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.
Rico Visser at the University of Amsterdam in the Netherlands and his …