How do we find out the gravity of planets
By observing the orbits of their satellites.
About all you could want about gravity can be found at:
For planets, go to the center column about 2/3 of the way down, and click on the single-word "Gravity".
Gravity dictates the orbits of the planets around the sun (and as Janus said, the moons around the planets). By measuring those orbits, you can calculate the planet's gravity. See the link I provided in the other topic you started.
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