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If two planetary bodies were tidally locked to each other (e.g. pluto and charon) then could there ever be a situation where the smaller of the two bodies had the 'heavier' gravity? Could you ever weigh more on the smaller planet?

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    $\begingroup$ Gravity is a function of the mass so your smaller body would have to be denser to have a stronger gravity $\endgroup$ – Riff Nov 7 '16 at 11:40
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    $\begingroup$ That @Riff said. You just need to make it so that the smaller body has a higher mass (or the same mass, but being smaller means you are closer to the center of gravity which makes for a higher force of gravity on you). $\endgroup$ – MichaelK Nov 7 '16 at 11:42
  • $\begingroup$ Aside from the density there is also the angular momentum to consider. $\endgroup$ – user6760 Nov 7 '16 at 12:37
  • $\begingroup$ @user6760 Not with tidally locked planets, which can't rotate fast enough to have a significant effect on surface gravity. $\endgroup$ – Mike Scott Nov 7 '16 at 13:31
  • $\begingroup$ @Mike Scott: if the orbital plane is tilted and eccentric... $\endgroup$ – user6760 Nov 7 '16 at 13:32
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Gravity depends on the mass of a body. So you can adjust the radius of a planet as you see fit, just also adjust the density of the object to compensate.

But when it comes to orbits, the more massive planet would be the primary planet, a satellite is always the least massive. However, technically, two gravitationally connected bodies actually orbit a single point in between them - their centre of mass - it only appears that one orbits the other because one is more massive and the centre of mass is much closer to that body. Pluto and Charon are of very similar mass and actually orbit a point in between them.

Of course this depends on your perspective, standing on Charon you might think that Pluto orbits it, and vice-versa. Humans of course thought everything orbited the Earth for millennia

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Tidal lock is irrelevant. All you're asking is if a planet can have a heavier surface gravity than a larger planet, to which the answer is a definite yes. Just for example, Uranus is much larger than the Earth, but its "surface" (cloud-top) gravity is only 89% of the Earth's, because it's much less dense.

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  • $\begingroup$ That was probably the wrong wording to use! Sorry. I meant, could the surface gravity ever be higher on the satellite of a planet, rather than the planet itself? $\endgroup$ – R. Tierney Nov 7 '16 at 14:38
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    $\begingroup$ @R.Tierney The same answer applies. If a body exactly like the Earth was a satellite if Uranus, it would have a higher surface gravity than its primary. $\endgroup$ – Mike Scott Nov 7 '16 at 14:46

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