Closing the Gap: A Test of the Streaming Instability Using Simulations and Observations of the Cold-Classical Kuiper Belt Size Distribution

How planetesimals form is a major open question in studies of planet formation, as the growth of smaller solids must overcome barriers such as radial drift, fragmentation, and bouncing. The leading model is that planetesimals form by the gravitational collapse of dense particle clumps that have been aerodynamically collected via a process known as the streaming instability (SI). While the SI has passed several observational tests, a critical test remains: a successful comparison with the size distribution of Solar System planetesimals. In this work, we conducted a series of SI simulations for a range of input parameters and combine a more careful accounting of initial planetesimal mass with more advanced statistical techniques compared to previous studies in order to compare the simulation outcomes against observations of the most primordial population in the solar system: the Cold Classical Kuiper Belt (CCKB). Using both cumulative distribution function (CDF) model parameters and distribution comparison methods like the Kolomogorov-Smirnov test, we show that, when planetesimal masses are measured at formation and analyzed with modern statistical methods, the SI-predicted size distributions can quantitatively reproduce the high-mass end of the observed CCKB size distribution, though the extent of the agreement depends on the physical parameters chosen, providing further support for the SI theory of planetesimal formation.