There’s been numerous sci-fi stories in which there’s another Earth in the same orbital plane as our Earth, but on the opposite side of the Sun moving at the same rate. Because of this, the characters on both planets can (usually) never see the other planet.
Could this happen? Would there be any ill effects of this?
Taking the Earth as an example, how many could be in the same orbital plane before their gravitational pulls adversely effects the others?
That title should read “in the same orbit”.
For one thing it sounds as if you are referring to 1969’s “Journey To The Far Side Of The Sun”.
As for another Earth precisely 180º away from us in Earth’s orbit ? That would be VERY difficult to achieve. Think of the recent transit of Venus, the last of which occurred about 120 years ago. That is one the RARE times that the Sun Earth and another planet are in the same orbital plane.
Also, (as most Sci-Fi films overlook), even though we can’t see a planet doesn’t mean we can’t detect it. Neptune was discovered by its “perturbation” of the other planetary orbits. SO, if there were an Earth directly opposite of us (at the “Far SIde of the Sun” if you will), we certainly would have detected its presence by now.
As for deleterious gravitational effects on the Earth? Probably negligible, but I’ll leave that up to the more knowledgable astronomers here.
I wonder what Bizarro SDMB would be like…
Any planet in the same orbit as Earth, but on exactly the other side of the sun, would pretty quickly be drawn out of that position by the gravitational effects of the other planets in the Solar System. I’ve seen … somewhere … claims that it would take as little as thirty years before such a “Counter Earth” would be far enough out of its original position to be detectable by astronomers … and it’s been rather more than thirty years since the Solar System was formed.
(I should look for a cite for that, shouldn’t I … ?)
Could Earth and the other planet be at different Lagrange Points?
Consider that the solar system is over 4 billion years old. It’s unlikely that two equal-sized planets would form on opposite sides of the sun at the same orbital radius in the first place. Even if that had somehow happened, it’s unlikely they would stay 180° apart for over 4 billion revolutions. (The planets’ orbits are pretty close to constant, but they’re not that constant.) Before long — a few millennia maybe — they would catch up to each other. Then, depending on the distance of approach, they would either collide, or if they missed, one of them would be ejected from the solar system while the other was sent into, or nearly into, the sun.
Incidentally, I have another problem with that movie (Journey…) besides the ones being discussed. The movie’s premise is not merely that there’s another Earth-like planet opposite from us, but that it’s an exact mirror image. As in, if I spill a cup of coffee here, and make a carpet stain the shape of Wisconsin, then on the other Earth, there’s a mirror-image duplicate of me spilling his coffee at the exact same instant, making a carpet stain the shape of Wisconsin, reversed. And he’s reversed too, as is the surrounding room, and all the lettering on the signs and in the books. The two Earths are supposed to be atom-for-atom mirror-world duplicates of each other.
However, the universe doesn’t share that same perfect symmetry, or anti-symmetry as the case may be. Here on Earth #1, as we stand outside at night, I might point up at the sky and say “Hey look, there’s Orion.” Meanwhile, on Earth #2, there’s a duplicate of me saying and doing the same thing — but there’s no Orion up there. They’re on the other side of the sun after all.
So, it would be easy to know which Earth was authentic, and which was the (creepy) phony.
INAN astronomer (is anyone here?!)…but I doubt that it’s possible for the formation of two such planets to occur. At least, many many times less probably than it was for the Earth to exist in a way to support organic life. Which I mention because improbability doesn’t preclude a system as described existing somewhere in the Universe. But the probability of the two existing and being capable of supporting life are obvioulsy compounded.
Nope, the Lagrange points of stability depend on the three bodies being very different in mass. That is, M[sub]1[/sub] >> M[sub]2[/sub] >> M[sub]3[/sub].
L4 and L5 are stable, but then you’d be able to see the anti-earth.
The one directly accross from us is L3, and is very unstable. Its orbit would get out of whack in less than a year.
More on LaGrange points here.
And that too. 
If it’s of any help…Effects of adding another planet to the inner solar system?
How far away from L3 would another planet need to be to avoid such drastic instability? Could it potentially still be within the angle disguised by the sun?
As Bytegeist pointed out, there is L3 stability if the body there is the same mass as the Earth. The stabilty depends on M1<<M2<<M3.
D’oh, that should say “there is NO L3 stability…”
The idea is certainly older than that, there was an early story of Doctor Who (written but never filmed) using the idea in 1963. Probably wasn’t original then.
Anyone have an earlier example?
Of all places, Gamera vs. Guiron, but that was 1969.
For some distance, the further away it get from L3, the more unstable is its position.
The L4 and L5 Lagrange points are the only stable ones. They are the only places where a small body is “encouraged” to stay in place by the masses and the dynamics of the other two bodies. The other three L-points are points of unstable equilibrium. No object without its own propulsion system, activated frequently and intelligently, could remain there.
Moreover, the L4 and L5 points don’t even exist at all unless the mass inequality holds: that the primary object (the Sun or Earth) is much more massive than the second object (the Earth or Moon), which in turn is much more massive than the third object (the asteroid, the space probe, or what have you). A twin Earth definitely violates this constraint, unless perhaps it’s made from paper maché and chicken wire. And these were rare materials in the early solar system.
The only way to have another planet hidden on the other side of the Sun from us is to have its orbital plane almost exactly match ours (a little unlikely, but it could happen), and to have its orbital period match ours to one part in a billion or so (astronomically unlikely). But even if things started in that state, the planets’ orbital periods fluctuate chaotically over time. There would then be a “random walk” in the space of orbital periods, and therefore in the relative angular velocities as well, and before you know it one of the planets is catching up to the other one.
I wish I’d studied more astronomy!
So, basically, if there was another Earth on the opposite side of the sun, even under optimal conditions, the two planets would likely smash into each other eventually (assuming one isn’t flung out of the solar system), right?
Shouldn’t it be “there is no L3 stability.” Period?