Nebular hypothesis

The Nebular Hypothesis: A Journey Through Time

Imagine peering into the vast expanse of space, where stars and planets dance in harmony. The nebular hypothesis is like a cosmic storybook, explaining how our Solar System came to be. This theory suggests that the Sun and its planets formed from a spinning disk of gas and dust, much like a whirlpool forming from a bathtub drain.

Origins and Evolution

But where did this idea come from? The nebular hypothesis was first proposed by Immanuel Kant in 1755. He envisioned the Sun as a result of a contracting cloud of gas, with planets forming from smaller clumps within it. Later, Pierre Laplace refined this concept in 1796, suggesting that the Solar System formed through a series of shedding rings.

Could Swedenborg have had similar ideas even earlier?

The Protostellar Disk

Stars form within giant molecular clouds. These clouds collapse and fragment over millions of years, leading to the formation of protostars. The process is akin to a spinning top that gradually slows down as it loses its initial angular momentum.

Protostellar Development

A protostar forms from the collapse of a giant molecular cloud. It takes around 100,000 years for this process to occur. As the core compresses, it heats up and eventually becomes a seed for a star. Surrounding this core is a disk that accretes onto it, creating bipolar jets as the gas spreads out.

How does material lose angular momentum during accretion?

The Formation of Planets

Once the protostar reaches the classical T Tauri stage, the disk begins to dissipate. Dust grains form close to the center, containing crystalline silicates. These grains stick together, forming larger particles and eventually planetesimals measuring 1 km across or more.

Gravitational Instability

The formation of giant planets is a complex process involving gravitational instability within the disk. This leads to rapid formation of gas giants like Jupiter and Saturn in massive disks, while Uranus and Neptune’s formation remains a mystery due to their late start.

Planetary Embryos and Growth

The growth of planetary embryos follows two stages: oligarchic accretion and the merger stage. Oligarchs dominate growth, with smaller bodies unable to catch up. This process leads to a limited number of Earth-sized planets.

Giant Planet Formation

Two theories exist for giant planet formation: disk instability and core accretion. The core accretion model is the most promising, involving two stages: core formation and gas accretion. Giant planets form through a similar process to terrestrial planet formation but with ice-rich planetesimals.

Exoplanet Discoveries

The discovery of thousands of exoplanets has challenged our understanding of planetary systems. Hot Jupiters, warm-Jupiters, super-Earths, and tightly packed inner planets have unique orbits that hint at their formation processes.

Migration and Resonance

The migration of super-Earths is likely Type I due to smaller mass. Resonant orbits indicate migration occurred in some systems, while spacing suggests instability after gas disk dissipation. The absence of Super-Earths in the Solar System may be due to Jupiter blocking their inward migration.

Conclusion

The nebular hypothesis remains a cornerstone of our understanding of planetary formation. From the initial collapse of molecular clouds to the final stages of planet growth, this theory provides a framework for explaining the birth and evolution of our Solar System and beyond. As we continue to explore the cosmos, new discoveries will undoubtedly refine and expand upon these ideas.