In the core-accretion model , gas-giant planets form solid cores which then accrete gaseous envelopes .
Tidal interactions with disk gas cause a core to undergo inward type-I migration in 10 ^ { 4 } to 10 ^ { 5 } years .
Cores must form faster than this to survive .
Giant planets clear a gap in the disk and undergo inward type-II migration in < 10 ^ { 6 } years if observed disk accretion rates apply to the disk as a whole .
Type-II migration times exceed typical disk lifetimes if viscous accretion occurs mainly in the surface layers of disks .
Low turbulent viscosities near the midplane may allow planetesimals to form by coagulation of dust grains .
The radius r of such planetesimals is unknown .
If r < 0.5 km , the core formation time is shorter than the type-I migration timescale and cores will survive .
Migration is substantial in most cases , leading to a wide range of planetary orbits , consistent with the observed variety of extrasolar systems .
When r \sim 100 m and midplane \alpha \sim 3 \times 10 ^ { -5 } , giant planets similar to those in the Solar System can form .