Nebular hypothesis

The nebular hypothesis posits that planetary systems, including our own, originate from the gravitational collapse of a giant molecular cloud (GMC). These immense interstellar clouds, primarily composed of molecular hydrogen and helium, are not static but possess inherent instabilities. When a region within a GMC becomes sufficiently dense, perhaps triggered by a supernova shockwave or galactic collision, its self-gravity overcomes internal pressure, initiating a collapse.

As the cloud contracts, conservation of angular momentum causes it to spin faster and flatten into a protoplanetary disk surrounding a central protostar. This protostar continues to accrete mass from the disk, eventually reaching the temperature and density required for nuclear fusion, thus becoming a star. The formation of the central star is a rapid process, typically taking around a million years.

The surrounding protoplanetary disk, however, is the cradle for planet formation, evolving over tens to hundreds of millions of years.

The protoplanetary disk is a dynamic environment where the raw materials for planets are processed. Initially very hot, the disk cools over time, particularly during the T Tauri stage of the young star. This cooling allows for the condensation of refractory materials (like silicates and metals) closer to the star, and volatile ices (water, methane, ammonia) further out, beyond the 'frost line.' Dust grains within the disk begin to aggregate through electrostatic forces and gentle collisions, forming larger aggregates.

These aggregates grow into kilometer-sized planetesimals. If the disk is massive enough, these planetesimals undergo runaway accretion, rapidly growing into Moon- to Mars-sized planetary embryos. In the inner solar system, these embryos experience violent collisions and mergers, eventually forming the terrestrial planets.

The timescale for this inner planet formation is relatively quick, completing within approximately 100 million years.

The formation of gas and ice giants follows a different, more complex pathway, occurring beyond the frost line where abundant ices contribute significantly to core mass. Planetary embryos here can grow much larger, reaching several Earth masses. The critical step is reaching a 'critical mass' threshold (around 5-10 Earth masses) that allows them to gravitationally capture vast amounts of hydrogen and helium gas from the surrounding disk.

This gas accretion begins slowly but accelerates dramatically once the core reaches about 30 Earth masses, a process known as runaway gas accretion. Planets like Jupiter and Saturn are thought to have accumulated the bulk of their mass in a mere 10,000 years. The process halts when the gas in the disk is depleted.

Notably, these massive planets can migrate significant distances within the disk during or after their formation. Ice giants like Uranus and Neptune are theorized to be 'failed cores,' forming too late when the gas disk had largely dissipated, thus limiting their gas envelope.

The nebular hypothesis was first articulated by Immanuel Kant in 1755 and later independently developed by Pierre-Simon Laplace in 1796. Their original models proposed that a rotating solar nebula collapsed and flattened, with planets forming from the remaining material in rings. While the fundamental concept of formation from a rotating disk remains, modern astrophysics has refined the details significantly.

The 'solar nebular disk model' (SNDM) is the current iteration, incorporating concepts like core accretion for giant planet formation, disk evolution, and planet migration. The hypothesis successfully explains key observations such as the coplanarity and near-circularity of planetary orbits, the direction of planetary rotation matching the Sun's, and the compositional gradient across the solar system. Its applicability has been extended to exoplanetary systems, suggesting that planet formation is a ubiquitous consequence of star formation throughout the cosmos.