In the vast cosmic dance that leads to the birth of planets, a crucial mystery has long intrigued astrophysicists: how do tiny dust grains within protoplanetary disks evolve into the sizeable planetesimals that seed planets? Recent groundbreaking experimental research by Dr. Holly L. Capelo and her team at the University of Bern has provided compelling evidence to illuminate a key process in this transformation, demonstrating for the first time in microgravity conditions the physics of shear-flow instabilities. This discovery could redefine our comprehension of planet formation, bridging gaps that theoretical models alone could not fully address.
Protoplanetary disks, swirling rings of gas and dust that encircle newborn stars, are environments where cosmic evolution unfolds at multiple scales. The journey from micron-sized dust particles to fully fledged planets is understood to involve a cascade of collisional agglomerations. Fine dust grains initially stick together through electrostatic forces, progressively growing to millimeter sizes. At the opposite end of the scale, planetesimals ranging from hundreds of meters to kilometers coalesce through gravitational collisions, eventually building terrestrial and giant planets. The intermediate stage, involving centimeter to hundred-meter-sized bodies, poses a formidable hurdle. In this size range, particles frequently rebound upon collision, fragment, or even evaporate due to close proximity to intense stellar radiation, stalling growth in a phenomenon commonly referred to as the “bouncing barrier.” This impasse has remained a significant puzzle for astrophysicists striving to unravel planetary genesis.
Theoretical explorations in recent decades suggested that hydrodynamical instabilities within the gas-dust mixture of the disk could serve as a catalyst to overcome this barrier. These instabilities, acting like fluid dynamical perturbations, can induce dust to coalesce into dense clusters, eventually collapsing into planetesimals. Among these, the shear-flow instability is particularly compelling as it arises at the interface between two fluid layers with differing velocities and densities — conditions intrinsic to protoplanetary disks. Prior to Capelo’s experiment, however, this mechanism was largely speculative, based on mathematical models and simulations without direct empirical validation, especially in the extraordinarily low-density, near-vacuum conditions of space.
Addressing this gap, the research team developed the TEMPus VoLA experiment, a uniquely designed instrument capable of probing dust-gas interactions in an exceptionally thin gas under microgravity. Employing the opportunities offered by parabolic flights that simulate near-weightless conditions for brief intervals of approximately 20 seconds, the experiment meticulously reproduces the delicate balance of forces present in space. The high-speed cameras integrated into the apparatus capture real-time dynamics of dust particles suspended in rarefied gas, isolating effects attributable solely to shear-flow without the confounding influence of Earth’s gravity.
Microgravity is essential because gravity on Earth causes sedimentation and convection in the gas-particle mixture, masking subtle instabilities that would occur in protoplanetary disks. By flying on specially adapted aircraft executing parabolic trajectories, the team recreated short bursts of near-zero gravity, allowing dust particles to be suspended in a stable manner akin to their natural state in space. Variations in gas density, dust concentration, and flow velocities were fine-tuned over multiple flights to identify precise conditions under which shear-flow instabilities manifest. This approach transcends theoretical predictions, providing tangible, visual confirmation of these phenomena.
Although the short duration of zero gravity in parabolic flights limits observation time, the experiment revealed distinct flow patterns signaling the early stages of instability formation. These characteristic structures reflect complex interactions between gas and dust particles, heralding transitions from laminar flow to turbulent states pivotal for density enhancement and clumping necessary for planetesimal creation. Such turbulence is thought to facilitate particle concentration by creating vortices and pressure traps, providing ideal niches for growth beyond the bouncing barrier. The findings thus substantiate long-held hypotheses that hydrodynamical effects, particularly shear-flow instabilities, are not mere theoretical curiosities but active agents sculpting early planetary architectures.
Recognizing the limitations of brief microgravity periods, Capelo’s team is advancing a more sophisticated version of the experiment designed for deployment aboard the International Space Station. The extended microgravity environment there would permit sustained observation of the instability’s full evolution into turbulence, unlocking insights inaccessible through Earth-bound methods or short-duration flights. This development heralds a new chapter in experimental astrophysics, enabling researchers to witness in situ processes that underpin planet formation with unprecedented clarity and precision.
Beyond enriching fundamental understanding, these experimental results carry significant implications for astrophysical modeling. Existing simulations of protoplanetary disks often struggle with resolution constraints, preventing them from accurately resolving the scale at which these instabilities operate. Data derived from the TEMPus VoLA experiment can be integrated to enhance physical models, refining inputs relating to gas viscosity, dust-gas coupling, and turbulence initiation. Enhanced models will improve predictions about planetesimal formation rates, disk evolution timelines, and hence the diversity of planetary systems emerging throughout the galaxy.
The corroboration of shear-flow instability under realistic protoplanetary conditions also sheds light on the origins of our own Solar System. While comets and asteroids serve as fossil records of early Solar System materials, they cannot directly reveal the intricate dynamical processes that governed their formation. Bridging this observational gap through laboratory and microgravity experiments adds a crucial dimension to comparative planetary science. Understanding how dust and gas behavior fostered planetesimals billions of years ago deepens appreciation of the material and energetic pathways culminating in Earth’s formation, and by extension, the conditions enabling life.
Realizing this ambitious research initiative necessitated a fusion of multidisciplinary expertise across Swiss academic institutions. The University of Bern spearheaded instrument development, leveraging technical precision in vacuum and optical engineering. The University of Zurich contributed crucial theoretical frameworks in planet formation, while ETH Zurich’s strengths in small body observation informed experimental parameters. Additionally, the UZH Space Hub, ESA/PRODEX programs, and the aerospace company Novespace brought essential experience in orchestrating and executing complex parabolic flight campaigns. This synergy exemplifies how collaborative networks catalyze breakthroughs unattainable by isolated efforts.
This Swiss-led breakthrough continues a proud legacy of Bernese contributions to space science, tracing back to 1969 when Prof. Dr. Johannes Geiss and his team at the University of Bern crafted the Solar Wind Composition experiment, which was deployed on the Moon by astronaut Buzz Aldrin. Since then, Bernese scientists have remained at the forefront of space missions with ESA, NASA, and JAXA, participating in both instrumentation and data analysis. The University of Bern now co-manages the CHEOPS mission for ESA and leads cutting-edge planetary formation modeling, further cementing its status among global leaders in space research.
To capitalize on these achievements, the University of Bern established the Center for Space and Habitability, a dedicated competence center focused on interdisciplinary exploration of planetary environments and habitability conditions. Furthermore, the integration within the National Center of Competence in Research (NCCR) PlanetS fosters cohesive national efforts to unravel planetary system formation. This institutional framework amplifies the impact of experimental and theoretical ventures such as TEMPus VoLA, shaping the frontiers of astrophysical knowledge.
As the TEMPus VoLA experiment paves the way for observing shear-flow instabilities under authentic cosmic conditions, scientists edge ever closer to unraveling the full narrative of planet formation. These results not only dismantle previous conceptual barriers but also open new horizons for exploring planetary origins both within our Solar System and beyond. The fusion of innovative experimentation, rigorous theory, and high-fidelity simulation propels this field into a dynamic era where cosmic origins are no longer confined to the realm of speculation but are illuminated through empirical evidence.
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Subject of Research: Not applicable
Article Title: Experimental evidence for granular shear-flow instability in the Epstein regime
News Publication Date: 17-Mar-2026
Web References:
DOI link
Image Credits: Courtesy of Holly Capelo
Keywords
planet formation, protoplanetary disks, shear-flow instability, microgravity experiments, dust aggregation, planetesimals, hydrodynamical instabilities, parabolic flights, TEMPus VoLA, astrophysical fluid dynamics, turbulence, cosmic origins
