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A Planets Mass


igosaur

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Quick question that has been bugging me for a while.

 

I was looking into the recent discovery of a new 'Earth like' world, Gliese 581 c, which has a mass 5 times that of the Earth.

 

However, its gravity would only be twice that of our planet.

 

Question: Why is the gravity not 5 times as much if the mass is?

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How do you quantify gravity? My guess would be that "twice of our planet" means that the acceleration due to gravity at the surface is twice that of earth. The 5-times mass can then be countered by a larger radius, i.e. you are comparing values at different distances from the respective centers.

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