  The core-accretion and disk instability models have so far been used to
explain planetary formation. These models have different conditions, such as
planet mass, disk mass, and metallicity for formation of gas giants. The
core-accretion model has a metallicity condition ([Fe/H] > &#8722;1.17 in the
case of G-type stars), and the mass of planets formed is less than 6 times that
of the Jupiter mass MJ. On the other hand, the disk instability model does not
have the metallicity condition, but requires the disk to be 15 times more
massive compared to the minimum mass solar nebulae model. The mass of planets
formed is more than 2MJ. These results are compared to the 161 detected planets
for each spectral type of the central stars. The results show that 90% of the
detected planets are consistent with the core-accretion model regardless of the
spectral type. The remaining 10% are not in the region explained by the
core-accretion model, but are explained by the disk instability model. We
derived the metallicity dependence of the formation probability of gas giants
for the core-accretion model. Comparing the result with the observed fraction
having gas giants, they are found to be consistent. On the other hand, the
observation cannot be explained by the disk instability model, because the
condition for gas giant formation is independent of the metallicity.
Consequently, most of planets detected so far are thought to have been formed
by the core-accretion process, and the rest by the disk instability process.
