From pebbles to planets

Thousands of millimetric grains assemble into a cluster after colliding with each other in microgravity for a few minutes. Shaken and charged on their way to space, the tiny particles bind together in lumps of about three centimetres.

Scientists are studying this collision-driven growth to understand how planets are born. Planets are formed when dust and rock in a disc around a young star collide and combine to form ever larger bodies. It takes millions of years for dust grains to become a planet.

The cosmic journey begins with a cloud of gas and dust. Dust particles collide and form aggregates, with gravity constantly puling more matter towards the growing clusters until they eventually become fully fledged planets.

The processes behind planetary formation are not yet fully understood, so astrophysicists at the University of Duisburg-Essen in Germany decided to remove Earth’s gravity from the equation. They had done some tests during a drop tower experiment, but nine seconds of microgravity were not enough.

The team wanted to observe the collision speed and electrical charge of the particles for longer on a suborbital flight. 

This experiment flew onboard SubOrbital Express-3, a MASER rocket launched from Esrange Space Center in Kiruna, northern Sweden, in 2022. The tiny particles enjoyed six minutes of undisturbed microgravity as the sounding rocket climbed to and descended from an altitude of 260 kilometres.

From ground control, researchers observed compact clusters growing to about three centimetres in size and measured the speed at which particles collided.

Particle speed and size turned out to be critical for the stability of the clusters. Too fast or too big, and the lump disintegrated with the particles bouncing off each other or breaking apart existing formations.

However, when half-millimetre grains travelled at 0.5 metres per second, they kept colliding, became electrically charged and attracted each other. Numerical simulations also showed that the collisions resulted in a strong electrostatic charge and attraction.

These findings suggest that both the grains sent to space in this experiment and the disc-shaped cloud around a young star would dissolve or break up under the same physical conditions. Researchers are now incorporating these results into physical models of protoplanetary discs and particle growth to better understand how planets are born.

Next stop: the International Space Station.

Full article in Nature Astronomy: ‘The growth of super-large pre-planetary pebbles to an impact erosion limit’.