2005ApJ...626.1045I

The origin of a recently discovered close-in Neptune-mass planet around GJ 436 poses a challenge to the current theories of planet formation. On the basis of the sequential accretion hypothesis and the standard theory of gap formation and orbital migration, we show that around M dwarf stars, close-in Neptune-mass ice giant planets may be relatively common, while close-in Jupiter-mass gas giant planets are relatively rare. The mass distribution of close-in planets generally has two peaks at about Neptune mass and Jupiter mass. The lower mass peak takes the maximum frequency for M dwarfs. Around more massive solar-type stars (G dwarfs), the higher mass peak is much more pronounced. Planets around G dwarfs undergo orbital migration after fully accreting gas, while those around M dwarfs tend to migrate before starting rapid gas accretion. Close-in Neptune-mass planets may also exist around G dwarfs, although they tend to be mostly composed of silicates and iron cores and their frequency is expected to be much smaller than that of Neptune-mass planets around M dwarfs and that of gas giants around G dwarfs. We also show that the conditions for planets' migration due to their tidal interaction with the disk and the stellar mass dependence in the disk mass distribution can be calibrated by the mass distribution of short-period planets around host stars with various masses.