The Streaming Instability and Planet Formation

by Prof. Jürgen Blum and Holly Capelo

When astronomers peer through their telescopes at the very youngest stars, they find them to be surrounded by huge disks full of dust and gas. When they look at older stars, oftentimes planets like those in our own Solar system are present. This poses the question, how does a mixture of dust and gas turn into a life-hosting planet like the Earth? It is reasonable to assume that the growth of planets happens from the bottom up: small solid bodies form the building blocks of bigger bodies. The problem, though, is that gravity is not a very strong force when it comes to small things. For example, we feel the Earth’s gravity, but we don’t feel a gravitational attraction to small things like the every-day objects we hold or to each other. Similarly, the solid dust grains in planet-forming disks can’t stick together due to gravity in order to build a planet.

Can the gas around young stars help in the planet growth process? A lot of scientific research is centered on this possibility, and if so, fluid instabilities most likely play a crucial role. We see from common examples like the Rayleigh-Taylor instability that a very evenly mixed solution can later fragment into drops or clumps. The streaming instability has a similar effect, causing a well-mixed dusty gas to form clumps that contain high concentrations of dust particles. If this kind of dust-clustering effect happens in the dusty gas mixture near young stars, then the clusters will contain enough mass for gravity to hold them together. The bound object that forms won’t be a full-fledged planet, but it will be about the size of an asteroid and can participate in later stages of planetary growth.

How do the clusters of dust particles form and grow? Drag forces play an important role. For example, when we feel a strong wind blow against us, our bodies exert a force back on the air that causes it to rush even faster around and past us.  If a few bodies are packed together, then the rush of air around the barrier they create is even faster. Pushing the air past the cluster tends to remove the resistance that the cluster of bodies faces as a whole.  If the cluster is traveling through a fluid, then the decreased air resistance means that it can travel faster than its member particles would individually. This little swarm of particles can then race around collecting more particles, growing bigger and bigger in the process. A close analogy to the streaming instability is a peloton of bicycle riders who draft off of the strongest cyclist amongst them. As a group, they slip along together much faster than a cyclist who faces the wind alone. Scientists currently favor this fluid instability as one the most efficient ways to help small dust grains come together in the process of assembling planets.