Pebble accretion

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Pebble accretion is the growth of planets by capturing drifting pebbles—small rocks from a few centimeters up to a meter across—in a young star’s gas disk. Gas drag slows the pebbles, helping them lose energy and be captured by growing bodies called planetesimals. This makes planet growth much faster because a larger area becomes able to grab material.

How it works
- In the disk, gas orbits a bit slower than the solids. Pebbles feel drag, drift toward the star, and frequently pass by planetesimals.
- If drag is strong enough, these pebbles slow down and become bound to the planetesimal, spiraling in and sticking to it.
- The capture efficiency depends on pebble size and the planetesimal’s mass. Small planetesimals accrete from a small region, while larger ones gain a bigger capture area and can grow quickly (runaway growth).
- Once a planet grows, its gravity plus gas drag expand the region from which pebbles can be captured. The process can switch from 3D accretion (from a layer of the disk) to 2D accretion (from the full disk height), speeding up growth.

Why it matters
- Pebble accretion can double an Earth-mass core in a few thousand years and form giant planet cores in a few million years, before the gas disk disappears.
- When a core becomes big enough to open a partial gap in the gas disk, the inward drift of pebbles slows or stops, ending pebble accretion. The core can then rapidly attract gas and become a gas giant; smaller cores may become ice giants or stay terrestrial.

Inner vs outer Solar System
- Outer Solar System: gas and pebbles are more favorable for rapid growth, helping form gas giants like Jupiter and Saturn.
- Inner Solar System: crossing the ice line changes pebble sizes and reduces the solid mass flowing inward, leading to slower growth and smaller planets like Mars and the current asteroid belt.
- The timing and amount of pebble delivery also affect how many giant planets form vs. many smaller, Earth-mass bodies.

Overall picture
- Pebble accretion speeds up planet formation and helps explain how giant planet cores can form quickly, while also offering a mechanism that makes the inner Solar System’s smaller planets and asteroid belt natural outcomes. The details depend on the disk’s density, the timing of pebble supply, and how rapidly pebbles drift and sublimate at the ice line.