Study Offers New Twist on the Cosmic Recipe for Stars and Planets

When a dense clump of interstellar matter collapses under its own gravity, it settles into a rotating disk that channels the matter towards a growing star at its center. After some time, gas and dust in the disk form planets. Scientists do not fully understand how this process works. Standard protoplanetary disk models do not accurately predict the size of disks we can currently observe and offer no explanation for what determines a disk's mass, size, and correlation to the star's mass in its center.

For Dartmouth professor and astrophysicist Paolo Padoan, searching for an explanation has shaped much of his recent work. His latest research, published in Nature Astronomy's April 2025 online issue, uses novel insights of angular momentum in supersonic turbulence to show that a key assumption in protoplanetary disk models needs to be revised.  

One fundamental assumption of standard disk models is that, by the end of the star-forming gravitational collapse, a disk is fully formed and can only lose mass to its star, the environment, and the planets it forms. Under this framework, a disk's visible size is controlled primarily by internal processes (a scenario known as 'viscous evolution').

Padoan has long been skeptical of models that ignore interactions with their larger environment. In particular, the capture of surrounding gas by the stars' gravity, a process known as Bondi-Hoyle accretion, seemed too fundamental to ignore. Since observed disk sizes are too large to be explained only by the gravitational collapse, Padoan wondered if it was possible to gain enough angular momentum - and therefore size - from a star's parent gas cloud to solve the discrepancy.  

In his new publication, Padoan worked with his research partner, Liubin Pan at the Sun Yat-sen University (Guangzhou, China) and several others, to model the large gas clouds surrounding young stars. Their team derived a new analysis of how angular momentum scales in supersonic turbulence of a gas cloud. Supersonic turbulence creates random and extreme density fluctuations. Padoan and Pan found that strong density fluctuations actually create greater angular momentum within the gas than previously expected. The team tested their new analytical derivation with numerical simulations and reached the same conclusion: they found much greater angular momentum when there were strong density fluctuations within the gas. 

While the spatial resolution of their simulation is not granular enough to analyze individual disks, it still allows researchers to estimate how much of the gas cloud's mass and angular momentum are captured by protoplanetary disks. Padoan and Pan found that on the scale where the gas can be captured by disks, via Bondi-Hoyle accretion, the angular momentum of the gas relative to the star is many times larger—enough to explain the disk's observed angular momentum and size.  

Padoan and Pan's simulation suggests protoplanetary disks are primarily assembled by Bondi-Hoyle accretion from the parent gas cloud. They demonstrate that Bondi-Hoyle accretion can supply both the mass and the angular momentum necessary to explain the observed size of protoplanetary disks. This revised understanding of disk evolution also provides straightforward answers to long-standing discrepancies between observations and models involving disk masses, lifetimes, misalignments, and other phenomena. 

Padoan has introduced an exciting new perspective with his current publication. He plans to continue refining his work with higher-resolution simulations.

You can read Paolo Padoan's full publication here.