In uk.sci.astronomy message dNednQy4CJfjqcjVnZ2dneKdnZydnZ2d@supernews.
com, Sun, 15 Jun 2008 16:25:46, Jim Hawkins
posted:
If there were another planet, same mass as Earth and in the same orbit but
at the precise opposite orbital position, we would presumably never see it
because it would always be eclipsed by the sun.
Is there any way we can be sure that there isn't such a planet?


Even if the Universe consisted solely of the Sun, Earth in circular
orbit, and counter-Earth, the system would not be dynamically stable; it
would be like an infinitely-sharp pencil balanced on its point in an
infinitely-hard flat surface. That conclusion is independent of the
mass of counter-Earth.

Plus what others have said.

The argument that the Earth's orbit is not circular, and hence its
angular velocity is not constant, implies that a body in the same
orbital path would not remain diametrically opposite. But a body in an
orbit of the same shape but "pointing" in the opposite direction to
ours, if initially behind the Sun, would remain so until the effects of
perturbations and instability accrued. That's analogous to L4 & L5
still working for elliptical orbits.

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