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At what speed is our solar system expanding, and if it is, why don't the positions of the sun and planets change, or grow farther apart? Why is the Big Dipper still where it is?

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Our solar system isn't expanding, because it's bound by gravity. Even though space is expanding, the positions of objects in the solar system stay the same because gravity pulls them back.

This is true for all gravitationally bound objects, even galaxies and galaxy clusters.

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  • $\begingroup$ So basically; GRAVITY RULES. No gravity, no organic life then? $\endgroup$ – Mark Sottile Jan 18 '16 at 19:28
  • $\begingroup$ What do you mean by "bound by gravity"? Do you mean that, for example, a comet on a parabolic trajectory around our Sun will be affected by space expanding (since it is not bound to a closed orbit), but a comet on an elliptical trajectory will not? Or do you mean that if the gravitational force is of sufficient strength then you won't notice the effects due to space expanding? $\endgroup$ – mbeckish Jan 18 '16 at 19:31
  • $\begingroup$ I thing gravity is under appreciated. Without it we wouldn't have an atmosphere, or suns or planets. Thank God for Gravity. $\endgroup$ – Mark Sottile Jan 18 '16 at 19:35
  • $\begingroup$ In general the expansion of space competes with the attraction of gravity. At local over-densities (which are now galaxies, galaxy clusters, etc), the force from gravity can be strong enough to pull everything together faster than space can expand them apart. The recessional velocity due to the expansion of space scales linearly with distance, hence gravity dominates on small scales and expansion dominates at large scales (in between galaxies). $\endgroup$ – Judge Jan 18 '16 at 20:03
  • $\begingroup$ @Mark Sottile, yes indirectly. No gravity, no structure formation (cool visualizations of simulated structure formation), no galaxies, no solar systems, no us. $\endgroup$ – Judge Jan 18 '16 at 20:08
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And if you still need an answer for your speed of expansion (of the universe, because the solar system isn't expanding just by itself), it is measured to be about 74.3 km/s per megaparsec (a megaparsec being about 3 million light-years)

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  • $\begingroup$ Wow! That's really far ouut $\endgroup$ – Mark Sottile Jan 18 '16 at 19:31

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