Jupiter's mass is 2.5 times that of all the other planets in the Solar System combined—this is so massive that its barycenter with the Sun lies above the Sun's surface at 1.068 solar radii from the Sun's center. Although this planet dwarfs the Earth with a diameter 11 times as great, it is considerably less dense. Jupiter's volume is that of about 1,321 Earths, yet the planet is only 318 times as massive.[3][21] Jupiter's radius is about 1/10 the radius of the Sun,[22] and its mass is 0.001 times the mass of the Sun, so the density of the two bodies is similar.[23] A "Jupiter mass" (MJ or MJup) is often used as a unit to describe masses of other objects, particularly extrasolar planets and brown dwarfs. So, for example, the extrasolar planet HD 209458 b has a mass of 0.69 MJ, while Kappa Andromedae b has a mass of 12.8 MJ.[24]
Theoretical models indicate that if Jupiter had much more mass than it does at present, the planet would shrink.[25] For small changes in mass, the radius would not change appreciably, and above about 500 M⊕ (1.6 Jupiter masses)[25] the interior would become so much more compressed under the increased gravitation force that the planet's volume would decrease despite the increasing amount of matter. As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve. The process of further shrinkage with increasing mass would continue until appreciable stellar ignition is achieved as in high-mass brown dwarfs having around 50 Jupiter masses.[26]
Although Jupiter would need to be about 75 times as massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30 percent larger in radius than Jupiter.[27][28] Despite this, Jupiter still radiates more heat than it receives from the Sun; the amount of heat produced inside the planet is similar to the total solar radiation it receives.[29] This additional heat radiation is generated by the Kelvin–Helmholtz mechanism through contraction. This process results in the planet shrinking by about 2 cm each year.[30] When it was first formed, Jupiter was much hotter and was about twice its current diameter.[31]