Asteroids Were Born Big

How big were the first planetesimals? We attempt to answer this question by conducting coagulation simulations in which the planetesimals grow by mutual collisions and form larger bodies and planetary embryos. The size frequency distribution (SFD) of the initial planetesimals is considered a free parameter in these simulations, and we search for the one that produces at the end objects with a SFD that is consistent with asteroid belt constraints. We find that, if the initial planetesimals were small (e.g. km-sized), the final SFD fails to fulfill these constraints. In particular, reproducing the bump observed at diameter D~100km in the current SFD of the asteroids requires that the minimal size of the initial planetesimals was also ~100km. This supports the idea that planetesimals formed big, namely that the size of solids in the proto-planetary disk ``jumped’’ from sub-meter scale to multi-kilometer scale, without passing through intermediate values. Moreover, we find evidence that the initial planetesimals had to have sizes ranging from 100 to several 100km, probably even 1,000km, and that their SFD had to have a slope over this interval that was similar to the one characterizing the current asteroids in the same size-range. This result sets a new constraint on planetesimal formation models and opens new perspectives for the investigation of the collisional evolution in the asteroid and Kuiper belts as well as of the accretion of the cores of the giant planets.

💡 Research Summary

The paper tackles a fundamental question in planetary science: what were the sizes of the first planetesimals that gave rise to the present‑day asteroid belt? To answer this, the authors perform a suite of coagulation simulations in which planetesimals grow through mutual collisions, merging to form larger bodies and eventually planetary embryos. The key methodological innovation is to treat the initial size‑frequency distribution (SFD) of planetesimals as a free parameter. By varying the minimum size and the slope of the initial SFD, the authors generate a family of evolutionary outcomes and then compare the final SFD of the simulated asteroid population with the well‑constrained observational SFD of the real asteroid belt.

The observational benchmark consists of two main features: (i) a pronounced “bump” at a diameter of roughly 100 km, and (ii) an overall power‑law slope that extends from a few tens of kilometres up to several hundred kilometres. The simulations reveal that if the initial planetesimals are small—on the order of a kilometre or less—the collisional cascade quickly produces an excess of fragments and a steep depletion of bodies around 100 km. Consequently, the final SFD lacks the observed bump and fails to match the measured slope. In contrast, when the smallest initial planetesimals are set to about 100 km, the collisional evolution preserves a substantial population of bodies near that size, reproducing the bump and yielding a final SFD that closely follows the observed power‑law. Moreover, the simulations indicate that the initial SFD must already possess a slope similar to the present asteroid belt over the 100–several‑hundred‑km interval; otherwise the final distribution diverges from observations.

These results imply that planetesimals did not grow gradually from sub‑meter dust grains through a cascade of incremental accretion events. Instead, the data favor a scenario in which the solid component of the protoplanetary disk experienced a rapid “jump” from sub‑meter scales directly to multi‑kilometre (100 km‑scale) bodies. Such a jump is naturally produced by mechanisms like the streaming instability or gravitational collapse of dense particle clumps, which can concentrate solids and trigger the formation of large planetesimals in a single step. The authors further argue that the initial planetesimal population likely spanned a size range from ~100 km up to several hundred kilometres, and possibly even to ~1,000 km, with a relatively shallow SFD slope.

The implications of a “big‑born” planetesimal population are far‑reaching. First, it places a new constraint on any model of planetesimal formation: the model must be capable of generating a substantial number of ~100 km bodies without first producing a large reservoir of smaller objects. Second, the presence of such large bodies early on would have accelerated the accretion of the cores of the giant planets, providing a ready supply of massive building blocks. Third, the collisional evolution of both the asteroid belt and the Kuiper belt must be revisited, because a paucity of small impactors in the early stages would alter the timing and intensity of fragmentation events, potentially preserving more primordial large objects.

In the discussion, the authors suggest several avenues for future work. High‑resolution numerical studies of streaming‑instability‑driven clumping should be used to predict the exact initial SFD and size range, which can then be directly compared with the constraints derived here. Extending the analysis to the Kuiper belt will test whether the same “big‑born” paradigm applies to the outer Solar System. Finally, integrating the derived initial planetesimal distribution into models of giant‑planet core growth will help assess how much this early population speeds up the formation of Jupiter and Saturn.

In summary, the paper provides compelling evidence that the first planetesimals were already large—on the order of 100 km or more—and that their size distribution closely resembled the present‑day asteroid belt. This challenges traditional incremental growth models and supports rapid formation mechanisms, opening new research directions in planetary formation, collisional evolution, and the early dynamical history of the Solar System.