Have you ever looked up at the night sky and wondered — how did all of this come to be? Every planet you've ever read about, every moon, every rocky asteroid, and yes, even the ground beneath your feet, started as a microscopic speck of dust drifting around a newborn star. It sounds almost impossible. And for decades, a stubborn gap in our knowledge made it seem that way.
Welcome to FreeAstroScience, where we explain complex scientific ideas in clear, human language — because science belongs to everyone. We're glad you're here today. This story is one of the most exciting things to happen in planetary science this year, and we can't wait to share it with you.
A team of Swiss researchers, led by Dr. Holly L. Capelo at the University of Bern, has just provided the first experimental proof — obtained during zero-gravity parabolic flights — that a physical process called shear-flow instability genuinely occurs under conditions similar to those in planet-forming regions around young stars. Published in March 2026 in Communications Physics, a Nature Portfolio journal, this finding addresses a puzzle that has frustrated scientists for decades.
How do centimeter-sized pebbles grow into kilometer-sized building blocks of planets? That's the question this experiment answers. Stay with us to the end — we promise this turbulent tale will change how you think about the birth of worlds.
What's the Big Problem with Building Planets?
Planets are born inside protoplanetary disks — vast, swirling clouds of gas and dust that orbit around very young stars. Picture a cosmic pancake made of trillions upon trillions of tiny particles, stretching billions of kilometers across. That's the nursery where every planet in the universe starts its life.
The general story goes like this. At the smallest scale, fine dust particles — some tinier than a micrometer — bump into each other. They stick together through electrostatic forces. Slowly, they grow from specks to millimeter-sized grains.
At the largest scale, kilometer-sized rocky or icy bodies called planetesimals collide and merge. They snowball into bigger and bigger objects. The fastest-growing ones even grab gas from the disk, becoming gas giants like Jupiter.
So far, so good. But between these two stages, there's a problem. A big one.
The Meter-Size Barrier
Once dust clumps reach about a centimeter, the story breaks down. Instead of sticking together, they bounce off each other. Sometimes they shatter on impact. And if they drift too close to the host star, they simply evaporate
This is what planetary scientists call the "meter-size barrier." Getting from centimeter-sized pebbles to hundred-meter boulders seems almost impossible based on collisions alone. And this gap has puzzled researchers for decades .
Think of it like trying to build a sandcastle in the surf. You pack the sand together, but every wave knocks it apart. Something extra needs to happen — some force, some trick of physics — to hold things in place long enough for growth to take off.
The Fluid Idea
Since the early 2000s, theorists have pointed to a possible answer. They noticed that the gas-and-dust mixture in a protoplanetary disk doesn't just sit there — it behaves like a fluid . And fluids, as we know from watching clouds roll and rivers churn, can develop instabilities. These instabilities cause dust to clump into denser patches. The largest of those patches can collapse under gravity, forming planetesimals directly — no grain-by-grain stacking needed .
Several types of instabilities can happen, depending on conditions within the disk. One of the most important? The shear-flow instability .
The catch: until this experiment, nobody had ever proven it works under the ultra-thin gas conditions found in real protoplanetary disks.
What Is Shear-Flow Instability and Why Should We Care?
Let's slow down and unpack this concept. It's the heart of the whole discovery.
A shear-flow instability develops when two fluids with different speeds and densities slide past each other . Imagine two rivers meeting — one fast, one slow. At their boundary, turbulence stirs up. Small ripples grow into waves. Waves grow into rolling vortices. That's shear-flow instability in action.
The most famous version is the Kelvin-Helmholtz (KH) instability. You've probably seen it in nature without knowing its name. Those beautiful, wave-shaped clouds that look like ocean breakers rolling across the sky? That's a Kelvin-Helmholtz instability, forming where layers of air at different temperatures and speeds interact .
The Kelvin-Helmholtz Connection to Planet Birth
In a protoplanetary disk, dust particles settle toward the midplane — the disk's equator. As they accumulate, they create a dense layer that moves at a different speed than the gas above and below it . This velocity difference creates shear. And if that shear is strong enough, a KH-like instability can develop.
This instability can do two things:
- Concentrate dust into dense clumps, helping overcome the meter-size barrier.
- Generate turbulence that stirs the dust back out, preventing collapse .
Which outcome wins — clumping or dispersal — depends on the specific conditions: the dust-to-gas density ratio, particle sizes, and local velocities . Understanding this balance is essential for explaining how planets form.
But there was a serious gap. All of this existed only in mathematical models and computer simulations. Nobody had demonstrated it physically in the Epstein drag regime — the specific type of gas drag that applies in the extremely thin conditions of a protoplanetary disk, where the gas mean free path is larger than the dust particles themselves .
Dr. Holly L. Capelo and her team set out to change that.
How Do You Recreate Planet Birth on an Airplane?
You can't visit a protoplanetary disk. You can't build one in a regular lab, either — Earth's gravity ruins the experiment. On the ground, dust settles. Gas stratifies. The delicate dust-gas coupling you want to study gets drowned out by gravitational effects .
So what do you do?
You take your experiment to the sky.
The TEMPus VoLA Instrument
Starting in 2020, Dr. Capelo's team at the University of Bern designed and built a one-of-a-kind instrument: TEMPus VoLA (Timed Epstein Multi-Pressure Vessel at Low Accelerations) . It was purpose-built for parabolic flights — those special airplane maneuvers that create brief periods of weightlessness.
Here's what the instrument looks like:
- A cylindrical flow chamber, about 1 meter long with an 8 cm inner diameter .
- A vacuum system that drops internal pressure to about 0.9 millibar — roughly a thousandth of sea-level atmospheric pressure .
- High-speed cameras capturing 1,000 frames per second .
- A laser light sheet that illuminates a thin slice through the center of the chamber .
- A dust injector that feeds 10-micrometer silicon dioxide particles into the gas stream at a controlled rate .
The whole setup is secured in a flight-worthy rack that human operators can manage during the stomach-churning maneuvers of a zero-g flight .
Why Zero Gravity Changes Everything
During a parabolic flight, the aircraft climbs steeply at about 45 degrees, then dives. Each dive gives roughly 20 to 23 seconds of microgravity — a window where everything aboard floats freely .
"On Earth, gravity influences the behavior of the dust and gas," explained 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."
Remove gravity, and the experiment captures how dust and gas interact purely through their own drag forces. No contamination from settling. No buoyancy artifacts. Just the raw physics of two phases acting on each other — exactly what happens inside a protoplanetary disk .
The team flew several campaigns from the UZH Space Hub and through the European Space Agency (ESA). Each flight refined the conditions — gas pressure, particle injection rate, flow speed — to find the sweet spot where shear-flow instability would emerge .
What Did the Experiments Actually Show?
"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," explained Capelo .
That sentence carries decades of anticipation behind it. Let's break down what they found.
Pattern Formation: The Fingerprint of Instability
When the team analyzed particle velocity fields using particle image velocimetry (PIV), they didn't see the smooth, uniform flow you'd expect if dust particles simply rode along with the gas. Instead, they observed periodic patterns .
Regions of near-zero movement alternated above and below the midline of the flow. Those regions contained swirling motion — what physicists call non-zero vorticity — that grew more complex over time . This is precisely the signature of a Kelvin-Helmholtz-like instability. Small waves appear at the boundary between two fluid layers. The waves grow. Vortices form. The flow becomes more complex with each passing moment .
A purely laminar (smooth) flow wouldn't produce such patterns. The waves could only arise from the dynamic coupling between dust and gas — the particles collectively slowing the gas down, creating a density and velocity difference that triggered the instability .
A Frequency Spectrum That Tells a Story
To dig deeper, the team performed a wavelet analysis using the Morlet wavelet — a mathematical tool that reveals how oscillation frequencies evolve over time . The results painted a clear picture of an instability progressing through its stages:
- 4–5 seconds into measurement: Dominant frequencies concentrated above 300 Hz. Strong wave-like behavior. The instability was fresh and energetic. Maximum wavelet coefficient: 1.8 .
- 10–11 seconds: Frequencies drifted lower. Wavelet coefficients dropped to 0.9. The flow was growing more complex, with larger structures forming .
- 17–18 seconds: Lower frequencies dominated even further. Coefficients fell to 0.25. The system was evolving toward the kind of large-scale complexity that precedes full turbulence .
This progression — from sharp, high-frequency waves to broader, lower-frequency structures — matches theoretical expectations for how a KH instability develops from onset toward its fully turbulent state .
The smallest consistently observed wavelength? Approximately 3 centimeters — the critical wavelength at which coherent structures first emerge and grow .
The Mathematics of the Instability
The beauty of this experiment is that the results can be checked against well-known fluid dynamics equations. The two-fluid model treats dust as a pressureless fluid that exchanges momentum with the gas through drag forces . Below are the key equations that describe this system.
⚛ Gas Momentum Equation
∂ux / ∂t = (1 / tf) · (ρp / ρg) · (vx − ux)
The gas velocity ux changes over time in response to the dust. The ratio ρp/ρg (dust-to-gas density) controls how strongly the particles push back on the gas flow. The friction time tf sets how fast this coupling occurs.
⚛ Particle Momentum Equation
∂vx / ∂t = −(1 / tf) · (vx − ux)
Each dust particle accelerates (or decelerates) until it matches the gas velocity. The friction time tf determines how quickly particles couple to the gas — for this experiment, tf ranged from 3 to 9 milliseconds.
⚛ Phase Velocity of the Instability
vphase = (ρ1 v1 + ρ2 v2) / (ρ1 + ρ2)
The wave-like instability travels at a speed determined by the combined momentum of the gas layer (ρ1, v1) and the dust-laden layer (ρ2, v2). This is the center-of-mass velocity of the two-fluid system.
⚛ KH Instability Growth Rate
σ = kx · (v1 − v2) · √(ρ1 ρ2) / (ρ1 + ρ2)
The growth rate σ tells us how fast the instability amplifies. It depends on the wavenumber kx = 2π/λ (where λ ≈ 3 cm is the measured critical wavelength), the velocity difference between layers, and their density contrast. Larger density contrasts and velocity differences drive faster growth.
By combining their measured critical wavelength (~3 cm) and observed frequency range (300–500 Hz) with these equations, the team constrained the initial dust-to-gas ratio (ε) to values between 0.065 and 0.175 . These numbers are consistent with what we'd expect in planet-forming disks. The math works. The physics checks out.
The Key Numbers Behind the Discovery
The experiment operated in a very specific physical regime — one carefully designed to match the conditions in a protoplanetary disk. Here are the parameters that made this work :
A detail worth pausing on: the container-scale Reynolds number of just ~7 is far too low for turbulence to arise spontaneously . You typically need Re around 1,000 or more for that. So any complex flow behavior in this experiment came from one source alone — the interaction between dust and gas. That's what makes this result so clean and convincing.
Why Do Lab Experiments Matter for Astronomy?
You might ask: don't we have telescopes for this? Can't simulations handle it?
They help. But they're not enough — not yet.
Modern telescopes like ALMA and the James Webb Space Telescope can observe protoplanetary disks around distant stars. They can map the distribution of gas and dust, detect structures like rings and gaps, and track how disks evolve at different ages . But no telescope can zoom in far enough to see individual dust grains sticking together. That process happens on spatial and time scales far too small to detect from light-years away.
Computer simulations have made impressive strides, too. They can model dust-gas physics with increasing detail. But even our most powerful simulations can't yet resolve the smallest structures within a disk — the very structures where planet formation begins.
"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," said Dr. Antoine Pommerol of the University of Bern .
And then there's a deeper issue. We can't currently observe two-fluid hydrodynamics directly in astronomical data. Deviations from Keplerian rotation caused by pressure support — the kind of differential motion between dust and gas that drives these instabilities — remain extremely difficult to detect with today's observational precision .
That's exactly why controlled laboratory experiments like TEMPus VoLA are irreplaceable. They provide ground truth — a physical benchmark that theorists and observers can use to test, calibrate, and improve their models .
"Only experiments can 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," said Pommerol .
The experiment also tells us something profound about how turbulence itself originates in these disks. Scientists have long debated what drives turbulence in protoplanetary disks — and this work suggests that dust itself can be a source, not just a passive participant. By showing that shear instabilities can emerge from dust-gas interaction alone, without any external turbulence imposed, the team provides a physical basis for dust-driven momentum transport — a candidate mechanism that could explain angular momentum transfer in quiet regions of planet-forming disks .
What Comes Next? From Parabolic Flights to the ISS
There's a limitation the team openly acknowledges. Parabolic flights give only about 20 seconds of weightlessness at a time. That's enough to catch the instability in its early, pattern-forming stage. But it's not long enough to watch the full evolution into developed turbulence.
"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," said Capelo .
The instability didn't saturate within their 24-second measurement window. The wavelet power spectrum kept evolving — coefficients continued to drop, frequencies continued to shift — indicating the system was still growing in complexity . They caught the opening act, but the full performance is still ahead.
The Path to the International Space Station
The team's next step: build an improved version of TEMPus VoLA for the International Space Station (ISS) . Up there, continuous microgravity would last minutes or hours instead of seconds. That's enough time to:
- Watch the full lifecycle of the instability — from stable flow to linear growth, through pattern formation, and into full turbulence .
- Run the experiment many times, systematically varying particle size, injection rate, and gas pressure .
- Employ 3D Lagrangian particle tracking instead of 2D PIV, allowing deeper analysis of velocity-density correlations, particle clustering, and relative velocities in high-density regions .
- Push dust-to-gas ratios higher to explore the threshold where streaming instability sets in — a related but different mechanism that's also thought to be key for planetesimal formation .
"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," said Capelo .
A Swiss Collaboration Worth Celebrating
Behind this discovery lies a collaboration that brought together the best of Swiss science:
- University of Bern — instrument design and construction, data analysis, and project leadership.
- University of Zurich — theoretical expertise in planet formation, including the Center for Theoretical Astrophysics and Cosmology.
- ETH Zurich — laboratory analysis of small solar system bodies and geochemistry.
The NCCR PlanetS, the Swiss Space Office, and ESA/PRODEX programs all supported the effort . The UZH Space Hub and Novespace provided the parabolic flight infrastructure .
"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," said Capelo .
This kind of cross-disciplinary teamwork — experiment builders, theorists, and observers working side by side — is how we'll answer the biggest questions about our origins.
A Growing Family of Microgravity Experiments
The TEMPus VoLA work joins a growing body of planetary science done in microgravity conditions . Other teams have used parabolic flights to study crater ejecta emplacement under reduced gravity, the sorting of materials in protoplanetary disks, and physical processes on icy moons . Future experiments could investigate landslides on Mars, volcanic flows on Io, and surface processes in the icy environments of the outer solar system .
Each of these experiments brings us closer to a "hands-on" understanding of phenomena we can't observe directly in space.
Final Thoughts
Let's step back and take in the full picture.
For decades, we've known the broad story of how planets form — dust sticks together, small things become big things, and eventually a world is born. But the middle chapter was missing. How does nature jump from centimeter-sized pebbles to kilometer-sized planetesimals? That question haunted planetary science like an unfinished sentence.
Now, thanks to a team of researchers flying a vacuum chamber on a diving airplane, we have the first experimental proof that shear-flow instability — a process long predicted by theory — actually occurs under the gas conditions found in real protoplanetary disks . Dust particles can collectively behave as a fluid, push back on the gas, create shear, and trigger the kind of turbulence that concentrates matter into denser and denser patches .
This doesn't answer everything. The team caught the instability in its early stages — the full turbulent development is still waiting to be observed . The ISS experiments ahead will tell us more. And there are other instabilities — streaming instabilities, resonant drag instabilities — that also play roles we're still working to understand .
But what we've gained is something priceless: empirical confirmation that the physics we've been assuming in our models actually works in reality. That's the foundation everything else is built upon.
The road from dust to planets is a long one — billions of years and trillions of collisions. Yet here we are, standing on a planet that was once nothing but a smear of particles in a young star's disk, asking how it all happened. If that doesn't stir something in you, nothing will.
This article was written for you by FreeAstroScience.com, where we take complex scientific ideas and explain them in plain, honest language. We believe science isn't a privilege — it's a birthright. And we believe in something the great painter Goya once warned us about: the sleep of reason breeds monsters. So keep your mind awake. Stay curious. Ask questions. Come back often — there's always more to learn, and you're never alone in wanting to understand the universe.
We'll see you in the next one. 🌌
📚 Sources & Further Reading
- University of Bern. "From Dust to Planets: A Turbulent Story." Press release, March 17, 2026. — unibe.ch
- Petersen, C. C. "Parabolic Flight Experiments Delve into Planetary Formation." Universe Today, March 23, 2026. — universetoday.com
- Capelo, H.L., Bodénan, J.-D., Jutzi, M. et al. "Experimental Evidence for Granular Shear-Flow Instability in the Epstein Regime." Communications Physics 9, 88 (2026). — doi.org/10.1038/s42005-026-02531-9
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