  We investigate the atmosphere and interior of the new transiting planet HD
149026b, which appears to be very rich in heavy elements. We first compute
model atmospheres at metallicities ranging from solar to ten times solar, and
show how for cases with high metallicity or inefficient redistribution of
energy from the day side, the planet may develop a hot stratosphere due to
absorption of stellar flux by TiO and VO. The spectra predicted by these models
are very different than cooler atmosphere models without stratospheres. The
spectral effects are potentially detectable with the Spitzer Space Telescope.
In addition the models with hot stratospheres lead to a large limb brightening,
rather than darkening. We compare the atmosphere of HD 149026b to other
well-known transiting planets, including the recently discovered HD 189733b,
which we show have planet-to-star flux ratios twice that of HD 209458 and
TrES-1. The methane abundance in the atmosphere of HD 189733b is a sensitive
indicator of atmospheric temperature and metallicity and can be constrained
with Spitzer IRAC observations. We then turn to interior studies of HD 149026b
and use a grid of self-consistent model atmospheres and high-pressure equations
of state for all components to compute thermal evolution models of the planet.
We estimate that the mass of heavy elements within the planet is in the range
of 60 to 93 M_earth. Finally, we discuss trends in the radii of transiting
planets with metallicity in light of this new member of the class.
