I've read that many stars and other celestial bodies are found to constitute binary systems where the two bodies spin around each other. But our Sun is one of the exceptions. Could it be possible that the Sun was also a part of a binary system long ago ?
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We can be pretty sure that the Sun wasn't part of a binary system, because in a binary system there is no mechanism for the stars to separate. In a system of three or more stars it's possible for the energy to be divided in a way that ejects one star while leaving the others bound. But this won't work for a system of only two stars.
I suppose it's conceivable that the Sun might have been ejected from a system of three stars, but again this is very unlikely. Firstly the ejection of the Sun would almost certainly have stripped it of all it's planets, or at least left them in wildly eccentric orbits. Secondly if the Sun had been ejected from another system you'd expect it to be moving fast relative to the stars around it, and this isn't the case.
answered Jun 14, 2012 at 16:38
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$\begingroup$ We could have a dark matter star companion! That's kind of exciting... $\endgroup$– kηivesJun 14, 2012 at 16:47
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$\begingroup$ Dark matter is very unlikely to form stars. See physics.stackexchange.com/questions/29852 for why this is. $\endgroup$ Jun 14, 2012 at 16:53
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$\begingroup$ If we had a (dark matter) star companion, we would see much less regular orbits within the solar system than we currently have, as well as strange movements of the sun. $\endgroup$ Jan 25, 2013 at 17:07