The frequency of collisions between objects in a disk, which magnetic fields can drastically reduce.The merger rate is the frequency with which objects in a disk — such as planets in a protoplanetary disk or stars in a galactic disk — collide and merge with each other. This rate is a key parameter in models of planetary formation, galaxy evolution, and gravitational wave astronomy. Magnetic fields can drastically reduce this rate by creating turbulence and pressure that prevent objects from approaching each other closely enough to merge.
In a disk, objects orbit at different speeds depending on their distance from the center. Differential rotation causes them to approach each other, but mergers only occur if their relative velocity is low enough for gravity to bind them. Magnetic fields in the disk can generate instabilities that stir up the gas and dust, increasing relative velocities and making mergers less likely. This effectively lowers the merger rate.
Merger rates are used in simulations of planet formation and in predicting the frequency of gravitational wave events from merging black holes or neutron stars.
In some protoplanetary disks, magnetic fields can reduce the merger rate so much that planet formation is delayed by millions of years.
