Electricity may be more important to making planets than we thought. We aren’t sure how tiny particles come together to build baby planets, but dropping glass beads from the top of a tall tower has shown that it may be with some help from static electricity.
The very first seeds of planets are made of micrometre-sized grains of dust, which bump into one another as they orbit a star and stick together in fluffy clumps. As more and more tiny grains stick together, the clumps start to compact, until they are no longer fluffy and start to bounce off one another like billiard balls instead of sticking. This happens when the clumps are millimetres across and is called the bouncing barrier.
In order to build a planet, those millimetre-sized dust bunnies have to overcome the bouncing barrier and get bigger. It has been suggested in the past that this may be enabled by static electricity – as the dust particles collide and rub together, they gain electric charges that can encourage them to stick together.
Tobias Steinpilz at the University of Duisburg-Essen in Germany and his colleagues investigated this using the Bremen drop tower, a hollow turret about 120 metres high that acts as a vacuum chamber in which falling objects behave like they would in the microgravity of space.
They used the tower’s built-in catapult to throw a chamber containing 0.4-millimetre glass beads up towards the top of the tower, then allowed it to fall, watching it with a high-speed camera installed within the falling chamber. They found that the beads did gain electrical charge from static electricity and stuck together in clumps up to several centimetres across.
“When you have charged particles and they form centimetre-sized clusters like we observed in our experiments and our simulations, we can close this gap in size caused by the bouncing barrier,” says Steinpilz. The particles are then free to clump together even more with the help of gravity and eventually become planets.