A growing protoplanetary core encounters both a nearby planetesimal and a nearby pebble of similar mass. Which is more likely to be captured, and why?
AThe planetesimal, because its larger size increases the gravitational cross-section
BThe pebble, because aerodynamic drag bleeds away its kinetic energy, causing it to spiral onto the core rather than fly past
CBoth are equally likely; capture probability depends only on the core's mass, not the impactor size
DThe planetesimal, because pebbles are too small to feel the core's gravity at distance
Pebbles are strongly coupled to disk gas through aerodynamic drag. As a pebble drifts near a core, drag dissipates its kinetic energy, causing it to settle onto the core rather than following a ballistic trajectory past it. The effective capture cross-section for pebbles scales with the Hill sphere and pebble stopping time — far larger than the core's physical size. A planetesimal, by contrast, has weak gas coupling and follows nearly ballistic orbits, so it must nearly directly impact the core to be captured.
Question 2 Multiple Choice
Pebble accretion is considered to solve the 'timescale problem' in giant planet formation. What is that problem?
AGas disks around young stars dissipate in ~3–10 million years, but classical planetesimal accretion is too slow to grow a giant planet core before the gas disappears
BPebbles drift too quickly through the disk, preventing core growth entirely unless the disk is unusually massive
CGiant planet formation requires a minimum disk temperature that most young stellar systems cannot achieve
DPlanetesimals are too numerous, causing so many collisions that cores are ground down rather than grown
Gas giants need a core of roughly 10 Earth masses to capture a gas envelope, but the disk lifetime is only ~3–10 million years. Classical planetesimal accretion in the outer solar system was calculated to take tens of millions of years to build such a core — far longer than the disk survives. Pebble accretion resolves this because the drag-enhanced capture cross-section and the inward drift of pebbles act as a continuous conveyor belt, growing cores orders of magnitude faster.
Question 3 True / False
Pebble accretion works efficiently precisely because pebbles are aerodynamically coupled to disk gas, giving a growing core an effective capture radius far larger than its physical size.
TTrue
FFalse
Answer: True
This is the central mechanism. Drag-dominated motion causes pebbles that pass within a core's extended gravitational influence to lose energy to gas friction and spiral inward rather than continuing on ballistic paths. The effective capture cross-section scales with the Hill sphere and the pebble stopping time, making it vastly larger than the geometric cross-section that governs planetesimal capture.
Question 4 True / False
Pebble accretion requires an unusually massive protoplanetary disk to supply enough solid material for rapid giant planet core growth.
TTrue
FFalse
Answer: False
This is a common misconception. Pebble accretion is efficient precisely because it works in typical disk conditions — the efficiency gain comes from the drag-enhanced capture geometry, not from requiring more material. Because pebbles drift inward continuously and are captured with high efficiency, even a thin disk can supply enough pebbles to grow a core rapidly.
Question 5 Short Answer
Explain why pebble accretion can grow giant planet cores orders of magnitude faster than classical planetesimal accretion.
Think about your answer, then reveal below.
Model answer: In planetesimal accretion, capture requires a near-direct collision or strong gravitational deflection, so the effective cross-section is close to the core's physical size plus modest gravitational focusing. In pebble accretion, aerodynamic drag dissipates the kinetic energy of pebbles as they pass near the core, causing them to spiral onto the core from a much larger distance. The effective capture radius scales with the Hill sphere and the pebble stopping time, far exceeding the physical radius. Additionally, pebbles continuously drift inward through the disk, delivering a steady supply without requiring the core to gravitationally scatter each particle.
The key insight is that drag-dominated motion transforms the physics of capture. Planetesimals follow nearly ballistic orbits and require nearly direct hits; pebbles effectively 'fall' onto the core from a large surrounding region. This transforms accretion from a slow, collision-by-collision process into a rapid, drag-assisted funneling process.