From dust to planets: planetesimals and pebble accretion

4 problems

• What is the birth size distribution of planetesimals?

  Open

  If planetesimals form by sudden gravitational collapse, they should be born big, around 100 kilometers. The size mix at birth determines how fast planets can grow afterwards.

• How do solids grow past the drift and fragmentation barriers?

  Open

  Pebbles and boulders feel a headwind from the gas disk, so they lose orbital speed and spiral into the star in a few hundred orbits, and they shatter when they collide. Some process must form kilometer bodies faster than this.

• Do giant-planet cores grow mainly by pebble accretion?

  Contested

  A growing core can sweep up drifting pebbles efficiently because gas drag slows them as they pass. Whether this, or collisions with kilometer bodies, built the cores of Jupiter and Saturn is debated.

• When does the streaming instability collapse pebbles into planetesimals?

  Partially resolved

  Pebbles drifting through gas can bunch up through a feedback between their drag and the gas flow (the streaming instability), and dense bunches collapse under gravity. This only works if there are enough pebbles and the gas is calm enough, and the exact conditions are uncertain.