Wouldn't other factors aside from size influence a planets gravity?

Question

Lets say that earth had a heavier core for instance, wouldn't that have increased its gravity, and couldn't a larger solid planet have a light gravity depending on how heavy or light its core was?

Answer 1

Gravity depends entirely on mass, not size. So you're on the right track.


*****
I thoght about this question a little more and would like to add that two planets with the same mass but of different sizes could have different gravitational forces on their surfaces because the closer you get to the core of the planet, the stronger the gravity. So you would feel heavier on a smaller planet than on a bigger one with the same mass.

Answer 2

The force of gravity you feel from an object is determined by mass and distance. The further you are from the centre of the mass, the less force you feel due to gravity. The massive gravitational attraction of a black hole, for instance, arsies becase the black hole is so much smaller than the star that collapsed to form it that you can get much much closer to the centre of mass, and therefore the centre of gravitational attraction, than you could before.

So let's try a thought experiment. You have three planets. One is the Earth. The second planet is the same size as Earth but made entirely of iron, and the third is the same size as Earth but made entirely of polystyrene packing material. If you travel to the iron planet you'll feel much heavier because the mass of the planet is much greater, even though the size is the same. Converseley, if you go to the polystyrene foam planet you'll feel much lighter because the mass is considerably less.

But there's a 'real world' example of this phenomenon in our own back yard, astronomically speaking. Saturn is a gas giant planet. It has a diameter about nine times that of Earth and a mass 95 times greater than Earth's. That means it has a much lower density than Earth. If you assume the same density, then as you increase diameter by a certain factor, the volume (and therefore the mass) goes up by the cube of that factor. That is to say, if you double the diameter, you would multiply the mass by eight (2x2x2). Clearly Saturn doesn't add up. A 9x Earth diameter planet of the same density would be 729 times greater in mass. Saturn must have a much lower density than Earth.

Saturn is a gas giant, so does not have a solid surface that you could stand on. However, it is the convention to define the 'surface' of a gas giant planet as the distance from the centre at which the pressure of the atmosphere is equal to the pressure of Earth's atmosphere at sea level. Using that definition, the calculations show that an object at Saturn's 'surface' actually feels more or less the same force due to gravity as an object on the surface of Earth, even though the planet is so much larger.

So yes, size is not the only factor influencing gravity.

Answer 3

Density is indeed a factor.

Two planets with the same mass but wildly different densities will have significantly different diameters.

The further away you are from the center of mass, the higher you are in the gravity well. The lower the effect that gravity will have on you.

As has been mentioned, the centrifugal effects experienced by an extremely fast rotation can also affect the perceived weight of an individual on the surface of such a planet.... however the gravitational pull will still be the same.. just offset by the rotational effect.

Answer 4

The size of a planet will effect its surface gravity. For example, if the earth's mass was the same but it was only half as big across, the surface gravity would be 4 g's (g = 1 / r²). Rate of spin will effect gravity near the equator. If the earth was spinning at a rate of about 1.4 hours per spin, centrifugal force would exactly cancel out gravity on the surface.

Answer 5

Size doesn't matter. Mass is what matters. Gravity is a well created in the fabric of space/time. The greater the mass, the greater the well depth. However, the density is not an issue, as you are not dealing with a two dimensional fabric, it is the mass, centred at a particular point. A planet of a trillion trillion tonnes in mass, would warp space time the same as a singularity of the same mass.

Answer 6

G'day,

Thank you for your question.

Mass is an important factor in influencing a planet's gravity. A denser planet would have a greater gravitational force than a similar size planet that was not as dense.

Regards

Answer 7

size doesnt matter, it is mass which affects gravity, if the earth weighed the same, but was 1/2 the size, then the gravity would still be the same

Answer 8

true, size as a measure of length is one, but you are right in that the mass of the planet is also a factor, also the rotation will have an impact as to how we are affected by the gravity.

Answer 9

The combined factors for gravity are
1)Mass
2)Rate of spin (Centrifugal Force)
3)Other Planets (Masses) in vicinity.

Answer 10

When I come in close contact with a heavenly body, It does affect my size. So perhaps other heavenly bodies may also have some affect.