Like the other planets, Jupiter probably began to form when dust and ice particles in a primeval nebula around the early Sun condensed and coalesced due to collisions. Jupiter was in a perfect location for growing large: far enough from the Sun that solids could condense, and close enough that there was plenty of material to accrete. Once Jupiter's "embryo" (including rocky material now in its core, several times as massive as the Earth) became large enough, its gravity pulled together a surrounding envelope of gas. Although Jupiter was not large enough to begin nuclear burning, the compression of its own gravity generated a tremendous amount of heat when the planet formed. Even now, 4.6 billion years later, Jupiter radiates at least 60% more energy than it receives from the Sun. Jupiter is primarily composed of hydrogen and helium, in similar proportion to their abundance in the Sun and the solar nebula. There is no solid surface under the atmosphere, only a gradual transition to liquid. About one-fourth of the way in, pressures and temperatures are so high (two million times the pressure at the Earth's surface and 10,000� C) that the liquid hydrogen becomes metallic�that is, the molecules are stripped of their outer electrons. Astronomical observations have suggested a slight depletion in atmospheric helium, compared with solar abundance, which was attributed to precipitation of helium in the metallic hydrogen, raining down toward the core. However, measurements made by the Galileo space probe within Jupiter found no depletion of the hydrogen, at least at its entry location.