Turbulence Inhibits Early Planetesimal Formation

A major open question in planetary science and astrophysics is: when exactly does planet formation begin? Within the context of planetary science, it is known that CAIs formed 4.6 Gyr ago, but Kuiper Belt Objects (KBOs) may have formed more than 1 Myr later as radioactive decay of ²⁶Al would have caused melting of the ice that is still present in these bodies. However, in the astrophysical context, there is mounting evidence that planets (and thus planetesimals) should form very rapidly (< 1 Myr; while the protosolar disk is still embedded in its natal envelope).

Here, we present a state-of-the-art 1D model of dust growth in a very young, massive disk that is still embedded in its natal envelope. By simulating a wide range of dynamical processes, including infall from the envelope, magnetically and gravitationally induced turbulence (necessary to produce accretion rates in agreement with observational constraints), and thermal evolution, we find that the dust grain growth is severely limited by turbulent fragmentation at these early stages and at radii less than 20 AU (see Fig 1). Despite this limited grain growth, we are still able to form CAIs at a time remarkably similar to their expected formation in the protosolar nebula.

We conclude with two implications:  First, if planetesimal formation does occur rapidly, the planet-forming disk should be significantly less turbulent. In this case, accretion would need to be driven by a non-turbulent mechanism, such as large-scale magnetically launched winds.  Our future work will address this question. The second possibility is that planetesimal formation may be extremely difficult at these early times due to very limited grain growth. This hypothesis strongly agrees with observations of KBOs suggesting they formed relatively late (> 1Myr after CAIs). Future work should also address this possibility and how to reconcile these implications with observations of planets and their formation environments around other stars that suggest planet formation proceeds rapidly throughout the young disk phase.

Figure 1 - Mid-plane dust-to-gas ratio versus maximum Stokes number (proportional to particle size) at several radial locations in a 1D model) for dust growth in a young, massive, turbulent disk (colored curves near the bottom of the plot).  The curved red line is the regime in which the dust-to-gas ratio and Stokes number are such that streaming instability can readily occur.  The horizontal dotted line (from Carrera et al. 2025) is the minimum dust-to-gas ratio needed for the feedback mechanism from Carrera et al. 2025, in which dust growth and the streaming instability provide a positive feedback route towards planetesimal formation. We find that planetesimal formation is not possible in our model, which either suggests that such young disks are not very turbulent (if planetesimal formation does happen rapidly) or that planetesimal formation takes longer than 1 Myr.  

References:

Carrera, D.; Davenport, Lim, J.; Eriksson, L. E. J.; Lyra, W.; Simon, J. B.; Positive Feedback: How a Synergy Between the Streaming Instability and Dust Coagulation Forms Planetesimals”, submitted to A&A Letters, eprint arXiv:2503.03105.