The gravity numbers of other planets in the solar system refer to the gravitational force they exert on an object on their surfaces. If the Earth's gravity number is 1, then the planet Mercury has a gravity number of 0.3, Venus has a gravity number of 0.9, Mars has 0.3, Uranus has 0.8, Neptune has 1.1, Saturn has 1 and Jupiter has 2.3.
A planet that is very massive exerts a huge gravitational force only if it is also very compact. The equation that describes gravitational force increases with mass and decreases significantly as the radius of the gravitating object grows. This is why the planet Jupiter, which is much more massive than Earth, only exerts 2.3 times Earth's gravitational pull.
The sun is an object that is far more massive than Jupiter and even though its radius is also bigger, it is still compact enough to have a gravity number of 27. An example of a very massive object with a very small radius is a white dwarf star, which is the size of the Earth with the mass of the sun. It has a gravity number of 1.3 million. A neutron star is even more massive than the sun and has a radius far smaller than Earth. It has a gravity number of 140 billion.