The Unseen Journey: Dust to Planets
How does fine dust aggregate into building blocks that ultimately form entire planets like our Earth? A research team led by the University of Bern, with the participation of ETH Zurich, the University of Zurich, and the National Center of Competence in Research (NCCR) PlanetS has provided the first experimental evidence—obtained during parabolic flights in zero gravity—that a key physical process, known as shear-flow instability, actually occurs under conditions similar to those in planet formation regions. The study thus addresses an important gap in our understanding of the very first steps of planet formation.
Understanding Protoplanetary Disks
Planets form within protoplanetary disks: vast disks of gas and dust orbiting around very young stars. From the finest dust grains to fully formed planets, several distinct physical processes occur:
- Collisions between fine dust particles, leading to clumping and growth to several millimeters.
- Formation of planetesimals, rocky or icy bodies of a few hundred meters to kilometers in size.
- Formation of giants through the accretion of gas by the fastest planetesimals.
The Barrier to Growth
However, a "barrier" exists that prevents further growth from centimeter-sized to 100-meter-sized boulders. Most planet-forming scenarios struggle at this stage, where clumps tend to bounce off each other, break apart upon collision, or even evaporate when drifting too close to their star. This barrier has puzzled scientists for decades.
The Advanced Mechanism: Shear-Flow Instability
Theoretical models have suggested that because the gas-dust mixture behaves like a fluid, various hydrodynamic instabilities can develop, leading to clumping of dust into denser clouds.
Experimental Evidence from Parabolic Flights
Using a unique experiment that exploits the microgravity conditions during parabolic "0g" flights, a team led by Dr. Holly L. Capelo from the Space Research and Planetology Division at the Physics Institute of the University of Bern has shown experimentally that shear-flow instabilities can indeed form—even in extremely thin gas. The study is published in Communications Physics.
Flight Path: Zero Gravity Experiments
To understand these flow instabilities, the team developed the TEMPus VoLA experiment in 2020. The experiment includes high-speed cameras tracking dust particle behavior in a thin gas under vacuum conditions:
"On Earth, gravity influences the behavior of the dust and gas," explains Prof. Lucio Mayer from the University of Zurich. "Only conditions that simulate the absence of gravity allow us to probe an extremely dilute flow regime, similar to the gas and dust disks orbiting around young stars."
The team systematically varied conditions to test when shear-flow was triggered during several flights arranged through the UZH Space Hub and ESA. Consequently, they recreated the conditions existing in the planet-forming regions of protoplanetary disks, confirming that the shear-flow instability can indeed occur in reality.
Looking Towards the Future
Though parabolic flights offer limited microgravity time, the team plans to develop a more advanced version of the experiment suitable for the International Space Station (ISS). There, longer periods in microgravity could allow for the observation of turbulence, adding a crucial piece to the puzzle of planet formation.
The Collective Power of Collaboration
This pioneering experiment reflects the exceptional collaboration among Swiss institutions. Each participated, leveraging its unique strengths to push the boundary of knowledge in planetary formation:
- University of Bern: Instrument design and building expertise.
- University of Zurich: Planet formation theories and computational modeling.
- ETH Zurich: Observation and laboratory analysis experience of small solar system bodies.
Conclusion
The findings from the TEMPus VoLA experiment not only validate theoretical models but also provide vital insights into the early states of planetary formation. By confirming that shear-flow instability can manifest under protoplanetary disk conditions, the research lays the groundwork for more sophisticated future studies, which could ultimately elucidate how our solar system and Earth emerged from a primordial cloud of dust and gas.
For an in-depth exploration of this research, see the publication in Communications Physics.
