The water abundance in a planetary atmosphere provides a key constraint on the planet ’ s primordial origins because water ice is expected to play an important role in the core accretion model of planet formation .
However , the water content of the solar system giant planets is not well known because water is sequestered in clouds deep in their atmospheres .
By contrast , short-period exoplanets have such high temperatures that their atmospheres have water in the gas phase , making it possible to measure the water abundance for these objects .
We present a precise determination of the water abundance in the atmosphere of the 2 M _ { \mathrm { Jup } } short-period exoplanet WASP-43b based on thermal emission and transmission spectroscopy measurements obtained with the Hubble Space Telescope .
We find the water content is consistent with the value expected in a solar composition gas at planetary temperatures ( 0.4 - 3.5 \times solar at 1 \sigma confidence ) .
The metallicity of WASP-43b ’ s atmosphere suggested by this result extends the trend observed in the solar system of lower metal enrichment for higher planet masses .