Summary
Highlights
This section introduces the solar nebula hypothesis, which explains how solar systems, including our own, form. The theory must account for observations such as planets orbiting in the same direction and plane, different planet types, and the origin of various celestial bodies like asteroids and comets.
The solar nebula hypothesis posits that solar systems evolve from a flattened disk of gas and dust within an interstellar cloud. These vast clouds, common in our galaxy, consist mostly of hydrogen and helium, with a small percentage of dust grains. The collapse of such a cloud, once triggered, leads to the formation of stars and planetary systems.
An interstellar cloud, typically stable, can be triggered to collapse by events like a supernova. As it collapses, gravity pulls material inward, and due to the conservation of angular momentum, the cloud spins faster, flattening into a protoplanetary disk. Most material forms the central star, while the remaining gas and dust in the disk are where planets form. Such disks have been observed around other forming stars.
Temperatures within the protoplanetary disk vary, with hotter regions closer to the forming sun and cooler regions further out. This temperature gradient dictates which materials condense from gas into solid or liquid forms. Iron and silicates condense at higher temperatures closer to the sun, while ices form at cooler, outer regions. These condensed solid flakes then begin to stick together through static electricity and gravity, a process called accretion, eventually forming larger bodies known as planetesimals.
As planetesimals grow through collisions, they heat up and eventually melt internally. In the cooler outer disk, more solid material (rock, metal, and ices) is available, allowing planets to grow larger and faster. Once they reach about ten Earth masses, these outer planets can capture hydrogen and helium from the disk, forming gas giants. Inner terrestrial planets, being smaller, cannot retain these lighter gases.
After planets form and their surfaces cool, remaining planetesimals striking them create craters. This 'cratering' is evident on geologically inactive bodies like the Moon and Mercury. Moons of large planets often form in miniature solar systems, while many smaller moons, especially those of Mars and jovian planets, are captured asteroids. Earth's Moon is unique, believed to have formed from a giant impact.
Planetary migration, particularly of Uranus and Neptune, scattered many planetesimals, forming the Oort cloud (comets) and influencing the orbits of other bodies like Pluto. This migration also caused the 'late heavy bombardment,' a period 4.2 billion years ago with increased cratering, suggesting comets brought water to Earth through inward migration. This migration theory helps explain observed solar system configurations and events.
The final stage involves atmosphere formation. Outgassing from volcanic activity and vaporization of comets/asteroids on impact likely formed the atmospheres of inner terrestrial planets, primarily composed of carbon dioxide, water vapor, and nitrogen. Outer planets, however, captured their hydrogen and helium-rich atmospheres directly from the solar nebula due to their larger size and gravity.