IT HAS long been a mystery why planets tend to spin in the same direction as their stars do. The answer may lie in whether they are made from huge rocks or pebbles.
The standard model of planetary growth states that planets coalesce from giant rocks that are kilometres across. But models of that process result in planets that barely spin at all because similar amounts of boulders hit the fledgling planet from all sides.
Advertisement
Rico Visser at the University of Amsterdam in the Netherlands and his colleagues examined an alternative model of planetary formation called pebble accretion. In this, planets form from lumps that are between a millimetre and a metre across. These rocks are more affected by friction from gas surrounding stars than bigger boulders would be.
Visser and his colleagues simulated individual pebbles on trajectories around a growing planet. “For pebbles to be swept up by the protoplanet, the key is long-lasting encounters of pebbles with protoplanets,” he says.
“It has long been a mystery why planets tend to spin in the same direction as their stars”
The team’s models suggest that small rocks circulating between a nascent planet and its star keep being pushed back towards the planet by the friction they experience from the surrounding gas. This increases the time they spend near the planet.
This means that when the rocks fall onto the protoplanet, they tend to hit it at an angle that spins it in the same direction that the star is turning – called a prograde direction. Rocks coming in from the other side of the planet, away from the star, tend to fly past without colliding.
