  We examine the distributions of eccentricity and host star metallicity of
exoplanets as a function of their mass. Planets with M sin i >~ 4 M_J have an
eccentricity distribution consistent with that of binary stars, while planets
with M sin i <~ 4 M_J are less eccentric than binary stars and more massive
planets. In addition, host star metallicities decrease with planet mass. The
statistical significance of both of these trends is only marginal with the
present sample of exoplanets. To account for these trends, we hypothesize that
there are two populations of gaseous planets: the low-mass population forms by
gas accretion onto a rock-ice core in a circumstellar disk and is more abundant
at high metalliticities, and the high-mass population forms directly by
fragmentation of a pre-stellar cloud. Planets of the first population form in
initially circular orbits and grow their eccentricities later, and may have a
mass upper limit from the total mass of the disk that can be accreted by the
core. The second population may have a mass lower limit resulting from
opacity-limited fragmentation. This would roughly divide the two populations in
mass, although they would likely overlap over some mass range. If most objects
in the second population form before the pre-stellar cloud becomes highly
opaque, they would have to be initially located in orbits larger than ~30 AU,
and would need to migrate to the much smaller orbits in which they are
observed. The higher mean orbital eccentricity of the second population might
be caused by the larger required intervals of radial migration, and the brown
dwarf desert might be due to the inability of high-mass brown dwarfs to migrate
inwards sufficiently in radius.
