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Duplicate Earth and Moon, where should I put it?

This question partly arises from the criticisms of stories that claim a hidden planet obscured by the sun, how it would have been detected and if sharing earths orbit it would eventually approach earth, potentially wreaking havoc.

Interactions with other planets slows/speeds one more than the other and they collide. This all arises from the debunking of other stories and mythologies claiming there is another earth in exact orbit but other side of sun. Well that is said to be impossible. Would have been detected by many means. That concept is fairly well covered here. https://www.youtube.com/watch?v=U-P_EjZKbaM

I am NOT claiming there is a duplicate Earth in our orbit hiding behind the sun. But I am considering scifi-magically creating one and I'm wondering in what sort of orbit I would place it in our solar system, how long it would be before it would threaten disaster and potentially collide with earth. I mean if that's 5 billion years away when the sun turns red giant...

I realize it would not be a stable orbit in relation to earth (What would you call an orbit that remains roughly 180 degrees away aside from variances caused by elliptical orbit?) A half year away. How long before the gravity of passing objects such as Venus, Jupiter and Mars makes a duplicate earth collision possible?

Would it be a less disaster prone situation if instead of second-earth sharing the same orbital path, it's elliptical orbit was 90 degrees out of phase and tilted? Where best to locate the dual planet (with moon)?

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    $\begingroup$ Where would I put a backup, I mean, duplicate copy of Earth? No where near this solar system. $\endgroup$
    – Draco18s no longer trusts SE
    Commented Jan 11, 2016 at 20:45
  • $\begingroup$ I'm not exactly sure what you are really going for here. $\endgroup$
    – bowlturner
    Commented Jan 11, 2016 at 20:52
  • $\begingroup$ There is a sci fi method of creating said duplicate earth. (which may as well be magic in this context) $\endgroup$
    – Jazzylee
    Commented Jan 11, 2016 at 20:54
  • $\begingroup$ What I'm going for. I'm hoping to plop another earth out there in an orbit that will be at least be similar to original earths. But I'd like to do it in a way that they don't end up smashing into one another at least for a few billion years. Also with an eye toward avoiding other calamities such as comets - though if nothing else I can have the sci fi tech handle it. $\endgroup$
    – Jazzylee
    Commented Jan 11, 2016 at 20:57
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    $\begingroup$ Given a duplicate earth with the same mass, orbital speed and orbit diameter, but placing that orbit perpendicular to that of our earth, such that the two planets only intersect each other's orbits at two points. Then placing each planet along its orbit in such relation that each reaches the opposite intersection point at the same time. Would that cut down the likelihood of a planetary collision? Would it buy enough time for either or both of the planets to evolve intelligent life? $\endgroup$
    – Henry Taylor
    Commented Jan 12, 2016 at 1:51

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In the linked video the idea of Counter-Earth is presented.

The Counter-Earth idea is a planet of the same size and similar characteristic to earth that would be going around the sun on the same orbit... but on the "opposite side", located at the L3 Lagrangian point. In such way that from Earth, Counter-Earth is hidden behind the sun all the year.

Yet the L3 (along with L1 and L2) are saddle points. That means that they are in unsatable equilibrium. And any small influence will make the orbit of the object diverge. This will inevitably happen due to the influence of Venus and Mars.

I don't have the means to calculate if it would end up close to Earth, fall to L4 or L5, to ejected from the orbit.

Assuming an object could be kept in place in L3 - for example an space station that corrects its course to stay on L3 - it can be detected with space age technology - or even before that. There would be perturbations that could have been measured over the years and solved to indicate the presence of the object with the technology and math of the 19th century.

Also, 21th century space telescopes would have detected it. In particular STEREO which is designed to have its own orbit around the sun to study it.

On the other hand L4 and L5 allow stable orbits for small objects. These objects are known as trojans. The objects placed in these locations need to be smaller to keep the system stable. We have an example with Jupiter trojans that are divided across both points. There would be objects in both L4 and L5, but not on equal proportion. While objects in these location have stayed there for centuries, they may be unstable in the long run. It should also be pointed out that on the case of jupiter the influence of the next and prior planets is lesser than that from Venus and Mars for Earth.

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  • $\begingroup$ Apparently, a small planet can not be placed on the opposite side of the sun... worldbuilding.stackexchange.com/questions/8651/… $\endgroup$
    – Mikey
    Commented Jan 12, 2016 at 0:31
  • $\begingroup$ @Mikey I did search more about it to confirm that, and you are right... map.gsfc.nasa.gov/mission/observatory_l2.html - updated the answer. $\endgroup$
    – Theraot
    Commented Jan 12, 2016 at 2:24
  • $\begingroup$ I don't care for this story if the other earth is detectable or not. Just looking for a reasonably stable orbit that wont interfere with earth. Then again, maybe I just go with the predictable chaos that could ensue. ie earth gets ejected from orbit. Wonder what time frame that could occur in? $\endgroup$
    – Jazzylee
    Commented Jan 12, 2016 at 16:13
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    $\begingroup$ Lagrange points makes sense only if mass of third body is negligible in comparison to mass of two main bodies. Thus, using them when you have two planets of similar mass makes no sense, and leads to false results. $\endgroup$
    – Mołot
    Commented Oct 15, 2017 at 15:16
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Counter-Earth is too massive to stay in any of Earth's L points. As Theraot points out only L4 and L5 are stable for objects much smaller than the main body. Another Earth would be too massive, and also Jupiter is too massive so it would also perturb Earth's L points.

So you need to move Jupiter.

Move Jupiter to Earth's orbit and then put Earth and Counter-Earth as Jupiter's trojans in Jupter's L4 and L5 points. For bonus points use Venus as the Counter-Earth so the inner solar system is free from any massive perturbers. Mercury and Mars are probably too small and far away to affect this setup.

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Given today's level of technology it would be impossible to not know about the other Earth that has the same orbital period that we do within a few years or less no matter where you place the other Earth.

If you placed the other Earth exactly opposite from this one, then the period of time before the worlds collide should be on the order of thousands to millions of years.

If you are willing to have the orbits clearly visible to each other then you can have a system that would be stable over millions to billions of years.

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