My dad told me his theory of another planet just like earth in our solar system. So he believes that there is another planet on our orbit exactly on the other side of the sun that's just like earth. He says that they move at the exact same speed in the same orbit around the sun, never to meet eachother. I...
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answers:
Brigalow Bloke say: He's pulling your leg.
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ANDRE L say: No, not in the least.
We have sent out spacecraft that have viewed the whole of the inner Solar System and found no such planet. It's an old popular fictional concept, but basic orbital mechanics absolutely makes it impossible, as there is just no way for the orbital perturbations to balance out between the two planets.
There was a fun movie made about this concept, though, titled Doppleganger, AKA Journey To The Far Side Of The Sun, which came out in 1969.
The additional problem is that, even if a mirror Earth could exist, there would be no reason why life on it would be at like it is here. Evolution here has taken many steps of differentiation, that if we were to start it all over again, on the Earth as it was 4 billion years ago, it's a sure thing that the final result of Evolution 2.0 would be very, very different from what we got from 1.0.
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quantumclaustrophobe say: Well, it was a rumor, not really a theory, for a while; there was even a movie about it; but, over time, micro-differences in each planet's orbit would've brought the two together, and that never happened... Voyager took a 'family portrait' of our whole solar system in the 90's - and, Earth was the only planet in our orbit...
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Dump the liberals into Jupiter say: That theory is easy to disprove. Earth's orbit is an ellipse. Things moving along elliptical orbits move faster near periapsis and slower near apoapsis, due to the conservation of angular momentum.