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Okay, there are two habitable worlds in the Solar System. One of them is Earth. Human history goes mainly the way it really went. Then, there is another planet with a sentient, equally advanced species of humanoids on it.

EDIT The other habitable planet would be on a separate orbital path from the Earth.

My questions are these:

  • Where could I put this other planet.....I considered having it in place of the asteroid belt, is this too far away. Mars has to also exist in its current location and state, as well a Venus.

  • How might us and this species interact? Would we more likely trade or go it war over something petty?

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    $\begingroup$ possible duplicate of Mars as "Earth sister" $\endgroup$
    – Serban Tanasa
    Feb 24, 2015 at 16:56
  • $\begingroup$ Nope, they want Mars around too. They need to check out what makes a planet habitable for humans $\endgroup$
    – user3082
    Feb 24, 2015 at 16:59
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    $\begingroup$ No....in the question I think I made it clear I want the other habitable planet (let's call it Terra 2) to be separate from Mars. $\endgroup$
    – Jax
    Feb 24, 2015 at 17:10
  • $\begingroup$ Does it have to be another planet? Given the known structure of our solar system it would be easier to wedge in a habitable moon around Jupiter or Saturn. Alternately, would it be okay to adjust Venus, Earth and/or Mars's orbit to fit the new planet? $\endgroup$
    – IchabodE
    Feb 24, 2015 at 17:31
  • $\begingroup$ I am considering pushing mars farther out. $\endgroup$
    – Jax
    Feb 24, 2015 at 17:43

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It would definitely be possible. It would not, however, have its own orbit.

Right now, the inner solar system is pretty full, gravitationally speaking, without much in terms of room between the inner planets for more additional planets. Your new planet, Terra II, would have to share its orbit with one of the existing planets.

How can two planets share an orbit?

In our solar system, most things that share an orbit with other things do so either as moons of other planets, or by hanging out in their Lagrange points, like the Trojan asteroids do with Jupiter. You could stick an Earth-sized planet in the L4 or L5 Lagrange points, and it would be gravitationally stable.

This isn't however, the most likely way for Terra II to come to exist. While stable, the Lagrange points don't tend to be home to large bodies, probably due to the fact that the formation of a large body involves lots of energetic collisions, which are great for knocking things out of subtle gravity wells like the Lagrange points.

What's more likely is that Terra II would act as a binary partner for one of the other inner planets, such as Venus or Earth. Picture the Moon, if you will. Now picture it the size of Earth. Both planets would be large enough for an atmosphere, and the binary system would be gravitationally stable. While I don't know of any planets that have been found (so far) in such a system, we've found stars that orbit each other like this. The Castor system, for example, consists of two pairs of binary stars that orbit each other, and which are in turn all orbited by a more distant pair of binary stars. Planets could do the same.

How would we communicate?

Well, if both planets had intelligent life at the same time, we could do so with radio waves. We'd probably have sent a manned mission there, as well, since "We're talking to some guys on Mars and want to go say hi," is a more compelling argument than, "We want to make human footprints there, because America."

Unfortunately, intelligent life is extremely rare. It's more likely that only one planet would be inhabited by sentient creatures. We'd still have some strongly compelling reasons to go visit, but it's unlikely that we'd find anything there that resembles us.

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    $\begingroup$ "You could stick an Earth-sized planet in the L4 or L5 Lagrange points, and it would be gravitationally stable." No, you can not add planet sized things to lagrange points and expect stability, it changes the system entirely, it won't even be a lagrange point after that. Lagrange points are for significantly smaller objects, not planets. $\endgroup$
    – Samuel
    Feb 25, 2015 at 18:11
  • $\begingroup$ Why would changing the size of the bodies perturb the system? The L4 and L5 Lagrange points represent the equilinear solutions to a circular three body problem. As such, they are maxima of effective gravitational potential for the system and will exist and be stable regardless of the sizes of any of the bodies in the system. $\endgroup$
    – ckersch
    Feb 25, 2015 at 18:30
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    $\begingroup$ The third body in the circular restricted three-body problem must be significantly smaller than the other two. The statement "[L4 and L5] will exist and be stable regardless of the sizes of any of the bodies in the system" is clearly wrong, except where the ratio of the masses remains the same. Just consider an extreme case, place a supermassive black hole in L4 or L5 of the Earth-Sun, do you still argue that the system will be stable? Clearly, there is a maximum size for the third body to be stable; it must be small. $\endgroup$
    – Samuel
    Feb 25, 2015 at 18:48
  • $\begingroup$ It doesn't matter how massive the bodies are, so long as no body is inside another body's Roche limit. In an equilangular solution to a circular three body problem (not necessarily a circular restricted tree body problem), the resultant force on each body from the other two bodies acts through the barycenter of the system. This is true even if one of the bodies is a black hole. Why would it change? I can put the equations up here to prove it later, if you'd like. $\endgroup$
    – ckersch
    Feb 25, 2015 at 19:14
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    $\begingroup$ I don't know what a non-restricted circular three body problem is, please describe it. I'm not saying you can not get a stable orbit out of three bodies of arbitrary mass, I'm saying you can not add an arbitrary amount of mass to the Earth-Sun lagrangian points and expect stability. The equations aren't going to help you, but please add them if I'm wrong. $\endgroup$
    – Samuel
    Feb 25, 2015 at 19:45
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The habitable zone is linked in here. Also, larger than Mars (see same link) if you want humanoids.

Short answer: it would have to be located between Earth and Mars.

Earth is on the edge of the habitable zone. This says Earth is on the inner edge of the habitable zone.

I don't know if it would be feasible for it to have formed there (due to gravitational constraints), it would probably have to have been a capture.

Remember, no Counter-Earths.

Interaction

No (large-scale) trade, nor war. Given current tech, it's way too expensive to do either. We could trade information, however. Also, perhaps seeds or animals. Animals might do better than plants, due to evolutionary constraints. Possibly run ex-pat enclave, or maybe an embassy - but that would be a long-term appointment, and maybe a life-time one too.

Also, it would've changed Earth history. Sometime around when we started 'discovering' canals on Mars, or started monkeying around with radio technology we would've had a target (and they as well). Probably would've started a space race and or a focus for creative energies outside of warfare.

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  • $\begingroup$ Could you link your answer to why "no Counter-Earths"? I know there isn't one, I'm just fuzzy on why there couldn't legally be one. $\endgroup$
    – IchabodE
    Feb 24, 2015 at 17:33
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    $\begingroup$ @MBurke the counter-point is unstable. If you put a copy of the earth in the same orbit directly opposite of us, then one or both of earths would get pulled out of those orbits in comparably quick time. $\endgroup$
    – Peteris
    Feb 24, 2015 at 18:48
  • $\begingroup$ Once again, a citation would be useful. My understanding is that this is a Lagrange point (specifically L3), an area of inherent stability. Also, it has not yet been stated that this has to be Earth-size. $\endgroup$
    – IchabodE
    Feb 24, 2015 at 18:52
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    $\begingroup$ @MBurke For placing a planet at Sun-Earth L3, please see What are the possibilities of a dwarf planet orbiting opposite Earth's orbit?. $\endgroup$
    – user
    Feb 24, 2015 at 19:05
  • $\begingroup$ @Mburke Well, it's to have sentient humanoids on it - it has to be larger than Mars, and/or roughly Earth-size or larger - see habitable planet considerations. $\endgroup$
    – user3082
    Feb 24, 2015 at 19:59
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As a location, I would suggest that it be a larger world with Mars as its "moon". If large enough, it would have a thicker atmosphere, so would be able to have a warmer atmosphere in spite of being on the outer edge of the goldilocks zone.

Now, to your actual question.

There would be war. There would be trade. There would be all manner of interactions.

Basically take any example from the age of exploration between any distant countries. There will be similarities. Even if significantly different from each other, it wouldn't increase the negativity too much, as humans tended to treat humans of "inferior" cultures as "non-human" anyway.

Hopefully by the time the technology progressed to allow physical contact, social progression will allow a more peaceful interactions.

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