Near-infrared transit photometry of the exoplanet HD 149026b

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 $R_p = 0.813^{+0.027}_{-0.025}$ $R_{\rm Jup}$, 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.