![]() If this theory is correct, the largest example would be an asteroid calledġ6 Psyche-named after the Greek goddess of the soul, Psyche, and because it was the 16th member of the asteroid belt to be discovered (in 1852). These materials combined to form a rare kind of world. Astronomers hypothesize that a series of “hit and run” impacts caused these bodies to lose most of their mantles, leaving behind only a small quantity of silicate rock and a large amount of metal. Some collisions had enough energy to completely obliterate a protoplanet, leaving behind debris that contributed to the asteroid belt that now exists between the orbits of Mars and Jupiter.īut a few protoplanets may have escaped either of these fates. ![]() In each of those protoplanet collisions, the metallic cores were battered and remixed with silicate mantle material, later separating again after being melted by the heat of accretion. Some merged and grew into even larger protoplanets, eventually forming what became the familiar planets we know today. And over the next 20 million or so years, many planetesimals crossed paths and collided. In this way, vast amounts of iron and nickel alloys were trapped deep inside these planetesimals, forever hidden from direct scrutiny.Īt this time, the solar system was still relatively crowded despite its vast size. These lighter materials eventually cooled to form mantles of silicate rock around heavy metallic cores. The denser components of that melt-iron and other metals-settled to the center, leaving lighter silicates to float up toward the surface. Once they had attained such a size, heat from the decay of the radioactive elements within them became trapped, raising temperatures enough to melt their insides. Some of those planetesimals, as astronomers call them, grew to be hundreds of kilometers across as they swept up more dust and gas within the swirling solar nebula. But within a few million years, that churning cloud of primordial material collapsed under its own gravity to form hundreds, or maybe thousands, of infant planets. When our solar system was very young, there were no planets-only a diffuse disk of gas and dust circled the sun.
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