to fuse hydrogen and become a star, its diameter is sufficient as the smallest red dwarf may be only slightly larger in radius than Saturn. Jupiter radiates more heat than it receives through solar radiation, due to the Kelvin–Helmholtz mechanism within its contracting interior. This process causes Jupiter to shrink by about /yr. At the time of its formation, Jupiter was hotter and was about twice its current diameter. Internal structure Before the early 21st century, most scientists proposed one of two scenarios for the formation of Jupiter. If the planet accreted first as a solid body, it would consist of a dense core, a surrounding layer of liquid metallic hydrogen (with some helium) extending outward to about 80% of the radius of the planet, and an outer atmosphere consisting primarily of molecular hydrogen. Alternatively, if the planet collapsed directly from the gaseous protoplanetary disk, it was expected to completely lack a core, consisting instead of denser and denser fluid (predominantly molecular and metallic hydrogen) all the way to the centre. Data from the Juno mission showed that Jupiter has a diffuse core that mixes into its mantle. This mixing process could have arisen during formation, while the planet accreted solids and gases from the surrounding nebula. Alternatively, it could have been caused by an impact from a planet of about ten Earth masses a few million years after Jupiter's formation, which would have disrupted an originally solid Jovian core. It is estimated that the core takes up 30–50% of the planet's radius, and contains heavy elements with a combined mass 7–25 times the Earth. Outside the layer of metallic hydrogen lies a transparent interior atmosphere of hydrogen. At this depth, the pressure and temperature are above molecular hydrogen's critical pressure of 1.3 MPa and critical temperature of . In this state, there are no distinct liquid and gas phases—hydrogen is said to be in a supercritical fluid state. The hydrogen and helium gas extending downward from the cloud layer gradually transitions to a liquid in deeper layers, possibly resembling something akin to an ocean of liquid hydrogen and other supercritical fluids. Physically, the gas gradually becomes hotter and denser as depth increases. Rain-like droplets of helium and neon precipitate downward through the lower atmosphere, depleting the abundance of these elements in the upper atmosphere. Calculations suggest that helium drops separate from metallic hydrogen at a radius of ( below