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Name: theta sig
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Date: 1993 - 1999


Question:
Is a binary orbit possible? I mean, if there was another planet where Earth would be in six months of roughly the same mass, would it affect Earth's orbit any? Are there examples of binary orbits(not suns) in the cosmos?

Any intelligent response would be appreciated.


Replies:
There was an old sci-fi movie about a twin planet to the Earth locked into orbit exactly opposite the Earth. I can't think of any reason why that wouldn't be possible. There is no evidence of such a planetary system, but then there is no evidence of any planetary system outside our own; but that should change soon after the Hubble Space Telescope is repaired late this year.

John (e-mail hawley)


I think the movie was titled "Journey to the Far Side of the Sun". If currently-accepted theories of planetary formation are correct, it is extremely unlikely that "twin" planets would form on opposite sides of the Sun. These theories say that tiny "planetesimals" condensed out of the primordial solar dus t cloud; over time, some of these bumped into their neighbors and stuck together. Eventually some of these became big enough to attract others gravitationally, and ultimately one which became the Earth dominated, sweeping out all of the matter in the vicinity of its orbit. Unfortunately, we have (so far) only one solar system to test the theories on.

How do we know there isn't a second Earth on the opposite side of the Sun? Well, Earth's orbit isn't perfectly circular; it's an ellipse, and the Earth doesn't move at a constant speed in its orbit, so even if at a given time there were an Earth-sized body in the same orbit directly opposite the Earth, it would not stay directly opposite. I don't know if such a planet would ever be visible (it might always be lost in the glare of the Sun), but its presence would still show up in the grav- itational perturbations it would exert on the other planets (including the Earth).

RC Winther



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