Planet formation starts with tiny, micron-to-millimeter-sized specks of cosmic dust swirling in the massive, gaseous disks around newborn stars. These dust grains are too small for gravity to influence their initial aggregation into larger clumps. Non-gravitational forces must act as the primary glue to bridge this gap between dust and planetary building blocks. The initial mystery of how these minute particles first stick together is solved by understanding how they acquire an electric charge and the molecular forces that take over once they make contact.
How Dust Particles Become Charged
Cosmic dust grains are rarely electrically neutral due to the energetic environments of space, such as protoplanetary disks. The most common charging mechanism is photoemission, where high-energy ultraviolet (UV) radiation from the nearby star strikes the dust surface. This radiation knocks electrons free, causing the grain to lose negative charge and acquire a net positive electrical charge.
A second mechanism involves the surrounding plasma, a mix of free electrons and positively charged ions. Dust grains in these regions collect electrons from the plasma, which move much faster than the heavier positive ions. Since they acquire more electrons than they lose, the dust grains often develop a net negative charge. Micron-sized dust particles can acquire a surface potential in the range of 1 to 10 Volts, depending on the location and energy of the solar wind.
A third process is triboelectric charging, which occurs when two dust grains collide and rub against one another. Similar to generating static electricity, this friction causes a transfer of electrons between the two surfaces. The collision can result in one grain becoming positively charged and the other negatively charged. This sets the stage for subsequent electrostatic attraction and the beginning of accretion.
Electrostatic Attraction in Space
Once dust particles acquire an electric charge, the resulting electrostatic force becomes the dominant mechanism for clumping them together over short distances. This force follows the principle that objects with opposite electrical charges attract one another, as described by Coulomb’s law. In a protoplanetary disk, a mixture of positively and negatively charged grains can arise due to the varying charging mechanisms.
The attraction between oppositely charged particles can be significantly stronger than gravity at the scale of micron-sized dust, pulling the grains toward each other. This attraction allows the dust to clump into millimeter- to centimeter-sized aggregates, often called “pebbles.” The efficiency of this growth depends heavily on the velocity of the collisions.
If the collision speed is too high, the electrostatic attraction can be overcome, leading to bouncing, fragmentation, or erosion. Experiments suggest that particles must collide at very low velocities, often less than 1.5 feet per second, for the static electricity to successfully make them “sticky.” Electrostatic forces can also create a “charge barrier” when all particles acquire the same charge, leading to mutual repulsion that temporarily halts growth. Despite this, electrostatic forces dominate over self-gravity in the early stages of disk evolution, accelerating the growth of small particulates.
The Power of Molecular Attraction
While electrostatic forces bring charged particles together over a distance, a second interaction is necessary for the final, permanent “stick.” This is the Van der Waals force, a form of molecular attraction that acts only when the particles are almost touching. This force is not dependent on the net charge of the particle, but rather on the instantaneous, fluctuating electrical polarization within the molecules themselves.
Electrons are constantly in motion, and the electron cloud around a molecule may be momentarily denser on one side. This temporary imbalance creates an instantaneous dipole, a tiny, fleeting separation of charge. This dipole then induces a corresponding dipole in a nearby molecule, creating a synchronized attraction between the two. This component, known as the London dispersion force, is the main force responsible for the final cohesion of non-polar dust grains.
The strength of the Van der Waals force is highly dependent on distance and drops off rapidly. It is negligible until particle surfaces are separated by only a few atomic diameters. This force acts as the strong, short-range glue that converts a low-velocity electrostatic encounter into a permanent sticking event, preventing aggregates from bouncing apart. Electrostatic attraction provides guidance, and Van der Waals provides final adhesion, working together to drive the accretion that leads to planetesimals.