| MadSci Network: Astronomy |
Indeed, if another planet was following an orbit around the Sun exactly symmetrical to that of Earth's, it could never be seen directly from the Earth. As you said, flying a space probe away from the Earth and looking around the Sun might be one way of checking for the existence of such a planet. Another way would be to look for perturbations of the orbits of Venus and/or Mars by the gravity of that hidden planet. That would only work if the planet was massive enough, an asteroid would not generate enough gravity to tug Mars or Venus around significantly.
This said, the existence of such an object is very unlikely. We already know that the 8 other planets of the Solar System did not form in symmetrical pairs so why would Earth? Also it would not be consistent with present ideas about planet formation which involve the condensation of a ring of material from a protoplanetary disk. Each budding planet sweeps the material in a ring and so there is nothing left for another planet at the same distance from the Sun to be made up from.
marc herant, astro moderator, mad scientist network
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