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.
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. At one end, the fine dust particles collide and clump together electrostatically, growing in size up to a few millimeters. At the other, planetesimals - rocky or icy bodies of a few hundred meters to a few kilometers in size - collide, merge and aggregate, slowly growing into rocky or icy planets, and for the fastest ones eventually accreting gas to become giants. In between, however - from centimeter-sized to hundred meter-sized boulders - most planet-forming scenarios hit a "barrier" that prevents further growth. At these sizes, clumps tend to bounce off each other, break up in the collision process, or even evaporate when drifting too close to their star. This barrier has puzzled scientists for decades.
Since the turn of the century, however, theoretical models have proposed an additional mechanism that could fill the gap. Because the gas-dust mixture behaves like a fluid, various hydrodynamical instabilities can develop in it and cause the dust to clump into denser clouds, leading for the largest ones to eventually form planetesimals. Each of these instabilities arises under specific conditions and in different regions of the disk, and can affect it in distinct ways. One of these instabilities suspected to play an essential role is the shear-flow instability, which occurs at the interface between two fluids with different properties - in this case mainly velocity and density. However, whether such shear-flow instabilities really occur or not under the extremely tenuous gas conditions in protoplanetary disks had never been proven experimentally. Using a unique experiment that exploits the micro-gravity conditions during parabolic or "0g" flights, a team led by Holly L. Capelo from the Space Research and Planetology Division at the Physics Institute of the University of Bern has now shown experimentally that shear-flow instabilities can indeed form - even in extremely thin gas. The study has just been published in
Flying an experiment in zero gravity
To investigate these flow instabilities which can, depending on the exact conditions, either foster or hinder dust clumping into planetesimals the team around Capelo started developing the TEMPus VoLA experiment in 2020, with a one-of-a-kind instrument at its core. Funded by the NCCR PlanetS and the Swiss Space Office, it was designed and assembled at the University of Bern, in collaboration with the University of Zurich and ETH Zurich. The instrument is equipped with high-speed cameras that track the behavior of dust particles in an extremely thin gas under vacuum conditions. It was built specifically for parabolic flights, a special type of flight that provides micro-gravity. "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." During parabolic flights, a specially adapted aircraft follows a trajectory in which it repeatedly climbs and dives at angles of about 45 degrees. Each dive phase provides weightlessness for around 20 seconds, while the climb simulates stronger gravity than on Earth. During several flight campaigns from the UZH Space Hub and the European Space Agency (ESA), the team systematically refined and varied the conditions of the experiment to test when shear-flow was triggered. "To sum up, we recreated the conditions that arise in the planet-forming regions of protoplanetary discs, and we managed to demonstrate that this theoretically proposed shear-flow instability is not just a mathematical construct, but can actually occur in reality," explains Capelo.
However, parabolic flights only offer very short phases of weightlessness. "Once the instability starts, we noticed characteristic patterns developing in the flow of the material. Yet, the limited micro-gravity time prevents us from observing how these patterns evolve into fully developed turbulence," explains Capelo. The team is therefore developing a more advanced version of the experiment to operate on a space station such as the International Space Station (ISS). There, much longer periods in micro-gravity would allow turbulence to be observed, adding another crucial piece to the puzzle of planet formation.
To the origins of the Solar System
To understand how planetary systems form, astronomers rely on a range of elements. Modern telescopes can observe the protoplanetary disks orbiting a star, and determine the properties of the gas and dust within it and understand their global evolution by studying disks of different ages. On the theory side, computer simulations describe mathematically and physically the disk evolution and planet formation. None of them, however, has yet the capacity to study disks with a resolution high enough to distinguish the smallest structures within it. "In our Solar System, comets and asteroids bear witness of the early stage of our system and provide clues to the composition and structure of planetesimals, but we still cannot probe their early evolution," says Dr. Antoine Pommerol of the University of Bern. "Only experiments can bridge this knowledge gap and reveal the crucial details of the dust and gas movement on spatial and time scales so small that they cannot be observed directly in the cosmos." The new experiment not only provides a direct confirmation that a long-theorized phenomenon can occur under protoplanetary disk-like conditions, it will also help to improve theoretical models and refine simulations. "This, in turn, will lead to a better understanding of the overall picture of planetary systems formation - and ultimately how our own Solar System, and Earth itself, formed billions of years from a simple cloud of dust and gas", says Capelo.
The fruits of national collaboration across Switzerland
"Bringing such a pioneering experiment to life was a major challenge," says Capelo. While the NCCR PlanetS funded the initial development of the project, each participating institution contributed a unique expertise to it: from the instrument building proficiency of the University of Bern, to the planet formation theory knowledge of the University of Zurich, and the experience of the ETH Zurich in the observation and laboratory analysis of small solar system bodies.
The expertise of the UZH Space Hub, ESA/PRODEX programs, and Novespace in preparing and conducting parabolic flights was also a key component of the project. "Overall, the ability of Swiss institutions to join forces efficiently and collaborate closely on this project led to its remarkable success and to breakthroughs in the investigation of the fundamental physics of planet formation. These results pave the way to hopefully observe such mechanisms operating in the cosmos," concludes Capelo.
