The transiting exoplanet HD 149026b is an important case for theories of planet formation and planetary structure , because the planet ’ s relatively small size has been interpreted as evidence for a highly metal-enriched composition .
We present observations of 4 transits with the Near Infrared Camera and Multi-Object Spectrometer on the Hubble Space Telescope within a wavelength range of 1.1–2.0 \mu m. Analysis of the light curve gives the most precise estimate yet of the stellar mean density , \rho _ { \star } = 0.497 ^ { +0.042 } _ { -0.057 } g cm ^ { -3 } .
By requiring agreement between the observed stellar properties ( including \rho _ { \star } ) and stellar evolutionary models , we refine the estimate of the stellar radius : R _ { \star } = 1.541 ^ { +0.046 } _ { -0.042 } R _ { \sun } .
We also find a deeper transit than has been measured at optical and mid-infrared wavelengths .
Taken together , these findings imply a planetary radius of R _ { p } = 0.813 ^ { +0.027 } _ { -0.025 } R _ { Jup } , which is larger than earlier estimates .
Models of the planetary interior still require a metal-enriched composition , although the required degree of metal enrichment is reduced .
It is also possible that the deeper NICMOS transit is caused by wavelength-dependent absorption by constituents in the planet ’ s atmosphere , although simple model atmospheres do not predict this effect to be strong enough to account for the discrepancy .
We use the 4 newly-measured transit times to compute a refined transit ephemeris .