A pebble accretion model for the formation of the terrestrial planets in the Solar System

Abstract

Pebbles of millimeter sizes are abundant in protoplanetary discs around young stars. Chondrules inside primitive meteorites—formed by melting of dust aggregate pebbles or in impacts between planetesimals—have similar sizes. The role of pebble accretion for terrestrial planet formation is nevertheless unclear. Here, we present a model where inward-drifting pebbles feed the growth of terrestrial planets. The masses and orbits of Venus, Earth, Theia (which later collided with Earth to form the Moon), and Mars are all consistent with pebble accretion onto protoplanets that formed around Mars’ orbit and migrated to their final positions while growing. The isotopic compositions of Earth and Mars are matched qualitatively by accretion of two generations of pebbles, carrying distinct isotopic signatures. Last, we show that the water and carbon budget of Earth can be delivered by pebbles from the early generation before the gas envelope became hot enough to vaporize volatiles.

Terrestrial planet formation from lost inner solar system material

“…5C ). Thus, contrary to some recent studies ( 7 , 9 , 10 ), we conclude that the terrestrial planets could not have formed by accretion of large masses (~40%) of CC pebbles from the outer solar system. Instead, the terrestrial planets could only have formed by pebble accretion if the vast majority of the pebbles had an NC isotopic composition.…”

“…Instead, the terrestrial planets could only have formed by pebble accretion if the vast majority of the pebbles had an NC isotopic composition. To this end, it has been suggested that CC material initially resided far from the Sun and that it reached the terrestrial planet region by radial drift only after some time t , before which the planets grew by accreting NC pebbles ( 7 ). Within this framework, the fraction of CC material in the terrestrial planets would be higher the earlier the change in pebble composition from NC to CC in the terrestrial planet region would have occurred.…”

“…Within this framework, the fraction of CC material in the terrestrial planets would be higher the earlier the change in pebble composition from NC to CC in the terrestrial planet region would have occurred. For instance, to match the purported high CC fraction in the terrestrial planets ( 9 , 10 ) in this particular model would require this change to have occurred at ~3.8 million years (Ma) after solar system formation ( 7 ). However, the low CC fraction in the terrestrial planets determined in the present study indicates that this assumed change in pebble composition would have to have occurred at ~5 Ma, which is the assumed lifetime of the disk in this model ( 7 ).…”

“…More recently, an alternative model has been proposed, in which planets grow by accreting “pebbles” from the outer solar system, which drift sunward through the disk because of gas drag ( 2 , 3 ). Pebble accretion is very effective at forming giant planet cores ( 4 , 5 ) and may have also formed the terrestrial planets of the solar system ( 6 , 7 ). Determining which of these two processes governed the formation of the terrestrial planets of the solar system is crucial for understanding the solar system’s architecture and dynamical evolution and for placing planet formation in the solar system into the context of general planet formation processes, such as those observed in exoplanetary systems.…”

How drifting and evaporating pebbles shape giant planets

“…However, in the models of Brouwers & Ormel (2020) the pebbles evaporating in the planetary atmosphere do not allow such a fast transition into runaway gas accretion, while the study of Ormel et al (2021) shows that envelope pollution significantly reduces the time at which the planet reaches the cross over mass for runaway gas accretion in line with Venturini et al (2015). Furthermore, Johansen et al (2021) shows that water-rich pebbles entering the planetary Hill sphere can evaporate high up in the planetary atmosphere, where recycling flows (Lambrechts & Lega 2017;Cimerman et al 2017) could transport the water vapor away from the planet, preventing the buildup of a high Z envelope.…”