planetesmaal

Planetesmaal refers to the hypothetical process by which planets are formed from the protoplanetary disk of dust, gas, and other solid particles orbiting a young star. This theory is widely accepted in modern astronomy and explains the formation of our Solar System and other planetary systems observed around stars.

The process begins with the collapse of a molecular cloud triggered by external forces such as shockwaves from a nearby supernova or galactic collisions. As the cloud collapses under gravity, it flattens into a rotating disk known as a protoplanetary disk. Within this disk, small solid particles, or planetesimals, begin to form through the sticking together of dust grains and ice crystals. These collisions cause particles to grow gradually over time through a process called accretion.

Once planetesimals reach a critical mass, their gravitational pull becomes strong enough to attract more material, leading to rapid growth. Over millions of years, these bodies merge into larger protoplanets. Some protoplanets may migrate inward or outward due to interactions with the disk or other bodies, altering their orbits. Gas giants, such as Jupiter and Saturn, form from the accumulation of hydrogen and helium gas in the outer regions of the disk, while rocky planets like Earth develop closer to the star where temperatures are too high for volatile compounds to condense.

The planetesimal theory helps explain the distribution of planetary sizes, compositions, and orbital characteristics within a solar system. Observations of young stars with surrounding disks, as well as computer simulations, support this model. Variations in disk conditions, such as temperature gradients and chemical composition, influence the types of planets that form, contributing to the diversity of exoplanetary systems discovered in recent years.