Questions: Planetary Formation: The Nebular Hypothesis

Question 1

The frost line in a protoplanetary disk is significant primarily because it marks the boundary where:

• A. The disk becomes opaque to solar radiation, blocking further heating.
• B. The Sun's magnetic field can no longer strip away gas, allowing giant planet formation.
• C. Water ice and other volatile compounds can remain solid, enabling the accretion of larger, more massive planetesimals.
• D. Gravitational collapse of the disk into a second star becomes possible.

Answer: (C)

Inside the frost line, only high-temperature refractory materials (silicates and metals) can condense — too little solid material to build large planetary cores quickly. Beyond the frost line, water ice and other volatiles (CO₂, ammonia, methane ices) can also solidify, dramatically increasing the surface density of solid material. This allowed the outer solar system to build massive solid cores fast enough to gravitationally capture the abundant hydrogen and helium gas, forming the gas and ice giants.

Question 2

The planets of our solar system formed rapidly — within a few thousand years — as the solar nebula contracted and collapsed.

• T. True
• F. False

Answer: False

Planetary formation was a slow, violent process spanning tens of millions of years. Dust grains first stuck together electrostatically to form pebbles, then kilometer-scale planetesimals grew by accretion over millions of years, and final assembly of terrestrial planets involved giant impacts (including the Moon-forming impact on early Earth) that extended the process to roughly 50–100 million years after the Sun formed. Only the giant planet cores may have grown quickly — if they formed too slowly the nebular gas would have dissipated before they could capture it.

Question 3

Why are the inner planets rocky while the outer planets are gas and ice giants, even though all planets formed from the same solar nebula?

Model answer: The frost line divided the disk into two compositional zones. Inside the frost line, temperatures were too high for volatile ices to condense, so only rocky and metallic materials were available as solid building blocks — producing small, dense terrestrial planets. Beyond the frost line, water ice and other volatile compounds could solidify, greatly increasing the solid surface density and allowing much larger cores to form. These massive cores were then able to gravitationally capture the surrounding hydrogen and helium gas before it dissipated, producing the gas and ice giants.

This question targets the core insight of the frost-line model: the same nebula produced compositionally different planets solely because of the temperature gradient. The abundance of solid material available for accretion — and therefore the maximum planet mass achievable before the gas disk dispersed — depended entirely on heliocentric distance relative to the frost line.