  Celestial bodies with a mass of M ~ 10 M_Jup have been found orbiting nearby
stars. It is unknown whether these objects formed like gas-giant planets
through core accretion or like stars through gravitational instability. I show
that objects with M <~ 4 M_Jup orbit metal-rich solar-type dwarf stars, a
property associated with core accretion. Objects with M >~ 10 M_Jup do not
share this property. This transition is coincident with a minimum in the
occurrence rate of such objects, suggesting that the maximum mass of a
celestial body formed through core accretion like a planet is less than 10
M_Jup. Consequently, objects with M >~ 10 M_Jup orbiting solar-type dwarf stars
likely formed through gravitational instability and should not be thought of as
planets. Theoretical models of giant planet formation in scaled minimum-mass
solar nebula Shakura--Sunyaev disks with standard parameters tuned to produce
giant planets predict a maximum mass nearly an order of magnitude larger. To
prevent newly formed giant planets from growing larger than 10 M_Jup,
protoplanetary disks must therefore be significantly less viscous or of lower
mass than typically assumed during the runaway gas accretion stage of giant
planet formation. Either effect would act to slow the Type I/II migration of
planetary embryos/giant planets and promote their survival. These inferences
are insensitive to the host star mass, planet formation location, or
characteristic disk dissipation time.
