The authors provide a rigorous critique of the pebble accretion scenario, highlighting isotopic, chemical, dynamical, and chronological inconsistencies. However, the study adopts a somewhat binary approach by contrasting “classic” and “pebble accretion” models as mutually exclusive. In reality, a hybrid formation model, where planetary embryos form via pebble accretion in localized disk environments and then grow further via collisions and planetesimal accretion, remains physically plausible and has support in recent literature. This intermediate possibility is acknowledged but not thoroughly tested or modeled in the study.
Moreover, the paper argues that isotopic constraints preclude significant CC (carbonaceous chondrite-like) material from entering the inner disk, thereby ruling out widespread inward drift of outer-disk pebbles. However, it does not fully explore whether localized or pressure-trapped pebble populations (e.g., within midplane dust rings) could have contributed significantly to early embryo formation, while still preserving NC isotopic signatures.
Thus, a more balanced and quantitatively modeled investigation of mixed accretion pathways, involving local pebble growth followed by classical impacts, would strengthen the argument and provide a more nuanced understanding of terrestrial planet formation. Ignoring this middle ground risks oversimplifying a complex and multistage process.