How do rocky objects between 1cm and 1m accrete to form planetesimals?

Question

I am having a hard time gaining an intuitive understanding of some of the middle stages of planetary formation from a protoplanetary accretion disk.

I understand that microscopic dust particles may accrete through something like electrostatic forces. This makes sense on an intuitive level as we often observe very small objects sticking to one another, like clumps of dust 'just forming' in the corner of a dusty basement. I am guessing that gravity doesn't come into play here since the particles are too small.

However, I can't quite visualise how objects that are greater than 1cm and smaller than 1m can accrete, particularly when they are formed from rocky substances. This seems to defy intuition, as electrostatic forces don't act to join objects at this scale - you can't just stick rocks together. It doesn't seem to be a matter of energy either, since smashing rocks together results in either one of them breaking apart rather than sticking together.

Is there a known mechanism by which rocky objects within between 1cm and 1m accrete to form larger objects? I vaguely remember reading about how gas clouds may have been involved.

Answer 1

This is indeed a tricky problem, and the accretion of pebbles to form planetesimals is a big question in planetary science. You are right to say that small particles can stick together through electrostatic forces if collisions are gentle enough. But as particles increase in size, bouncing and fragmentation seem to present a barrier to growth. This is overcome somewhat in the outer disk, where icy particles tend to 'stick' more readily than rocky particles. Nevertheless, planetesimal growth requires a more thorough explanation.

One of the most popular solutions to this problem is a mechanism called the streaming instability. To understand how this works, we first need to know that dust particles within a protoplanetary disk undergo radial drift, due to the drag force exerted on them by gas in the disk. This causes them to move at 'sub-Keplerian' velocities and and lose angular momentum, drifting in towards the host star.

The idea behind streaming instability is that particles exert a frictional force back onto the gas, causing the gas to accelerate to near Keplerian velocities. This reduces the headwind on the dust particles, causing them to drift in more slowly. As particles further out in the disk start to 'catch up', the local density increases exponentially, forming clumps that become self-gravitating. These clumps grow vary quickly and eventually collapse, enabling planetesimals to form over very short timescales.

Streaming instability can create planetesimals in a variety of sizes, typically of the order ~100km, but even up to the size of objects such as Ceres. It is particularly favoured to take place at snowlines, which are the radial regions of the disk where particular molecular species 'freeze-out'.

For a more detailed explanation I highly recommend this recent review by Raymond & Morbidelli (2020).