Quote:
Originally Posted by Steven Lake
There's also something else people fail to consider. Gravity is inversely proportional to mass. The more heavy elements you have, the heavier the gravity will be. Why do you think Jupiter can be 11 times more massive than Earth and yet have only 2.5x's the gravity of Earth? It's because Jupiter has no heavy metals. The heaviest known elements in Jupiter's atmosphere are helium (10%) and hydrogen (90%) and given that we have elements as dense as plutonium on our planet, plus a massive sphere or iron at the planet's core, our gravity is naturally heavier.
If you go to Mars, our nearest neighbor, which is slightly over half the size of Earth, it has only about 1/3rd our gravity, and the only reason it has that is because of the very high iron and lead content on the planet. (it's not called the "red" planet for nothing) So there are a lot of factors that affect the gravity of a planet. The best thing to do in cases like that is simply to go with my first suggestion and avoid really getting into the details of the size, gravity, etc of the planet.
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I'm sure you meant to say the gravity is *directly* proportional to mass.
Assuming Newtonian physics, the gravitational equation is F=G*m1*m2/r^2,
where F is the gravitational attraction force, G is the gravitational constant, m1 and m2 are the masses of the two bodies (in this case, the planet's mass would be several orders of magnitude larger than anything on it) and r is the distance between the respective centres of gravity (in this case the radius of the planet).
THe important thing here is that size is *not* the only factor determining mass. The density, which is determined by the composition of the planet, is a huge factor. Hence the differences between the rocky planets (inner 4) and the gas giants (outer 4).