<P> Theoretical models indicate that if Jupiter had much more mass than it does at present, it would shrink . For small changes in mass, the radius would not change appreciably, and above about 500 M (1.6 Jupiter masses) the interior would become so much more compressed under the increased pressure that its 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 was achieved, as in high - mass brown dwarfs having around 50 Jupiter masses . </P> <P> 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 . Despite this, Jupiter still radiates more heat than it receives from the Sun; the amount of heat produced inside it is similar to the total solar radiation it receives . This additional heat is generated by the Kelvin--Helmholtz mechanism through contraction . This process causes Jupiter to shrink by about 2 cm each year . When it was first formed, Jupiter was much hotter and was about twice its current diameter . </P> <P> Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen . Beyond this basic outline, there is still considerable uncertainty . The core is often described as rocky, but its detailed composition is unknown, as are the properties of materials at the temperatures and pressures of those depths (see below). In 1997, the existence of the core was suggested by gravitational measurements, indicating a mass of from 12 to 45 times that of Earth, or roughly 4%--14% of the total mass of Jupiter . The presence of a core during at least part of Jupiter's history is suggested by models of planetary formation that require the formation of a rocky or icy core massive enough to collect its bulk of hydrogen and helium from the protosolar nebula . Assuming it did exist, it may have shrunk as convection currents of hot liquid metallic hydrogen mixed with the molten core and carried its contents to higher levels in the planetary interior . A core may now be entirely absent, as gravitational measurements are not yet precise enough to rule that possibility out entirely . </P> <P> The uncertainty of the models is tied to the error margin in hitherto measured parameters: one of the rotational coefficients (J) used to describe the planet's gravitational moment, Jupiter's equatorial radius, and its temperature at 1 bar pressure . The Juno mission, which arrived in July 2016, is expected to further constrain the values of these parameters for better models of the core . </P>

What is jupiter's core made up of