Questions: Protoplanetary Disk Structure and Evolution

Question 1

The solar system's giant planets (Jupiter, Saturn, Uranus, Neptune) all formed beyond the snow line, while the rocky planets formed inside it. What is the primary reason for this pattern?

• A. The inner disk was too hot for any solid material to exist at all
• B. Solar wind stripped gas from the inner disk before any planets could capture it
• C. Beyond the snow line, water ice condenses and adds to the solid inventory, roughly tripling the surface density of planet-building material and enabling cores to grow massive enough to capture thick gas envelopes
• D. The inner disk rotated too quickly for solid particles to clump together into planetesimals

Answer: (C)

The snow line is the critical boundary because it marks where water vapor condenses into solid ice grains. Inside the snow line, only rock and metal are solid, so the surface density of solid material is relatively low. Beyond it, ice roughly triples the available solid mass per unit area. This abundance of solids lets planetary cores grow to the ~10 Earth-mass threshold needed to gravitationally capture a thick hydrogen/helium gas envelope — producing the gas and ice giants. Rocky planets form inside where there simply isn't enough solid material for such large cores.

Question 2

An astronomer observing a protoplanetary disk with ALMA detects a prominent ring-and-gap structure. What does a gap in the disk most likely indicate?

• A. A region where no disk material ever formed due to random density fluctuations in the original nebula
• B. The location of a forming planet or a pressure effect that has cleared or concentrated material in that orbital zone
• C. Where the disk ends and interstellar space begins
• D. A zone where ice has fully sublimated, leaving only gas

Answer: (B)

Ring-and-gap structures revealed by ALMA are among the most exciting features of observed protoplanetary disks. A gap is carved when a forming planet clears material from its orbital neighborhood through gravitational interaction, or when magnetic or pressure effects concentrate material into rings. Each gap potentially marks active planet formation. This is direct observational evidence that disk structure is not uniform — it has sharp features reflecting real physical processes — and that planet formation is already underway while the disk still exists.

Question 3

Protoplanetary disk structure is essentially static — snow lines remain at fixed orbital distances throughout the disk's lifetime.

• T. True
• F. False

Answer: False

Disk structure evolves significantly over the disk's few-million-year lifetime. As the central star's luminosity changes and the disk loses mass through accretion and photoevaporation, temperatures throughout the disk drop and the snow line migrates inward. This migration means the compositional zones available for planet building shift over time. A planet forming early in the disk's life encounters different conditions than one forming later. The disk is a dynamic, evolving system, not a fixed template.

Question 4

Outside the snow line, the surface density of solid planet-building material is roughly three times greater than inside it, because ice adds to the rocky material already present.

• T. True
• F. False

Answer: True

Inside the snow line, only refractory materials (silicate rocks, metals) are solid. Outside it, water ice condenses and adds substantially to the solid mass available per unit area — roughly tripling it. This is the key reason the giant planets formed in the outer solar system and rocky planets in the inner solar system. The snow line is not just a temperature boundary; it is a solid-material boundary that fundamentally controls what kinds of planets can grow at a given orbital distance.

Question 5

Explain why the snow line is a critical boundary for determining what kinds of planets form at different distances from a star.

Model answer: The snow line marks the distance from the star where temperatures fall below ~170 K, allowing water vapor to condense into solid ice grains. Inside the snow line, only rock and metal are solid, limiting the surface density of planet-building material. Outside it, ice adds to the solid inventory, roughly tripling the available mass per unit area. This abundance of solids allows planetary cores outside the snow line to grow to ~10 Earth masses — the threshold needed to gravitationally capture large volumes of hydrogen and helium gas, forming gas or ice giants. Interior cores never reach this threshold due to limited solid material, producing only rocky terrestrial planets. The snow line thus divides the disk into compositionally and structurally different zones with fundamentally different planet-forming outcomes.

Additional snow lines exist for CO₂, CO, and N₂ at progressively greater distances, each marking further compositional transitions. But the water snow line is the dominant one because water ice is by far the most abundant condensable volatile in the disk.