Dust growth toward planetesimals is the first step of planet formation. However, there are some barriers against the planetesimal formation, which include dust depletion of the fast dust drift (Brauer et al. 2008). Previous studies proposed promising mechanisms of planetesimal formation, and one mechanism is disk instability including streaming instability (e.g., Youdin & Goodman 2005; Johansen et al. 2007) and secular gravitational instability (GI; e.g., Ward 2000; Youdin 2011; Takahashi & Inutsuka 2014, 2016). The disk-instability scenario requires a high dust-to-gas ratio (>0.01) of large dust grains (e.g., mm- or cm-size). Since the dust growth to such large grains results in dust depletion, some mechanisms to increase dust-to-gas ratio are necessary before the instabilities operate, e.g., dust trapping in pressure bumps or zonal flows (Bai & Stone 2014; Flock et al. 2015; Carrera et al. 2020, 2021).
We propose yet another mechanism named coagulation instability. Coagulation instability is driven by collisional dust growth and leads to dust concentration (Tominaga et al. 2021). Conducting numerical simulations, we find that its nonlinear growth locally increases the dust size and the dust-to-gas ratio, and the resulting dust-rich rings can become unstable to secular GI according to the growth condition in Tominaga et al. (2019) (see also, Tominaga et al. 2022a,b). Secular GI will lead to planetesimal formation via self-gravitational collapse of the dust rings (Tominaga et al. 2020; Pierens 2021; Takahashi et al. 2022).
In addition, we also revisit the growth condition of secular GI by conducting vertically global linear analyses since the previous studies treated only the radial motion using the razor-thin disk model. We show that the previous condition adopted above is applicable even when considering vertical structures. Therefore, our studies demonstrate that the combination of coagulation instability and secular GI is the promising mechanism of planetesimal formation.