Locked inside the littlest objects of the solar system—asteroids, comets, and meteorites—is a secret history. These small fry retain many characteristics they had when they formed 4.5 billion years ago. A series of studies in 2012 pried from them telling new clues about how Earth and the other major planets came to be.
One insight came from NASA’s Dawn spacecraft, which circled the asteroid Vesta in 2011 and 2012. Over the past year, using Dawn’s measurements, astronomers determined that Vesta has an iron core. That bolstered the theory that Earth and the other inner planets swallowed an earlier set of small, iron-cored planets. Vesta is a survivor of this original solar system.
Astrophysicist Alan Boss at the Carnegie Institution and his colleagues pushed further into the past by analyzing small particles inside primitive meteorites, which preceded the first mini-planets. They determined that the particles contained products of the decay of radioactive iron-60. The iron-60 could have originated in a nearby supernova explosion—but how, Boss wondered, did it end up the meteorites? To investigate, he ran a 3-D computer model, and in September he published his results
: A local supernova could have sprayed radioactive elements into the dusty gas cloud from which our solar system formed, and then triggered a shock wave that caused the cloud to begin collapsing. In Boss’s simulation, turbulent flows within the shock wave injected clumps of radioactive iron into the cloud, matching the observed properties of the meteorites.
Boss’s computer simulations also helped solve the mystery of the comet Wild-2. The comet is icy, but it contains particles that appear to have formed in a hot zone. Boss built a 3-D model that could account for the seeming paradox. It showed that the young sun was surrounded by a disk of gas and dust so agitated that particles in the hot, inner regions might be flung out to the cold, distant zone where comets formed. This turmoil also helped keep gas giants like Jupiter far from the sun, leaving space for rocky planets to form. Without such violent mixing, Earth might not have come to exist.