14.3 Formation of the Solar System

Key Constraints and the Solar Nebula

Any good theory of solar system formation has to explain several observable patterns. These patterns fall into three categories: motion, composition, and age. Together, they point toward a single origin story involving a rotating disk of gas and dust.

Constraints on Solar System Formation

Motion

The planets all orbit the Sun in the same direction (prograde) and in nearly the same flat plane. Their orbits are also close to circular. This strongly suggests everything formed from a single rotating, flattened disk of material.

• The Sun itself rotates in the same prograde direction, sharing angular momentum with the planets.
• Most planets also spin in the same direction they orbit, further supporting a common origin from one rotating disk.
• Venus and Uranus are exceptions. Venus rotates backward (retrograde) and Uranus is tipped on its side, likely due to massive collisions early in their histories.

Composition

There's a clear divide between the inner and outer solar system:

• Inner (terrestrial) planets are small and rocky, made of metals and silicates. They formed in a hotter region where lightweight volatile compounds like water and methane couldn't condense into solids.
• Outer (giant) planets are large and gaseous, composed mostly of hydrogen and helium. Their composition closely matches the Sun's, indicating they captured gas directly from the original nebula.
• The Sun is about 98% hydrogen and helium by mass, just like the gas giants. This shared composition supports the idea that the Sun and planets formed from the same cloud of material.

Age

• Radiometric dating of meteorites gives ages of about 4.6 billion years, making them the oldest known solid objects in the solar system.
• Since meteorites formed very early in the solar system's history, their age sets a firm lower limit on the age of the entire system.
• Models of star formation and the Sun's estimated age both suggest the whole process of forming planets took roughly 100 million years or less, which is fast on cosmic timescales.

Changes in the Solar Nebula

The solar system formed through a sequence of physical changes in a collapsing cloud of gas and dust called the solar nebula. Here's how that process unfolded:

1. Gravitational collapse. The nebula began collapsing under its own gravity. As it contracted, its density increased, accelerating the collapse further.
2. Flattening into a disk. Because the nebula was already slowly rotating, conservation of angular momentum caused it to flatten into a disk shape (a protoplanetary disk). Think of a spinning ball of pizza dough flattening out as it spins faster.
3. Heating. Gravitational energy converted into heat as material fell inward. The center of the disk grew hot enough for hydrogen fusion to ignite, and the Sun was born.
4. Cooling in the outer disk. While the center was blazing, the outer regions of the disk cooled enough for solid particles to condense out of the gas.
5. Particle growth. Tiny solid grains collided and stuck together through electrostatic forces and, eventually, gravitational attraction. Over time, these clumps grew into planetesimals, the kilometer-scale building blocks of planets.
6. Clearing the disk. As planets grew, they gravitationally swept up remaining gas and dust. Solar radiation and stellar winds from the young Sun expelled whatever was left, leaving behind the planets and smaller debris.

Planet Formation and Evolution

Terrestrial vs. Giant Planet Formation

The frost line (also called the snow line) is the key boundary. Inside it, temperatures were too high for ices to form. Outside it, water, methane, and ammonia could all freeze into solid particles.

Terrestrial planets (Mercury, Venus, Earth, Mars):

• Formed inside the frost line, where only metals and silicates could condense.
• Limited solid material meant these planets stayed relatively small.
• Grew through collisions and mergers of rocky planetesimals, a process called accretion.

Giant planets (Jupiter, Saturn, Uranus, Neptune):

• Formed beyond the frost line, where icy particles added to the supply of metals and silicates.
• Much more solid material was available, so cores grew larger and faster.
• The leading explanation is the core accretion model, which works in two stages:
  1. Icy and rocky planetesimals collided to build solid cores of roughly 10 Earth masses.
  2. These massive cores then gravitationally captured enormous amounts of hydrogen and helium gas from the surrounding disk.
• An alternative idea, the gravitational instability model, proposes that dense clumps in the disk collapsed directly into gas giants without needing a large core first. This model is still debated.

The composition difference between terrestrial and giant planets directly reflects where they formed. Rocky worlds come from the hot inner disk; gas-rich worlds come from the cold outer disk.

Post-Formation Solar System Events

The solar system didn't settle into its current arrangement right away. Several major events reshaped it after the planets formed.

Late Heavy Bombardment (LHB)

Between roughly 4.1 and 3.8 billion years ago, the inner solar system experienced a spike in asteroid and comet impacts. Evidence comes from dating the large impact basins on the Moon. The LHB likely scarred all the terrestrial planets with craters and may have delivered significant amounts of water and organic molecules to early Earth.

Giant Planet Migration

The giant planets probably didn't form where we see them today. Gravitational interactions with leftover gas and dust in the disk caused them to migrate, with Jupiter and Saturn shifting outward over time. This outward migration likely destabilized the orbits of nearby asteroids and comets, which may have triggered the LHB.

Formation of the Oort Cloud and Kuiper Belt

Not all icy planetesimals ended up in planets. The giant planets' gravity flung many of them outward:

• The Kuiper Belt is a disk-shaped region of icy bodies (including dwarf planets like Pluto) just beyond Neptune's orbit.
• The Oort Cloud is a vast spherical shell of icy objects extending up to about a light-year from the Sun. Long-period comets originate here.

Both regions are essentially leftover construction debris from the outer solar system.

Planetary Differentiation

Once planets grew large enough, their interiors separated into layers based on density. This process is called differentiation.

• Heat from radioactive decay and the energy of accretion melted planetary interiors.
• Denser materials (iron, nickel) sank to form a metallic core.
• Lighter materials (silicates) rose to form the mantle and crust.
• On Earth, this process produced the iron core responsible for our magnetic field and set the stage for plate tectonics.

Solar System Formation Theory

The nebular hypothesis ties all of these observations together into one framework. A collapsing, rotating cloud of gas and dust flattened into a disk, formed a star at its center, and built planets from the leftovers. Conservation of angular momentum explains the disk shape and prograde orbits. The condensation sequence explains why rocky planets formed close to the Sun and gas giants formed farther out, since different materials solidify at different temperatures. And planetary migration after formation explains why the final arrangement of the solar system doesn't perfectly match where everything originally formed.