Did the Terrestrial Planets Form by Pebble Accretion?
The formation of Earth and its neighboring terrestrial planets remains a hot topic in planetary science. Alessandro Morbidelli and colleagues present a thorough evaluation of two contrasting theories: the classical model and the pebble accretion model. The classical model proposes that planets formed from collisions and mergers of large rocky bodies called planetesimals, leading to planetary embryos that eventually collided to form the terrestrial planets over tens of millions of years. On the other hand, the pebble accretion model suggests that planets grew by gathering smaller, millimeter-to-centimeter-sized particles, called pebbles, from the protoplanetary disk during its lifetime. Morbidelli's team revisits observational and theoretical constraints to evaluate these models.
The Chemical and Isotopic Puzzle
The isotopic composition of materials offers critical clues about planetary origins. Terrestrial planets are rich in non-carbonaceous (NC) material, distinct from the carbonaceous (CC) material common in outer solar system objects. The pebble accretion model predicts that the radial drift of CC material should gradually influence the isotopic makeup of Earth and Mars. However, the data show that Earth and Mars have a low CC contribution—around 6% for Earth and virtually none for Mars. This suggests that significant CC material did not reach the inner solar system during the planets’ growth, contradicting a core assumption of pebble accretion.
Dynamics of Planetary Growth
One of the pebble accretion model's challenges is explaining the orbits and spacing of the terrestrial planets. If Earth, Venus, and Mars formed through pebble accretion, their masses and positions should reflect a pattern influenced by inward migration within the gas disk. However, the evenly spaced orbits of terrestrial planets don't align with such a migration scenario. The classical model, which assumes that planets formed after the gas disk dissipated, more naturally explains this.
Evidence from Planetary Chemistry
The bulk silicate Earth (BSE) shows a gradual depletion of volatile elements that vaporize at relatively low temperatures. This depletion pattern is challenging to reconcile with pebble accretion, which predicts a sharper, step-like depletion. The observed pattern aligns better with the classical model, where Earth forms from collisions of diverse planetesimals, each contributing different levels of volatiles.
Timing and the Role of Giant Impacts
Pebble accretion requires planets to grow rapidly within the 5-million-year lifespan of the gas disk. However, Earth’s tungsten isotopes suggest a slower growth timeline, requiring additional giant impacts after the gas disk dissipated. Evidence from noble gases (like helium and neon) further supports that Earth experienced multiple giant impacts, which are consistent with the classical model’s prediction of chaotic collisions among many planetary embryos.
Conclusion: A Hybrid or Classical Model?
Morbidelli’s team concludes that pebble accretion alone cannot explain the terrestrial planets' formation. Instead, a hybrid scenario where small pebbles contribute to early growth, followed by planetesimal accretion and giant impacts, may be more plausible. However, the classic model of planetesimal collisions remains the most consistent with the chemical, isotopic, and dynamical constraints. This work highlights the need for models to account for diverse evidence to uncover the solar system's history.
Source: Morbidelli
