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Astronomy

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    $\begingroup$ Even in galaxies withy central black holes, the gravity of the stars and gas (and dark matter) is enough to hold it together; the black hole is almost always a very small fraction of the total. (E.g., the Milky Way's central black hole is about one ten-thousandth of the combined mass of the Milky Way's stars.) $\endgroup$
    – Peter Erwin
    Commented Oct 15, 2018 at 7:08
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    $\begingroup$ Some references would be nice. $\endgroup$
    – pela
    Commented Oct 15, 2018 at 7:42
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    $\begingroup$ Upper limit of 1500 solar masses on any possible BH at center of M33: iopscience.iop.org/article/10.1086/323481/meta $\endgroup$
    – Peter Erwin
    Commented Oct 15, 2018 at 10:56
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    $\begingroup$ @T.E.D. That analogy doesn't quite work, because we know that the black holes in the centers of galaxies are just a tiny part of the mass of the whole galaxy. On the other hand, planetary systems are probably utterly dominated by the mass of the star(s) - and our solar system is already far more massive than most star systems. Of course, our observations of other planetary systems are still very limited, so there might be some surprises. But we already know you can have complex "planetary systems" without a star - just look at Jupiter or Saturn. $\endgroup$
    – Luaan
    Commented Oct 15, 2018 at 13:26
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    $\begingroup$ I think most galaxies - which are dwarf galaxies, don't have black holes. $\endgroup$
    – ProfRob
    Commented Oct 15, 2018 at 18:24