Nebular hypothesis

The nebular hypothesis is the leading scientific explanation for how the Solar System—and other planetary systems—came to be. It describes a process where the Sun was once surrounded by a swirling cloud of gas and dust, and the planets gradually formed as this material clumped together. The idea was first developed by Immanuel Kant in 1755, then refined by Pierre Laplace in 1796. While originally applied only to our Solar System, scientists now believe this same mechanism creates planetary systems throughout the universe. The modern version of this idea is called the solar nebular disk model (SNDM). It successfully accounts for several key features of the Solar System: the planets’ orbits are nearly circular, they all lie in roughly the same flat plane, and they revolve in the same direction as the Sun rotates. Although some parts of the original theory survive in current models, most of its details have been replaced.

According to the nebular theory, stars are born inside vast, dense clouds of molecular hydrogen known as giant molecular clouds (GMCs). These clouds are gravitationally unstable, so matter within them gathers into smaller, denser pockets. These pockets rotate, collapse, and eventually form stars. Star formation is a complex process that always produces a gaseous protoplanetary disk (or proplyd) around the young star. Under certain conditions—which are not yet fully understood—this disk can give rise to planets. Thus, planetary systems are now seen as a natural byproduct of star formation. A Sun-like star typically takes about 1 million years to form, and the surrounding protoplanetary disk evolves into a planetary system over the next 10 to 100 million years.

The protoplanetary disk is an accretion disk that feeds material into the central star. Initially extremely hot, the disk cools during what is called the T Tauri star stage. At this point, small dust grains made of rock and ice can form. Over time, these grains may stick together to create kilometer-sized planetesimals. If the disk is massive enough, runaway accretion begins, leading to the rapid formation—within 100,000 to 300,000 years—of Moon- to Mars-sized planetary embryos. Near the star, these embryos undergo violent collisions and mergers, eventually producing a few terrestrial planets. This final stage takes roughly 100 million to a billion years.

The formation of giant planets is more complicated. It is thought to occur beyond the frost line, where planetary embryos are composed mostly of various ices. As a result, these embryos are several times more massive than those in the inner disk. What happens after the embryo stage is not entirely clear. Some embryos appear to keep growing until they reach 5 to 10 Earth masses—the threshold needed to begin pulling in hydrogen and helium gas from the disk. Gas accumulation around the core is initially slow, lasting several million years, but once the protoplanet reaches about 30 Earth masses, the process accelerates into a runaway phase. Planets like Jupiter and Saturn are thought to gather most of their mass in just 10,000 years. Accretion stops when the gas supply runs out. Formed planets can also migrate over long distances during or after their formation. Ice giants like Uranus and Neptune are believed to be failed cores—they formed too late, when the disk had nearly vanished.

**History**

There is evidence that Emanuel Swedenborg first proposed parts of the nebular theory in 1734. Immanuel Kant, who was familiar with Swedenborg’s work, expanded on the idea in 1755 with his *Universal Natural History and Theory of the Heavens*. He argued that gaseous clouds (nebulae) rotate slowly, gradually collapse and flatten under gravity, and eventually form stars and planets. Pierre-Simon Laplace independently developed a similar model in 1796 in his *Exposition du système du monde*. He imagined the Sun originally had a vast, hot atmosphere that filled the entire volume of the Solar System. In his view, this protosolar cloud contracted and cooled, flattening and spinning faster as it did so. It shed a series of gaseous rings, and the planets condensed from this material. Laplace’s model was more detailed and on a smaller scale than Kant’s. While it dominated the 19th century, it ran into serious problems. The main issue was the distribution of angular momentum: the planets hold 99% of it, which the nebular model could not explain. As a result, astronomers largely abandoned this theory of planet formation at the start of the 20th century.

According to some accounts, a major critique came in the 19th century from James Clerk Maxwell, who is said to have argued that different rotation rates between the inner and outer parts of a ring would prevent material from condensing. However, both the critique and its attribution to Maxwell have been shown to be incorrect upon further investigation. The original error was made by George Gamow in some popular publications and has been repeated ever since. Astronomer Sir David Brewster also rejected Laplace’s idea. In 1876 he wrote that believers in the nebular theory considered it certain that Earth derived its solid matter and atmosphere from a ring thrown off by the solar atmosphere, which later contracted into a solid sphere, from which the Moon was then thrown off by the same process. He argued that under this view, the Moon must have carried off water and air from the Earth and therefore must have an atmosphere.