Researchers have experimentally confirmed a key mechanism in planet formation – a shear flow instability – even under the extremely thin gas conditions found in protoplanetary disks. The findings, published March 13, 2026, in Communications Physics, could resolve a long-standing puzzle in astrophysics regarding how dust and gas coalesce to form planets.
Planets originate within protoplanetary disks, vast structures of gas and dust circling young stars. The journey from microscopic dust grains to fully formed planets involves several physical processes. Initially, tiny dust particles collide and clump together through electrostatic forces, growing to a few millimeters in size. Subsequently, planetesimals – rocky or icy bodies ranging from hundreds of meters to kilometers in diameter – collide, merge, and accrete. These gradually grow into rocky or icy planets, with the fastest-growing eventually accumulating gas and becoming gas giants.
However, a “barrier” has challenged planet formation scenarios for centimeter-sized to roughly hundred-meter rocks. In this size range, clumps tend to bounce off each other during collisions, break apart, or even vaporize if they drift too close to their star. This obstacle has puzzled scientists for decades.
Since the early 2000s, theoretical models have proposed an additional mechanism to bridge this gap: hydrodynamic instabilities within the gas-dust mixtures of protoplanetary disks. These instabilities cause dust to clump into denser clouds, eventually forming planetesimals. Each instability operates under specific conditions and in different regions of the disk, influencing it in various ways. One such instability, suspected to play a significant role, is the shear flow instability, which arises at the interface between fluids with differing properties – in this case, different velocities and densities.
Until now, experimental verification of shear flow instabilities under the extremely low-density gas conditions of protoplanetary disks had remained elusive. A research team led by Dr. Holly L. Capelo of the Department of Space Research and Planetology at the University of Bern has now demonstrated that these instabilities can indeed form, even in extremely thin gas. The breakthrough came through a unique experiment leveraging the microgravity conditions achieved during parabolic flights, also known as Zero-G flights.
The experiment, called TEMPus-VoLA, was designed and built in collaboration between the University of Bern, the University of Zurich, and ETH Zurich, with funding from the NCCR PlanetS and the Swiss Space Office. It features high-speed cameras tracking the behavior of dust particles in an extremely thin gas under vacuum conditions, specifically developed for parabolic flights. “On Earth, gravity influences the behavior of dust and gas,” explained Professor Lucio Mayer of the University of Zurich. “Only under conditions that simulate the absence of gravity can we investigate an extremely rarefied flow regime that resembles the gas and dust disks around young stars.”
During parabolic flights, a specially adapted aircraft follows a trajectory repeatedly ascending and diving at approximately a 45-degree angle. Each dive phase provides about 20 seconds of weightlessness, although the ascent simulates stronger gravity than on Earth. Through multiple flight campaigns conducted by the UZH Space Hub and the European Space Agency (ESA), the team systematically refined and varied the experimental conditions to test when shear flow is triggered. “We recreated the conditions prevailing in the planet-forming regions of protoplanetary disks and were able to show that this theoretically proposed shear flow instability is not just a mathematical construct, but can actually occur,” Capelo stated.
However, parabolic flights offer only brief periods of weightlessness. “Once the instability sets in, we discover that characteristic patterns emerge in the flow of material. The limited duration of weightlessness, however, prevents us from tracking how these structures evolve into fully developed turbulence,” Capelo explained. The team is therefore working on an advanced version of the experiment for deployment on a space station, such as the International Space Station (ISS). There, the emergence and development of turbulence could be studied over significantly longer periods in weightlessness – another crucial piece of the puzzle in understanding planet formation.
Understanding the origins of our solar system relies on diverse research approaches. Modern telescopes observe protoplanetary disks around stars, and comparing disks of different ages reveals their properties and evolution. Theoretical computer simulations mathematically and physically describe disk development and planet formation. However, none of these simulations can examine disks with the high resolution needed to visualize the smallest structures within them. “In our solar system, comets and asteroids bear witness to the early phase of our system and provide clues about the composition and structure of planetesimals, but we cannot directly study their early development,” said Dr. Antoine Pommerol of the University of Bern. “Only experiments can close this knowledge gap and reveal the crucial details of dust and gas movement on such tiny spatial and temporal scales that cannot be directly observed.”
The new experiment not only provides direct confirmation that a long-theorized phenomenon can occur under protoplanetary disk-like conditions, but will also support improve theoretical models and refine simulations. “This, in turn, helps us better understand the overall picture of the formation of planetary systems – and it will help explain how our solar system and Earth itself arose from a cloud of dust and gas billions of years ago,” Capelo concluded.
The research highlights the power of national collaboration in Switzerland. While the NFS PlanetS initially funded the project’s development, each participating institution contributed its unique expertise: from the University of Bern’s instrument-building capabilities to the University of Zurich’s theoretical expertise in planet formation and ETH Zurich’s experience in observing and analyzing small solar system bodies.
The expertise of the UZH Space Hub, the ESA/PRODEX programs, and Novespace in preparing and conducting parabolic flights was also integral to the project’s success. “the ability of Swiss institutions to efficiently pool their strengths and work closely together on this project led to its remarkable success and breakthroughs in exploring the fundamental physics of planet formation. These results pave the way to hopefully observe such mechanisms in the cosmos as well,” Capelo added.
The University of Bern has been at the forefront of space research since the first moon landing, when Buzz Aldrin unfurled the Bern Solar Wind Composition Experiment (SWC) on July 21, 1969. The university regularly participates in space missions by major space organizations like ESA, NASA, and JAXA. With CHEOPS, the University of Bern shares responsibility for an entire mission with ESA. Bernese researchers are also at the forefront of modeling and simulating the formation and evolution of planets.
The successful work of the Department of Space Research and Planetology (WP) at the University of Bern has been strengthened by the establishment of a university competence center, the Center for Space and Habitability (CSH). The Swiss National Science Foundation has also awarded the University of Bern the National Research Centre (NFS) PlanetS, which it co-leads with the University of Geneva.
scientific contact:
Dr. Holly Capelo
Department of Space Research and Planetology (WP), University of Bern and NFS PlanetS
Phone
+41 31 684 36 87
Email Address:
Dr. Antoine Pommerol
Department of Space Research and Planetology (WP), University of Bern and NFS PlanetS
Phone +41 31 684 39 98
Email Address: 
Original publication:
Holly Capelo et al., “Experimental evidence for granular shear-flow instability in the Epstein regime”, in: Communications Physics
DOI: 10.1038/s42005-026-02531-9
https://www.nature.com/articles/s42005-026-02531-9
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