"OG" wrote in message
...

"Jim Hawkins" wrote in message
m...
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?

Jim Hawkins


It would be detectable due to its perturbation of the orbits of other
solar system objects.


Yes.

And perturbations by other planets would move it out from behind the sun
where it could be seen, and from time to time it would approach the
longitude of the Earth, when all hell would break loose due to tides while
the two passed each other. The situation would resemble the co-orbital
moons of Saturn, Janus and Epimetheus.

Even the "presumably" of the OP is wrong. At total eclipses of the Sun, any
Earth-similar planet on the opposite side of the Sun would be directly
visible more times than not. The Earth's orbit isn't circular and Kepler's
second law would apply, so it would not always be exactly 180 deg different
in true anomaly.

Objects like Cruithne (which has the same period as Earth) could not exist
in a captured orbit if an Earth-like planet had a similar orbit. This goes
back to OG's point, of course.

--
Mike Dworetsky

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