* The Earth’s orbit is elliptical. This means the other planet wouldn’t always be precisely behind the sun from our perspective.
* We have sent a number of probes into the outer solar system, some of which have taken composite panarama photographs of the entire system, and they have seen nothing.
* Each of the planets has a measurable gravitational effect on the other planets as well as on asteroids and comets. A planet on the other side of Earth’s orbit would be affecting Venus, Mars, and comets/asteroids in ways we could detect. None of these bodies behave in ways we can’t account for.
* None of the other planets in our solar system have “twins” in their orbits, and no planets we have found so far in other star systems have either. Why would Earth be unique in this regard?
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