Exoplanets form in discs of gas and dust around newly-formed stars. The solid particles we see in the interstellar medium (commonly referred to as “dust”) are microscopic, and the process(es) of planet formation must somehow assemble solid bodies as large as the Earth from these tiny building blocks. Our long-standing picture of planet formation is one of “bottom-up” growth, where solids grow through repeated collisions until they become large enough for gravity to take over. Typically this means growing to km sizes or larger, but we now recognise that collisional growth is not efficient beyond mm to cm sizes. How solids grow from “pebbles” up to “planetesimals” (solid bodies large enough to have significant gravity of their own) remains one of the most important unsolved problems in astrophysics.
One possible mechanism for forming planetesimals is to “trap” solids inside structures (such as rings or spirals) in the gas-rich protoplanetary disc. Particles of cm sizes (“pebbles”) are the most strongly influenced by drag forces from the disc gas, and tend to accumulate in regions of high gas pressure. One natural way of generating such structures is disc self-gravity, which occurs when young, massive gas discs become unstable and start to collapse under their own gravity. In extreme circumstances this process can even form planets directly, but the more usual outcome is the creation of spiral waves in the disc.
We used DiRAC’s Data Intensive at Leicester (DIaL) cluster to run state-of-the-art numerical simulations of self-gravitating protoplanetary discs, to investigate the dust physics in detail. Trapping can be driven both by the drag forces and by the gravity of the gas and dust. By repeating the same numerical experiment with different physical processes “switched off” we were able to isolate the key physics, and gain crucial new insight into the planetesimal formation process. We found that only when both drag forces and self-gravity are considered does dust becomes strongly trapped in spiral arms, and that the dust can collapse to form clumps even with the gas does not. This represents a viable mechanism for forming large (asteroid- or even lunar-sized) solid bodies very early in the protoplanetary disc’s lifetime, and may provide the kick-start needed to explain the formation of many observed exoplanets.
These results were published in The role of drag and gravity on dust concentration in a gravitationally unstable disc, S.Rowther et al., Monthly Notices of the Royal Astronomical Society, 528, 2490 (2024).
