We know that planetary systems are common throughout the galaxy, but there is a lot still to learn about how they form. Now, an old theory has been given a new spin, with modeling revealing how a nearby supernova could have triggered the collapse that led to the formation of the Solar System's planets – including Earth.
Supernova explosions have previously been observed producing shock waves that slam into interstellar clouds, seeding them with heavy elements. Such an event has been used to explain the presence of elements on Earth that are not forged during the normal lifespan of a star.
However, creating a clear picture of how these events occurred has proved challenging. Dr. Alan Boss and Dr. Sandra Keiser of the Carnegie Institution for Science believe they have created a viable model, which they have outlined in The Astrophysical Journal.
Boss and Keiser's work tries to explain the distribution of short-lived radioisotopes (SLRIs). An element is defined by a common number of protons in the nucleus of its atoms, but the number of neutrons varies, forming different isotopes whose half-lives depend on the stability the proton/neutron combination provides.
The staggering pressures at the hearts of supernovae form numerous isotopes of every natural element. Those with short half-lives decay before they can reach other star systems. Longer-lived isotopes, however, can be injected into the gas clouds forming new stars, and leave a trace in the products they become as they decay, even if the original isotopes are long gone. Boss and Keiser find evidence for the impact of a shock wave from a Type II supernova in the ratio in meteorites of Iron-60, which has a half-life of 2.6 million years, to its immortal cousin Iron-56.
Boss and Keiser have previously shown that if a supernova exploded near the cloud that would eventually become the Solar System, it would create indentations – which they call “fingers” – injected with radioisotopes.
In their latest work, the authors extend the model to explain the angular momentum (spin) that made dust clouds coalesce into planets, rather than simply collapsing into the Sun. They found that the fingers produce the spin, not only determining the composition of the planets, but creating the conditions for their existence.
"This was a complete surprise to me," Boss said. "The very fact that a rotating disk formed around our proto-Sun may have been a result of the spin induced by this shock front. Without spin, the cloud disappears into the proto-Sun. With spin, a disk suitable for planet formation is created."
When the pair modeled a gas cloud a little over twice as massive as the Sun being hit by shock waves 20 or 40 kilometers per second (45,000 or 90,000 mph), they found protoplanetary disks form in every case, indicating the supernova did not need a particular alignment with the direction of the cloud's pre-impact rotation.
This finding helps explain why planetary systems seem to be so common – any nearby supernova will do, rather than one in just the right location.