possible for the planets to come close to one another such that each planet pitches the other's moon out, leaving the two planets in orbit around one another, and that binary planet system in orbit around their sun?

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"Toward no crimes have men shown themselves so cold- bloodedly cruel as in punishing differences of belief." - James Russell Lowell

I would think that this is possible but VERY unlikely.

Suppose that the two systems approach from such an angle and velocity that each of the two "substantial moons" experience slingshot orbital assists from the "other" planet. The two moons could be accelerated and ejected from their systems and the two planets would experience loss of momentum sufficient for them to become a binary planet.

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/onyE

I think you're right .
This must be possible but the odds are very low.
I've simulated ( with Gravity Simulator by Tony Dunn) several systems till now to get a moon stabily captured by a planet ,. None of my trials was succesful till now allthough the initial orbital parameters were very close to a stable configuration

And is it possible that the two planets are born twins, orbiting each other from the beginning , a double birth ?

Is it significant that you're not considering a collision then the reforming of oblates (such as Earth-Theia leading to the Earth-Moon system)? It is suspected that that was possible. Had Theia been closer in mass to Earth than Mars then a collision might still have led to a system.

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Yonder is Dubhe seen on Earth tonight as it was in the days of Grover Cleveland's presidency whereas this way is Deneb seen as it was in the lifetime of Muhammed . If one somehow travelled to Deneb at very close to c then whenever you looked back you'd measure Earth as closer to you than the distance you would simultaneously measure between Earth and Dubhe.

The sort of 'accretion' theories that seem most popular to explain planet formation don't look like they would be able to produce 'twins'. But there are still lots of debate about how planets form so who knows....

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/onyE

While researching some information, I came across the following: "Use of the term accretion disc for the protoplanetary disc thus leads to confusion over the planetary accretion process, although in many cases it may well be that both accretion processes are happening simultaneously (e.g. T Tauri)."

Looking up protoplanetary disk, instead, I discovered the nebular hypothesis , and the following: "At the end of the planetary formation epoch the inner Solar System was populated by 50–100 Moon- to Mars-sized planetary embryos. Further growth was possible only because these bodies collided and merged, a process which took up to 100 million years. These objects would have gravitationally interacted with one another, tugging at each other's orbits until they collided, growing larger until the four terrestrial planets we know today took shape."

Upon reading this, I imagined that while the possibility of a binary planet remains low, it's certainly possible. Instead of requiring a highly unlikely encounter between two planets with sizeable moons, the situation of 50 to 100 planetary embryos made the possibility quite real.

So, with this - have any of your thoughts changed?

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"Toward no crimes have men shown themselves so cold- bloodedly cruel as in punishing differences of belief." - James Russell Lowell

I agree with those that say its unlikely but possible - & with the vast number of stars and planets that are out there surely there must be one or two examples in existence - although most likely a ve-eeery long distance away from us.

Then again we have two close examples that are nearly "double planets" like that in our own solar system : Pluto-Charon & Earth-Luna so maybe there're NOT that rare after all ... ?

I'd love to have such a pair of worlds discovered and imaged!

Robert L. Forward wrote an excellent "hard" SF novel 'The Flight of the Dragonfly' (New English Library, 1984) featuring two such (fictional) terrestrial planets (Eau & Roche - "water" & "rock") around Barnard's Star.

Aussie SF author Sean Williams in another novel had two gas giants in a similar situation although they were less integral to the plot.

Plus I'm sure there are other SF references folsk could offer.

You may enjoy reading those if you can find them ... then again they might have sparked this question!

Does it work better if one of the Earth mass planets is a moon of a Gas Giant to begin with and the other is the eccentric one with a large moon?

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Always challenge the assumptions

Here's one from OA
http://eg.orionsarm.com/xcms.php?r=o...=466c8fefdbfc5

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That's interesting you should mention this, as the dynamics of an Earth-sized planet with a Moon-sized Moon interacting with, say, a Jovian-sized planet with an Earth-sized moon could very well wind up with the Earth-sized planet forfeiting it's own moon and walking away with Jovian's moon.

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"Toward no crimes have men shown themselves so cold- bloodedly cruel as in punishing differences of belief." - James Russell Lowell

Mugs ,
although very unlikely , binary planets may form if there is enough chance to have multiple interactions between bodies .
I've set up a simulation of in total 200 bodies in an orginal circular path at 5AU around sun . An animation of what can happen can be seen here :
http://www.orbitsimulator.com/cgi-bi...m=1246783909/0

We have been here before... Several times have I said this; When you play these what if games. Any and all are possible however unlikely... Moon capture would be unlikely but not impossible. Many hours have I spent fiddling with the orbital machanics programs...http://www.orbitsimulator.com/cgi-bi...m=1246783909/0 That link of 'frankuitaalst's' is a good example... great.

Given the billions and billions of stars, however unlikely for it to occur in any one instance, it becomes exceedingly probably that there are millions of binary planets out there.

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"Toward no crimes have men shown themselves so cold- bloodedly cruel as in punishing differences of belief." - James Russell Lowell

I agree Mugs .
In a crowded environment capture is surely possible . But also the "de"capture , stealing is possible , as is shown in the animation in the link hereunder .

http://www.orbitsimulator.com/cgi-bi...1246783909/3#3

True, but given the number of moons in our solar system, I think the odds favor more being captured than released.

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"Toward no crimes have men shown themselves so cold- bloodedly cruel as in punishing differences of belief." - James Russell Lowell

Simple enthropy considerations would suggest the opposite.

What about the situation where a single large rocky body is impacted by another such that two distinct piles of debris re-accrete in the same vicinity and begin to orbit each other.

Is that more likely?

Rob

ah...

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"Toward no crimes have men shown themselves so cold- bloodedly cruel as in punishing differences of belief." - James Russell Lowell

I can imagine a situation where two equally sized impactors capture each other and become a double planet; this would be the same as the event that created the Moon, except that both planets would be the same size. However I think that even in a case like that one of the planets would end up bigger than the other one; perhaps much bigger.

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