  We explore the possibility that large-scale convection be inhibited over some
regions of giant planet interiors, as a consequence of a gradient of
composition inherited either from their formation history or from particular
events like giant impacts or core erosion during their evolution. Under
appropriate circumstances, the redistribution of the gradient of molecular
weight can lead to double diffusive layered or overstable convection. This
leads to much less efficient heat transport and compositional mixing than
large-scale adiabatic convection. We show that this process can explain the
abnormally large radius of the transit planet HD209458b and similar objects,
and may be at play in some giant planets, with short-period planets offering
the most favorable conditions. Observational signatures of this transport
mechanism are a large radius and a reduced heat flux output compared with
uniformly mixed objects. If our suggestion is correct, it bears major
consequences on our understanding of giant planet formation, structure and
evolution, including possibly our own jovian planets.
