A hypothetical planet has density r, radius R and surface gravitational acceleration g. If the radius of the planet were doubled, but the planetary density stayed the same, the acceleration due to gravity at the planet’s surface would be:
2g
1stcaseg=G(43πR3p)R2=43πGRpIIndcase:g′=G(43π(2R)3p)(2R)=2[43πRGp]∴g′=2g