Inner rocky planets formed in regions solar nebula hot enough to boil away ice Planet formation stopped when a protoplanet became so large its gravitational pull opened up a gap in the nebula - similar to gaps in Saturn's rings. When this happened there was no more material to attract. Present circular orbits - result from the average motions of many planetesimals Young planets consisted of chaotic mixture silicates, metals, trapped gases. Bodies grew to planetesimals as grains collided. Gas giants retained light gases.
T goes up over time.
Results in metals melting and sink to form core . Core separation may have started before initial accretion was complete. Separation of core from mantle may have been complete as early as 4.4 billion years ago. Sinking of metals raised T more, sinking faster. Atmospheres also possibly from infall volatile-rich meteorites.