Let me clarify on the title, because this is important. In recent years, people have been feeling less comfortable calling the only natural satellite orbiting Earth a moon. It may not be the largest satellite in the solar system, but in proportion to its mother planet, it takes top billing. In fact, there's been talk that we should start calling Earth and moon "a binary planet".

Also in relation to this question is iron. This metal makes up 84% of the Earth's core, and it's known to have magnetic properties. There is recent evidence saying that the moon also has iron in its core, but its smaller size means that it orbits the blue planet, not the other way around.

But if our recent exploration into the universe is any indication, there are some things in the cosmos that defy explanation and conventional understanding. So is it possible for a large planet the size of Earth with a core that is 84% iron to orbit a smaller planetary body the size of Ganymede with a core that is 100% iron from a distance of, say, 426,000 miles?

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    $\begingroup$ 84%-iron Earth would still outweigh 100%-iron Ganymede by a large margin. You need to switch rocky Earth with a "gas dwarf" to get what you want. $\endgroup$ – Alexander Jan 26 '18 at 5:51
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    $\begingroup$ In reality, the Earth and the Moon both orbit a point of mass that they share. The moon does not orbit the earth any more than the earth orbits the moon. If you're not satisfied by that, a small super dense planet would certainly not rotate around a large light planet (given their mass was very different). $\endgroup$ – Clearer Jan 26 '18 at 12:31
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    $\begingroup$ If two planetary bodies orbit around a common center of mass, it is possible for a larger but less massive planetary body to orbit farther from the center of mass than a smaller but more massive planetary body. There would have to be a large difference in the density of the two bodies which would have a large effect on their possible structure and habitability. In the most striking examples neither planetary body would be habitable for Earth like lifeforms. $\endgroup$ – M. A. Golding Jan 26 '18 at 21:54
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    $\begingroup$ If you want an explanation based on the speculation that "some things in the cosmos that defy explanation and conventional understanding", you will not get anything sensible here. Also, your question reads as if you don't have basic understanding of modern astronomy (things don't orbit each other, they orbit barycenter) and lack basic facts (84%-iron Earth would still outweigh 100%-iron Ganymede, as @Alexander mentioned). $\endgroup$ – Mołot Jan 26 '18 at 22:13
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    $\begingroup$ You were okay right up until you specified the '84% iron' bit., Edit that out, and the question might have a positive answer. For a larger planet to appear to orbit a smaller planet, the larger planet would have to consist of only a fraction of the mass of the smaller planet. it is not size, but mass that determines which one appears to rotate around which. Perhaps if the larger were only 1% iron, and 99% fluff. Think inertia. Which one is easier to 'move'. A figure skater twirling another skater. The 'heavier' (more massive) skater remains at the 'center'. $\endgroup$ – Justin Thyme Jan 27 '18 at 3:39

Yes and no. You could establish two planets of that size in an orbital system of that range. You could establish two planets of ANY size in an orbital system of ANY range. However, in such a system, one body does not stay in place while the other orbits it. In any orbital system, what really happens is that the two bodies involved orbit a common centre of mass. (That is to say, the average position of all the mass in the two bodies.) In the Earth/Moon system, the Earth is so much more massive than the Moon that this centre of mass is inside the Earth, meaning that the Earth appears to move barely at all, while the Moon appears to orbit around it. However, if both bodies were of a similar mass, they would both orbit a point about halfway between the two of them. If you had a more massive planet and a less massive one, then the centre of mass would always be closest to the more massive one.

You seem to be asking whether you could weight a smaller planet with iron to make it more massive than a larger one so that the larger planet has less mass. In the case of an Earth-sized planet and a Ganymede-sized one, a back-of-a-napkin calculation tells me that the Ganymede-sized one would need to be about 500 times as dense as the Earth-Sized one, simply for them to have the same mass. If you wanted their centre of mass to be inside the Ganymede-sized planet, it would need to be about 250 times as massive as the earth-sized one, making it (very approximately) 125,000 times as dense. Considering iron is only about 3 times as dense as granite, even if the entirety of one planet was made of iron and the other of rock, you could only achieve 3 times the density, instead of the 125,000 times you would need.

(Note that all the numbers I just spouted are probably only correct to the nearest order of magnitude, but that is good enough for what I needed them to show.)

However, at the end of the day, there is no problem having the two planets just orbiting a common centre of mass somewhere between the two. They would still be in an orbital system, which would work like any other orbital system. It is important to remember that one body never simply "orbits" another, but rather the two orbit eachother in any orbital system, even one where the masses of the bodies are enormously disparate. There is never a parent body and a child body, but rather both bodies, orbiting in tandem.

  • $\begingroup$ Exactly this, the math may or may not be anything like perfect (I'm not even going to check, it's not important) but the sense of what's here is spot on. $\endgroup$ – Ash Jan 27 '18 at 11:20
  • $\begingroup$ +1. BTW, if you want a system with a larger/less dense thing orbiting a smaller/more dense thing, think bigger than planets and go for a neutron star. $\endgroup$ – Royal Canadian Bandit Jan 29 '18 at 9:31
  • $\begingroup$ Ganymede has a diameter of about 3300, vs. Earth's 8000. Volume is proportional to the cube of the diameter. Figure Ganymede's diameter as about 40% of Earth's. and the volume is about 6.4%, meaning that Ganymede would only need to be 16-17 times as dense as Earth to have the same mass. I'm not saying this is feasible, only that the factor of 500 is way off. $\endgroup$ – David Thornley Jul 27 '18 at 18:29

Sure. If the smaller planet is very dense and the larger planet is not. Mass-wise no. But I don't think that's what you are after.

Check out this page https://en.wikipedia.org/wiki/Barycenter to get a feel for how planets of various sizes (Mass) might interact. The Gallery section is really nice.

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    $\begingroup$ if you want to answer the question, provide an explanation and not just a link $\endgroup$ – L.Dutch Jan 27 '18 at 17:55
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    $\begingroup$ Expecially the time-lapse animation of the Pluto-Charon system, with a barycenter outside Pluto. $\endgroup$ – jamesqf Jan 27 '18 at 19:15
  • $\begingroup$ Please don't ask questions in answer. This is not a forum. If you think question needs to be clarified, that's what comments are for. $\endgroup$ – Mołot Jan 27 '18 at 23:07
  • $\begingroup$ @Mołot edited. (I can't comment without more reputation) oh well lol $\endgroup$ – MParm Jan 27 '18 at 23:43
  • $\begingroup$ @Mołot It's a rhetoric question. It doesn't need to be answered by the OP. But, yes, normally questions shouldn't be asked in answers. $\endgroup$ – a4android Jan 28 '18 at 1:09

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