Let’s start at the beginning. Before humans, before Earth, before any of the planets existed, there were baby planets — planetesimals. Coalesced from dust exploded outward by the solar nebula, these blobs of material were just a few kilometers in diameter. Soon, they too aggregated due to gravity to form the rocky planets in the innermost part of the solar system, leaving the early details about these planetesimals to the imagination. Their mysterious identity is complicated by the fact that Mercury, Venus, Earth, and Mars are all different in chemical…
