The inner Solar System might have taken shape through a different process than what was long assumed. For decades, researchers operated under the premise that the rocky planets coalesced from a single belt of dust and debris in the early Solar System. However, newer simulations suggest the possibility of two distinct material reservoirs existing instead. These findings were recently published in The Astronomical Journal.
Simulations built on the idea of a singular disk or ring of material encircling the nascent Sun typically fail to replicate certain observed characteristics of our Solar System. For one, Earth appears to be composed of two distinct types of rock, which would be counterintuitive if everything originated from a single ring. Furthermore, single-ring models commonly result in Mercury and Mars being overly large, Venus and Earth positioned too closely together, and the elemental makeup of Earth and Mars being overly similar.
Bill Bottke from the Southwest Research Institute in Colorado, along with his collaborators, developed a series of detailed simulations tracing planetary formation from one unified source of material and tracking its subsequent evolution, yet the issues persisted.
“We spent six months on the computer, and nothing was working, so we made a desperate attempt. We thought, why don’t we try a second reservoir?” Bottke remarked while presenting the work at the Lunar and Planetary Science Conference in Texas. “It turned out that this model not only simulated the terrestrial planets effectively but also accounted quite well for several phenomena that were troubling us.”
The configuration that showed the best fit involved two separate disks: one situated around half the current Sun-Earth distance, and the second located roughly 1.7 times the Sun-Earth distance. Upon running the simulations, the resulting planets all achieved the appropriate sizes and spacing relative to one another.
This model also aligns with the observed compositions of Earth, the Moon, and Mars. “Our idea is that Earth formed predominantly from material [in the inner Solar System], with only the final accretion coming from the outer Solar System,” stated Jan Hellmann of the Max Planck Institute for Solar System Research in Germany during a separate presentation. If Earth formed from the inner disk with a minor contribution from the outer disk, as Bottke’s model predicts, this aligns with such expectations. Mars, conversely, would have largely formed from the outer disk, explaining the compositional contrast between the two planets.
Concerns remain that the model relies on highly specific initial conditions to accurately reproduce the Solar System’s interior, and the reason for these precise boundary values is not entirely clear. “Small alterations in the disk’s shape can lead to significant shifts in the layout of the terrestrial planets,” Bottke noted.
Researchers are currently focused on refining their simulation and exploring its broader implications for the Solar System. “We are allocating significant supercomputer time to test all plausible scenarios,” Bottke shared. If successful, this novel explanation could resolve numerous Solar System anomalies, ranging from peculiar asteroids to unexplained lunar rock samples.
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